Grid interactive: what benefits make it worthwhile?

danielhdanielh Solar Expert Posts: 69 ✭✭✭✭
I will be installing a 6.5 kw system (25 Helios 260w) on my suburban DC house. I was originally planning on a basic grid-tie, using microinverters or dc optimizers (due to intermittent shading from neighboring trees). Note that the local utility supports net metering, and this system will deliver about 50% of my annual electricity usage.

However, I have been thinking about some form of grid interactive system. These thoughts became especially poignant after my just concluded 3 day power outage.
During that time I ran a Generac 6500 generator, which kept the basics going (lights, fridge, tv, laptops, and a few fans). Nevertheless, it is loud/smelly/requires maintenance.... and is a gas pig (1/2gal hour so about $30/day in the 90+ degrees we have been enjoying)

Not having to use a generator for such blackouts (which seem to happen every 2 years) is a pleasant prospect. But is it worth the 5,6,8 thousand bucks extra for a better inverter/batteries/charger etc? At the upper end (8k, call it 5.6k after 30% fed rebate), probably not: if one pessimistically assumes 20 blackout days spread over the next 10 years, that's $280 a day! Even in DC that's a lot!

But I am realizing generator replacement is not the only advantage of a battery enabled system. There is also power cleaning, load shifting, and perhaps others. Power cleaning would mean I could stop spending $100+ /yr replacing UPS's (I have about 5 on phone lines, tvs, and several computers). Load shifting opens the possibility of time-of-use metering. And maybe a smaller system, something less than 8k extra would work (note that by extra, I mean costs above what the same sized grid-tie setup would cost). For example, maybe 4k would be on a "grid interactive" setup, the remaining 2.5k would be grid-tied (perhaps using microinverters).

I also wonder if it might be worthwhile to install grid-tie now and wait a few years to upgrade, since the choice of equipment seems to be betting better all the time.
But that means losing current investment ("discarding" a grid-tie inverter), AND missing out on the 30% fed subsidy (from installing batteries, etc with the panels).

So, what are peoples experience, and thoughts, on this?
For example, would setting up a central inverter (such as the outback radian) to provide good UPS services mean disabling grid-tie?
That could get expensive, since I suspect over half of my power production will not be needed when produced.

Thanks (btw: this is my first post here)
«13

Comments

  • tmarchtmarch Solar Expert Posts: 143 ✭✭
    Re: Grid interactive: what benefits make it worthwhile?

    IF and it's a big IF, you are close to a city and your grid continues to be as reliable as it has been, maybe it's not as good an investment as it is for us "country folk" that are miles from the nearest substation.
    Personally I'm adding a Sunny Island and some batteries to my system at less than 5K. That was my original plan and I see nothing to change my mind. My grid is reliable to the extent that I haven't experienced a prolonged outage of over a day for several years, but it like many is old enough a minor storm could lead to a extended outage.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Grid interactive: what benefits make it worthwhile?

    Welcome to the forum.

    You might be better off investing in a smaller, quieter, more fuel-efficient inverter-generator for occasional outages.
    Battery back-up power comes at no small expense, and if it goes unused most of the time it's a pretty big waste of money.
  • danielhdanielh Solar Expert Posts: 69 ✭✭✭✭
    Re: Grid interactive: what benefits make it worthwhile?

    The notion of spending $3k on a Hondaeque generator has crossed my mind. But doesn't solve the annoyance of having
    the equivalence of a nice new car sitting on your roof, that can't be used when most needed! More importantly, the other benefits of
    "grid interactive" capability, such as load balancing and cleaner power, are not provided.

    BTW: I think I messed up; I should of posted this in the grid-tied/grid-interactive forum. Is that something you can correct?
  • danielhdanielh Solar Expert Posts: 69 ✭✭✭✭
    Re: Grid interactive: what benefits make it worthwhile?
    tmarch wrote: »
    IF and it's a big IF, you are close to a city and your grid continues to be as reliable as it has been, maybe it's not as good an investment as it is for us "country folk" that are miles from the nearest substation.
    Personally I'm adding a Sunny Island and some batteries to my system at less than 5K. That was my original plan and I see nothing to change my mind. My grid is reliable to the extent that I haven't experienced a prolonged outage of over a day for several years, but it like many is old enough a minor storm could lead to a extended outage.

    That is a great unknown. Will changing climates mean more destructive storms (now I know what a "derecheo" is)?
    Will skimping on maintenance in our deregulated power grid mean they cause more damage, or produce dirtier power?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Grid interactive: what benefits make it worthwhile?
    danielh wrote: »
    BTW: I think I messed up; I should of posted this in the grid-tied/grid-interactive forum. Is that something you can correct?

