Sizing your backup: how to judge risk vs cost tradeoffs

ggunn · July 2012

Re: Sizing your backup: how to judge risk vs cost tradeoffs

danielh wrote: »

Sigh, is it true that matching a 6kw xantrex with 200AH of battery and 2kw of panels is not so good -- that it will lead to unnecessary cycling of the battery.

That it won't be straightforward to achieve the goal of keep the batteries ready for use as a UPS and for use during an outage backup (but to otherwise not use them), to
use available PV and the grid for local loads (when the grid is up), and sell excess PV to the grid?

Is this a setup that the Outback does a better job at providing?

Have you considered a Sunny Boy/Sunny Island system?

danielh · July 2012

Re: Sizing your backup: how to judge risk vs cost tradeoffs

Hi Bill et al

BB. wrote: »

The rules of thumb for battery/load sizing (100 AH @ 48 volts for 1kW of load/charging) is one rule of thumb. ...

Obviously, there is the important rule for Watt*Hours of load per day have to be replaced the next time the sun...

For example, a 3.3 kWH per day ...

Lets say run without a genset for 9 month of the year... 4.28 hours of sun minimum for February:
3,300 WH per day * 1/0.52 * 1/0.77 panel+controller derating * 1/4