    Easily fixed. :D

    What we're saying here is you have to weigh the costs and benefits very carefully because it inevitably is a lot of money to go with either a battery-based hybrid system like the Radian or XW or to add battery back-up with something like the SMA Sunny Island or even a small, stand-alone off-grid inverter. For instance you can use a standard GT array to charge batteries through the Xantrex 600 Volt charge controller, but that costs over $1,200 - the price of a Honda EU2000i. Then you still need batteries (which need to be replaced over time no matter what) and the off-grid inverter - which will add thousands more on to the price.

    You are right that the most economical approach for having battery back-up is to start with the hybrid GT system in the first place, rather than trying to add the ability later.

    And someone is inevitably going to come in here and mention AC coupling to make use of the straight GT for additional power generation. In my opinion such systems are too technical for the average homeowner to deal with.
  • BB.BB. Super Moderators, Administrators Posts: 32,027 admin
    Re: Grid interactive: what benefits make it worthwhile?

    More than likely, the local utility will raise prices (we range from $0.09 to $0.50 per kWH depending on residential plan and how much power we use... >~1,000 kWH per month is >$0.40 per kWH, under 300kWH per month around $0.09 to $0.13 per kWH)...

    I would love to install a Hybrid inverter system (GT+Off Grid)--But for my location, so far the last outage that was over a few hours was 50+ years ago (about a 1 week outage). It just does not make sense for me.

    Roughly, look at pricing for power:
    • $0.10 to $0.30 for Utility Power
    • $0.10 to $0.30 for Grid Tied Solar (no emergency backup)
    • $0.45 to $0.75 for Hybrid Solar (GT/Off Grid + battery bank)
    • $1 to $2+ per kWH for full off-grid solar

    In the end, look at your power needs--Both for the full home and emergency backup power. And then work on conservation--It is almost always cheaper to conserve a Watt*Hour vs generating a WH of power.

    If you are interested in Off-Grid solar PV system, see if you can get your power down to the 3.3 kWH per day (100 kWH per month) range... Systems are not too expensive and maintenance costs will not break the bank (plan on batteries lasting ~5-10 years, electronics needing replacement after ~10+ years, solar panels 25+ years).

    Conservation will be your friend in any case... It is almost always cheaper to conserve than to generate power. And for many people, they do not have a good location to install a 3-10kW solar array (shading from buildings/trees and even power lines all can "kill" solar PV power generation).

    If you need A/C for your location--If you can cool one room (well insulated, shaded from direct sun) with a Mini-Split A/C system, those are friendly for off grid solar and even generator supplied power.

    One way to approach the problem is design a system to run your home overnight on battery power and use genset/grid power to recharge... Eventually, you can add solar PV to help reduce fuel usage (again, for emergency loads rather than the whole home).

    I really believe in sizing the power system to your loads... For example, my home (no A/C, natural gas for heating) can run very nicely on a 1,600 watt genset (Honda eu2000i) on ~1-2 gallons of fuel per day--20 gallons will keep me in power for 10+ days. Plus I can use fuel from the car or even use the emergency fuel for the car if I need to evacuate (we are in earthquake country--So, there is a possibility our home would not be standing--so a large solar off grid/hybrid system may not be usable "after the big one").

    I would guess that there is a good chance your utility rates will go way up to better prepare for the next emergency--So conservation will be a good investment anyway.

    if you do not have a good sunny location for solar panels (lots of shading, even from overhead power lines), solar PV may never be a good solution for you... Add the cost of maintenance (replacing batteries every ~5-10 years, new electronics every 10+ years), and the generator looks better and better.

    Anyway, if you have "good enough" sun, and can cut your power usage back during an emergency (my suggestion would be around 3.3 kWH per day/100 kWH per month), you can have a off-grid capable system for a reasonable amount of money/costs.

    If A/C is needed, if you can use a single Mini-Split A/C system in a well insulated room that does not get a lot of heat from the sun, you can run that from a smallish genset and even from an off grid solar system.

    Note that reducing your loads through conservation (laptop computer vs desktop on 24x7, replacing lights with CFL/LED, looking at your entertainment system for energy usage, etc.) can also save on your cooling bill (less power "wasted" in the home, less waste heat the A/C need to move outside).

    It is not unusual for a family to reduce their energy use by 1/2 with insulation, energy start appliances, turning unused equipment off (DVR's, Sat-Receivers, Desktop Computer systems, etc. can use a lot of energy when in standby/people walk away for a bit) will all help a lot.

    A Kill-a-Watt type meter, whole house power monitor system, etc. can help you identify your loads and usage so you can spend money where it will help the most (links are starting points for your research).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • vtmapsvtmaps Solar Expert Posts: 3,739 ✭✭✭✭
    Re: Grid interactive: what benefits make it worthwhile?

    My 2 cents: stick to a basic grid tie system with a quiet inverter generator (honda or yamaha). The overall efficiency of battery based systems is about 50%. Therefore it is difficult to justify storing power to take advantage of TOU rates.

    I don't know why you are spending $100 year on UPS. Maybe you could install a 'central' UPS rather than having a bunch of smaller ones. If the UPS is powered by solar it will qualify for tax credits. You would want to put in a separate electrical panel that was wired to the outlets in your house where you wanted clean UPS power. In effect you have two independent solar systems, a larger grid tie, backed up by generator, and a smaller off-grid system. Both eligible for tax credits.

    Redundancy can be a good thing.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: Grid interactive: what benefits make it worthwhile?

    The bottom line. Take the money you would invest in batteries and battery infrastructure and invest it instead in more PV. (and perhaps a more efficient ). $30/day for a week is $210, for a fluke event is pretty cheap.

    The reality is the battery systems have limited capacity, and if you amortize the hours they are actually needed relative to thier cost, they re going to be way more expensive than a genny. The caveat however is if yard is very unreliable. A few times a year for a few hours or a couple of days,, genny wins, hands down. Images every week, for days and weeks,, maybe you have an argument.

    Tony
  • Ralph DayRalph Day Solar Expert Posts: 953 ✭✭✭✭
    Re: Grid interactive: what benefits make it worthwhile?

    "$30/day for a week is $210, for a fluke event is pretty cheap. "

    Can't help but think this week...what is a fluke event? Heavy storms tend to knock down lines and poles, which can only be repaired so fast. I heard some of our Ontario Hydro One crews are down helping repair some of last week's storm damage. That's nice, but how often and how far afield can one utility go to help (and still provide for their own customer base?).

    For anyone building new now I'd recommend at the least, a generator sub panel for critical loads and an inverter generator to suit. At the most...well, if it's someone else's money:cool: let's play!

    Ralph
  • mike95490mike95490 Solar Expert Posts: 9,363 ✭✭✭✭✭
    Re: Grid interactive: what benefits make it worthwhile?

    If you really want to spend $$$, you can get the Xantrex 600V charge controller, and have it charge 4, 12 deep cycle batteries wired for 48V. Add a small 48V inverter, and you can ride out power failures while the sun shines.

    But I think a Auto Throttle Inverter Generator would be the right thing to have.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • danielhdanielh Solar Expert Posts: 69 ✭✭✭✭
    Re: Grid interactive: what benefits make it worthwhile?
    vtmaps wrote: »
    My 2 cents: stick to a basic grid tie system with a quiet inverter generator (honda or yamaha). The overall efficiency of battery based systems is about 50%. Therefore it is difficult to justify storing power to take advantage of TOU rates.
    Excuse my ignorance, but what do you mean by ""overall efficiency is ... 50%". Do you mean that one can only safely draw 50% of rated power (say, 4.8 kwh from a 9.6khw battery bank). Or are there major losses in storing/extracting electricity from batteries that I am not aware of?
    vtmaps wrote: »
    I don't know why you are spending $100 year on UPS. Maybe you could install a 'central' UPS rather than having a bunch of smaller ones. If the UPS is powered by solar it will qualify for tax credits. You would want to put in a separate electrical panel that was wired to the outlets in your house where you wanted clean UPS power. In effect you have two independent solar systems, a larger grid tie, backed up by generator, and a smaller off-grid system. Both eligible for tax credits.

    Redundancy can be a good thing.

    --vtMaps
    I have several UPS, each costing between $60 to $120. One for wife computer, one for my computer, one for a server I run out of my house, one for the router/network switch/ phone, one for the kid's computer (acdtually, 2, but they are at college half the year), and one for the big screen TV. The batteries on these things die, my rough guess is they have a 2-3 year life. One can replace the batteries, but it isn't much of a savings.

    Central UPS on an off-grid system is an interesting idea... but it seems risky to dedicate panels to an off-grid system whose main purpose is clean power. First, what if it runs out of power (a week of gray winter weather). Conversely, what if it can't use or store the power (everyone is busy and not using their computer). That is why I was wondering if any one had experience with a grid-interactive system that would both provide clean power, and be able to get and sell power to the grid. For example, the Radian seems to have that capability, but not at the same time (so it kind of doesn't do what I want). But perhaps I am missing something.
  • danielhdanielh Solar Expert Posts: 69 ✭✭✭✭
    Re: Grid interactive: what benefits make it worthwhile?
    Ralph Day wrote: »
    "$30/day for a week is $210, for a fluke event is pretty cheap. "

    For anyone building new now I'd recommend at the least, a generator sub panel for critical loads and an inverter generator to suit. At the most...well, if it's someone else's money:cool: let's play!

    Ralph
    I did that back in January, before the idea of "get solar now!" took root in my brain (glad I did -- it came in useful this weekend) So I have a 7000w transfer switch, and a 6500w hunk of 'merican generator.
    It would be nice to use the existing transfer switch as the critical load subpanel for a grid-interactive system.

    What I am hearing in this thread is that it is a good idea to run a grid-interactive inverter to a sub panel with critical loads, and that connects to the main panel (for other household loads and for selling to the grid). Such a strategy keeps future upgrades (i.e. AC coupling) easier to implement.
  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: Grid interactive: what benefits make it worthwhile?

    In short, batteries are much less efficient than direct grid tie. There are a number of built pin efficiency issues. The first is the simple fact hath it takes ~20% more power to charge a battery than that battery will give back. Add in charge controller losses, loses associated with charging a nearly full battery. (if the battery is nearly full, the panels will simply not produce power into the batteries if it is not needed.

    Add all these things up, and y of can see how you can get to 50%.

    Tony
  • danielhdanielh Solar Expert Posts: 69 ✭✭✭✭
    Re: Grid interactive: what benefits make it worthwhile?
    icarus wrote: »
    The bottom line. Take the money you would invest in batteries and battery infrastructure and invest it instead in more PV. (and perhaps a more efficient ). $30/day for a week is $210, for a fluke event is pretty cheap.

    The reality is the battery systems have limited capacity, and if you amortize the hours they are actually needed relative to thier cost, they re going to be way more expensive than a genny. The caveat however is if yard is very unreliable. A few times a year for a few hours or a couple of days,, genny wins, hands down. Images every week, for days and weeks,, maybe you have an argument.

    Tony

    If riding out an outage or five is the only reason for batteryizing a residential pv installation, then I basically agree. The frustration of having "a nice new car" on the roof that can't be used when most needed -- is worth something, but not enough.

    But it may be that day-to-day benefits of "batteryizing" are more impotant? Perhaps they are on the scale of power outage convenience (a hudnred or two a year), but perhaps they are larger. This is what I am trying to suss out!
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Grid interactive: what benefits make it worthwhile?

    Actually the Outback Radian will do what you want. It is of the type of inverter known as a "hybrid", meaning that it can supply power like a "straight" grid-tie inverter while maintaining charged batteries to provide back-up power should the grid go down.

    vtMaps reference to 50% efficiency is in respect to a battery-based system, wherein the actual AC Watt hour produce is roughly half the STC rating of the array * the hours of good sun. So a 5 kW array in 4 hours does not produce 20 kW hours AC, but only about 10. This is due to the inefficiencies of battery charging/discharging and the Watt hour potential that is lost once the batteries are charged and there's no place for the power the panels could produce to go.

    With a standard grid-tie system the losses are far less. This is because every Watt hour the panels can produce has someplace to go: either consumed by the loads in-house or sold to the utility.

    A hybrid system will function near a standard GT's efficiency while operating in that mode, and like an off-grid system when the utility power goes down.
  • BB.BB. Super Moderators, Administrators Posts: 32,027 admin
    Re: Grid interactive: what benefits make it worthwhile?

    Solar panels, on average only generate about 82% of name plate rating, solar charger is about 95% efficient, batteries are around 80 to 90% efficient, inverters are about 85% efficient.

    All together about 52% efficient from solar panel name plate to your 120 vac load.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • danielhdanielh Solar Expert Posts: 69 ✭✭✭✭
    Re: Grid interactive: what benefits make it worthwhile?
    BB. wrote: »
    In the end, look at your power needs--Both for the full home and emergency backup power. And then work on conservation--It is almost always cheaper to conserve a Watt*Hour vs generating a WH of power.

    If you are interested in Off-Grid solar PV system, see if you can get your power down to the 3.3 kWH per day (100 kWH per month) range... Systems are not too expensive and maintenance costs will not break the bank (plan on batteries lasting ~5-10 years, electronics needing replacement after ~10+ years, solar panels 25+ years).

    Conservation will be your friend in any case... It is almost always cheaper to conserve than to generate power. And for many people, they do not have a good location to install a 3-10kW solar array (shading from buildings/trees and even power lines all can "kill" solar PV power generation).
    Most of the low hanging fruit is picked. 19 seer AC/HP just installed. Double & triple pane windows. R-38 attic insulation. CFL/LED everywhere (even several dimmable CFLS).
    Front end washing machine, energy eff (bosch) dishwasher. True, my computer usage is sloppy (I tend to leave CPUs running 24/7); but getting newer/better computers over time should help with that.

    In fact, it is the completion of these steps that led me to "time to think about solar".
    BB. wrote: »
    If you need A/C for your location--If you can cool one room (well insulated, shaded from direct sun) with a Mini-Split A/C system, those are friendly for off grid solar and even generator supplied power.

    One way to approach the problem is design a system to run your home overnight on battery power and use genset/grid power to recharge... Eventually, you can add solar PV to help reduce fuel usage (again, for emergency loads rather than the whole home).
    AC in our nation's capital is kind of a gotta-have for every day living. One can get by for a few days of outages, but living without it would not be fun. Doing just one room would be better than nothing, but I am spoiled, and want to feel mostly comfy everywhere in the house (we keep it around 78, and 50 % rh which is actually more important)

    I should say that I am much more interested in PV for "harnessing the sun" reasons than for outage protection (my current generator isn't at all elegant, but it is tolerable).

    BB. wrote: »

    It is not unusual for a family to reduce their energy use by 1/2 with insulation, energy start appliances, turning unused equipment off (DVR's, Sat-Receivers, Desktop Computer systems, etc. can use a lot of energy when in standby/people walk away for a bit) will all help a lot.

    A Kill-a-Watt type meter, whole house power monitor system, etc. can help you identify your loads and usage so you can spend money where it will help the most (links are starting points for your research).

    -Bill

    It will be interesting to see my power bills over the next months (since I have made a slew of conservation improvements in the last few months). And whole house monitoring (TED) will be part of my PV install.
  • danielhdanielh Solar Expert Posts: 69 ✭✭✭✭
    Re: Grid interactive: what benefits make it worthwhile?
    BB. wrote: »
    Solar panels, on average only generate about 82% of name plate rating, Chaffer is about efficient, batteries are around 80 to 90% efficient, inverted are about 85% efficient.

    All together about 52% efficient from solar panel name plate to your 120 vac load.

    Solar panels, on average only generate about 82% of name plate rating, solar charger is about 95% efficient, batteries are around 80 to 90% efficient, inverters are about 85% efficient.

    All together about 52% efficient from solar panel name plate to your 120 vac load.
    [fixed my typo's from phone post. -Bill]

    -Bill
    But the "% of rating" and "inverter efficiency" effect you whether you are using the power immediately, selling to the grid, or sending it to a battery bank? So isn't it more accurate to say "batteries are 80 to 90 % efficient, compared with using it right away". So the question is "is a ~20% loss so large that it isn't worthwhile for load balancing (i.e.; drain batteries for the heavy day time loads, and charge them at night).
  • danielhdanielh Solar Expert Posts: 69 ✭✭✭✭
    Re: Grid interactive: what benefits make it worthwhile?
    Actually the Outback Radian will do what you want. It is of the type of inverter known as a "hybrid", meaning that it can supply power like a "straight" grid-tie inverter while maintaining charged batteries to provide back-up power should the grid go down.

    vtMaps reference to 50% efficiency is in respect to a battery-based system, wherein the actual AC Watt hour produce is roughly half the STC rating of the array * the hours of good sun. So a 5 kW array in 4 hours does not produce 20 kW hours AC, but only about 10. This is due to the inefficiencies of battery charging/discharging and the Watt hour potential that is lost once the batteries are charged and there's no place for the power the panels could produce to go.

    With a standard grid-tie system the losses are far less. This is because every Watt hour the panels can produce has someplace to go: either consumed by the loads in-house or sold to the utility.

    A hybrid system will function near a standard GT's efficiency while operating in that mode, and like an off-grid system when the utility power goes down.

    The Radian is a choice, but I wonder how spiffy it is: can it chew gum (provide power in a grid outage) and hammer nails (act as a UPS) at the same time?7227

    That said ...

    I seem to be getting a message from you all: "if you can grid-tie, then it isn't worthwhile to use batteries for every day use -- storing and discharging them loses too much power." And "that if you want convenience for the occasional blackout, replace your genset with something good"
  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: Grid interactive: what benefits make it worthwhile?

    But it is not just the 20%! There re the other cumulative loses as well. As has been stated wi th grid tie, 100% of the output is used (or sent to the grid) (minus a few percentage of invert and wiring loses). With a battery based system you have basic battery charging loses (~20%) plus charge controller loses, plus loses as Bettie's come full, PLUS. Similar inverter efficiency as grid tie.

    As I said before, you may have a new caron your roof, but you maybe much further ahead net/net by simply using a genny during those times of outages. In the real world, multiple day outages are quite rare, and those longr than a few days are even rarer.

    A few dollars (or few hundred) in fuel (since you already have a genny) is likely to be waaay cheaper, and more reliable.

    Just my opinions.

    Tony

    Ps your last post overlapped mine. The bottom line with gennies, is use the best gennie for the right job. A diesel unit is probably going to be the most fuel efficient, but for those rare outages, may not be cost effective. Consider the proper size genny for the job, and pare down the needs to essentials and you can cut your fuel costs considers. For the rare us mmer outage, consider only partial cooling, for winter only partial heat etc

    T.
  • BB.BB. Super Moderators, Administrators Posts: 32,027 admin
    Re: Grid interactive: what benefits make it worthwhile?
    danielh wrote: »
    But the "% of rating" and "inverter efficiency" effect you whether you are using the power immediately, selling to the grid, or sending it to a battery bank? So isn't it more accurate to say "batteries are 80 to 90 % efficient, compared with using it right away". So the question is "is a ~20% loss so large that it isn't worthwhile for load balancing (i.e.; drain batteries for the heavy day time loads, and charge them at night).

    A hybrid inverter system ends up being a bit more efficient because you are 1) not cycling the batteries, and 2) have the grid -- which is, for all the world, an unlimited AC battery bank (so you can store all harvested power, and not just until the battery bank is full).

    The typical Grid Tied system is around 77% efficient (again, from solar panel nameplate to AC out to grid). Hybrid inverters tend to have a bit more losses, but are still better than off-grid system efficiency (no battery cycling, some hybrid inverters are very efficient compared to "typical" off grid inverters).

    If you time shift (use battery power during the day), you not only have the cost of battery charging (typical AC to DC charger is around 80% efficient, battery ~80-90% efficient, and re-invertering around 85-90+% efficient = ~54% worst case efficiency to power shift) and you are cycling the batteries, which have a limited cycle life too (around 500-2,000 cycles, depending on how deep you are cycling, type of battery, how expensive of battery, etc.)...

    So, It usually does not make a lot of sense to install a "hybrid" system just to take advantage of time shifted power use unless you have a huge difference in peak/off peak costs (probably on the order of $0.45 per kWH between peak and off peak power costs just to break even--but you can do your own calculations and see what you get--prices for hardware/panels/etc. change all the time).

    If you have a GT system and Time of Use metering, that is your "optimum" power shifting... I generate on sunny summer afternoons for $0.30 per kWH, and night time/off peak weekends, pay around $0.09 per kWH. I also try to not use power from noon-6pm summer weekdays (peak time). Cooking/washing/etc. is done during off peak (for the most part).

    If your utility does not support net metering (or has a poor plan for GT solar billing), you are, more or less, out of luck (in terms of saving money).

    Throwing a Hybrid Inverter + Battery Bank will give me emergency power capabilities, but it will not save me money.

    Glad to hear you went down the conservation road--You should be very happy with the results (if nothing else, the homes are usually much nicer, more even temperatures, quieter, fewer drafts, etc.).

    Still more than happy to work some numbers (battery bank, array size, gt vs off grid) based on your new power usage so we can figure out your options.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: Grid interactive: what benefits make it worthwhile?

    Has no one mentioned AC coupling as an option yet? I think this would give you a very nice solution since you can use a very efficient grid tied inverter (95%) for the 99% of the time you'll be spent connected to the grid. Then use a battery based inverter as a whole house UPS. While the grid is active the only efficiency loss is the panels and the grid tie inverter (the battery inverter is out of the equation since it's in pass-through mode). SMA has the cleanest solution for this, but xantrex and magnum both support some form of AC coupling.

    Using a charge controller will introduce 2 extra losses into the system: firstly the charge controller itself (around 97% efficient) and secondly the battery based inverter (which is always less efficient than a grid tie inverter), around 95% max efficiency. If the system is going to be grid connected 99% of the time, then I would rather design it for max efficiency during that time.

    Attachment not found.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Grid interactive: what benefits make it worthwhile?
    danielh wrote: »
    The Radian is a choice, but I wonder how spiffy it is: can it chew gum (provide power in a grid outage) and hammer nails (act as a UPS) at the same time?

    Yes; that is exactly what hybrid inverters do. Feed power from panels to grid when it's up, and supply power from batteries recharged by panels when the grid is down.
    That said ...

    I seem to be getting a message from you all: "if you can grid-tie, then it isn't worthwhile to use batteries for every day use -- storing and discharging them loses too much power." And "that if you want convenience for the occasional blackout, replace your genset with something good"

    Yes again; you do not want to regularly discharge and recharge batteries when you've got grid power. There simply is no point. Some people do this to "time shift" their load demands: recharging the batteries when utility rates are low and discharging them when rates are high. It's doubtful this is worthwhile in most cases, especially when the added cost of accelerated battery life depletion is included.

    When it comes to back-up power you have to consider your particular circumstances, i.e. frequency and length of outages and how critical the supply need is. For most, but not all, people an efficient generator is the most cost-effective solution.
  • mike95490mike95490 Solar Expert Posts: 9,363 ✭✭✭✭✭
    Re: Grid interactive: what benefits make it worthwhile?

    Any of the hybrid inverters (Xantrex, Radian, SMA) will still need a high voltage Charge Controller to maintain the battery if the grid goes down. AFIK, only the XW 600 can do that.

    A generator CAN feed the inverter, which will charge the batteries, if you don't want solar to do it.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: Grid interactive: what benefits make it worthwhile?
    mike90045 wrote: »
    Any of the hybrid inverters (Xantrex, Radian, SMA) will still need a high voltage Charge Controller to maintain the battery if the grid goes down. AFIK, only the XW 600 can do that.

    OR you could AC couple a grid tie inverter to the Xantrex or SMA instead of using a charge controller, as in the diagram posted above.
  • danielhdanielh Solar Expert Posts: 69 ✭✭✭✭
    Re: Grid interactive: what benefits make it worthwhile?
    Yes; that is exactly what hybrid inverters do. Feed power from panels to grid when it's up, and supply power from batteries recharged by panels when the grid is down.
    Umm... the "goal" is to have a big UPS and be able to sell power to the grid. With said UPS being active all the time -- not just when the grid is down. The Radian's literature states there are 6 modes. One of this is UPS mode, another is Grid-Tie. In UPS mode, selling back to the grid is NOT supported. In grid-tie mode, the switch over (should the grid go down) is about 2x the delay as in UPS mode -- which might not be quick enough to protect equipment (or to prevent computer reboot).

    I might be misunderstanding their literature? Or maybe some other company does this better, and doesn't make you choose one or the other.

    Another solution, suggestdd by vtmaps, is to dedicate a few panels to an independent "off grid" system, whose purpose would be to act as a UPS for a selected set of outlets. Interesting, but I wonder about the tradeoff between risk (not having enough power on those days everyone is working their computers) and waste (on those days the sun is shining, the batteries are full, and no one is home).
    Yes again; you do not want to regularly discharge and recharge batteries when you've got grid power. There simply is no point. Some people do this to "time shift" their load demands: recharging the batteries when utility rates are low and discharging them when rates are high. It's doubtful this is worthwhile in most cases, especially when the added cost of accelerated battery life depletion is included.
    In other words, the inclusion of batteries in a PV system for load shifting purposes, such as exploiting time of use pricing, is not worth it (unless the difference in price is quite high). That in net value terms the losses (from storing and then extracting power from the batteries) are substantially more gains (from shifting demand).

    Thus, demand shifting is NOT a good reason to build a system with batteries.
    When it comes to back-up power you have to consider your particular circumstances, i.e. frequency and length of outages and how critical the supply need is. For most, but not all, people an efficient generator is the most cost-effective solution.
    That has been my operating hypothesis (assuming the local grid continues its so-so reliability). Basically, I can deal with a generator as just another hassle (and upgrading the generator to honda equivalent would probably be more cost effective at hassle minimization then battery backup).
  • danielhdanielh Solar Expert Posts: 69 ✭✭✭✭
    Re: Grid interactive: what benefits make it worthwhile?
    BB. wrote: »

    If you time shift (use battery power during the day), you not only have the cost of battery charging (typical AC to DC charger is around 80% efficient, battery ~80-90% efficient, and re-invertering around 85-90+% efficient = ~54% worst case efficiency to power shift) and you are cycling the batteries, which have a limited cycle life too (around 500-2,000 cycles, depending on how deep you are cycling, type of battery, how expensive of battery, etc.)...

    So, It usually does not make a lot of sense to install a "hybrid" system just to take advantage of time shifted power use unless you have a huge difference in peak/off peak costs (probably on the order of $0.45 per kWH between peak and off peak power costs just to break even--but you can do your own calculations and see what you get--prices for hardware/panels/etc. change all the time).
    So your 54% is in addition (or in multiplication :)) to other losses. So that 1kw of panel produced power might mean ~0.8kwh of power directly delivered to the grid, but if you time shift it through a battery pack, then power to the grid is ~0.4kwh. Not a good ratio.

    But perhaps that is a bit too extreme? Wouldn't you avoid the inverter-to-ac step if you take PV power directly to battery? So you shouldn't count the losses as paort of the "77%". Still, that's probably not a big number: so I am ready to conclude that there are substantial power losses when one time shifts.



    BB. wrote: »
    Glad to hear you went down the conservation road--You should be very happy with the results (if nothing else, the homes are usually much nicer, more even temperatures, quieter, fewer drafts, etc.).
    l
    The humidity control on the new AC system is much better. 80 degrees is now quite tolerable! The interesting question is how well it will work as HP in cool weather -- will it beat my 20yr old ng boiler (in terms of cost).

    BB. wrote: »
    Still more than happy to work some numbers (battery bank, array size, gt vs off grid) based on your new power usage so we can figure out your options.
    -Bill
    TY :)
  • vtmapsvtmaps Solar Expert Posts: 3,739 ✭✭✭✭
    Re: Grid interactive: what benefits make it worthwhile?
    danielh wrote: »
    Another solution, suggestdd by vtmaps, is to dedicate a few panels to an independent "off grid" system, whose purpose would be to act as a UPS for a selected set of outlets. Interesting, but I wonder about the tradeoff between risk (not having enough power on those days everyone is working their computers) and waste (on those days the sun is shining, the batteries are full, and no one is home).

    I don't see much solution for the dedicated UPS solar panels going to 'waste' when the batteries are full. Best solution is to undersize the panels (explanation below) you only need to have 1 panel to get the tax credit.

    Now, about the UPS system... many off-grid systems use an 'inverter/charger'. A single unit that a acts as an inverter (powered by battery) or a charger (to charge that battery). These inverter/chargers always have an input for AC current (or they couldn't be chargers).

    Many folks (myself) use these off grid, in which case my AC input is from a generator. Other folks use these on grid, in which the grid is the AC source. In either case, when the AC stops the inverter/charger changes from charger to inverter quickly enough to be a good UPS.

    Therefore, I think you should hook up an inverter/charger type of UPS to the grid. In order to get the tax credits, take one panel and a cheap controller and hook it up to the UPS battery. Size your battery (which will almost always be fully charged) for a day of use. A battery that sits fully charged most of the time should be an AGM battery.

    When the grid is down for more than a day you will run your generator for general homestead use. At the same time, that same generator will be charging up your UPS battery.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • danielhdanielh Solar Expert Posts: 69 ✭✭✭✭
    Re: Grid interactive: what benefits make it worthwhile?
    stephendv wrote: »
    Has no one mentioned AC coupling as an option yet? I think this would give you a very nice solution since you can use a very efficient grid tied inverter (95%) for the 99% of the time you'll be spent connected to the grid. Then use a battery based inverter as a whole house UPS. While the grid is active the only efficiency loss is the panels and the grid tie inverter (the battery inverter is out of the equation since it's in pass-through mode). SMA has the cleanest solution for this, but xantrex and magnum both support some form of AC coupling.

    Using a charge controller will introduce 2 extra losses into the system: firstly the charge controller itself (around 97% efficient) and secondly the battery based inverter (which is always less efficient than a grid tie inverter), around 95% max efficiency. If the system is going to be grid connected 99% of the time, then I would rather design it for max efficiency during that time.

    Attachment not found.

    http://www.wholesalesolar.com/pdf.folder/inverter%20pdf%20folder/MagnumACcoupling.pdf helped me get the picture. It does seem interesting.

    But I wonder what's the point: why not have one good grid-interactive inverter rather than two separate inverters (one grid-tie and one battery). Especially in the ac coupling solution, if you want the panels to charge the batteries in grid-down times (which strikes me as a requirment) you have to have some kind of diversion load.

    Maybe the battery inverter does a better job at being a whole house UPS then the current crop of grid-interactive inverters?


    BTW: Cariboocoot, you were right ("someone is inevitably going to come in here and mention AC coupling") :)
  • mtdocmtdoc Solar Expert Posts: 600 ✭✭
    Re: Grid interactive: what benefits make it worthwhile?
    danielh wrote: »
    So your 54% is in addition (or in multiplication :)) to other losses. So that 1kw of panel produced power might mean ~0.8kwh of power directly delivered to the grid, but if you time shift it through a battery pack, then power to the grid is ~0.4kwh. Not a good ratio.

    Just an FYI/ comparison point - My Grid interactive system using an Outback GVFX 3648 with Midnite Classic/ AGM battery backup is 80% efficient - from production as measured by the CC to output of inverter (to either loads or selling to the grid). YMMV

    My system is wired so that almost all my 120 VAC loads come through subpanel/inverter. One critical 240 VAC load (septic pump) is powered via autotransformer. My other 240 VAC loads (Heat pump, water heater, dryer) are not backed up - except by other means -- woodstove, etc.

    I have no problem with any significant delay when grid power goes out - no computer reboots, etc.

    This system works well for me as back up for semi-unreliable -end of the line - grid power. Most of the time I'm selling to the grid when production exceeds the loads. My system will be unlikely to ever pay for itself but it helps that my utility pays 0.18 per KWH produced while only charging 0.065 per KWH consumed!

    BTW - my first post! Great forum - have learned lots reading here. Questions to come!
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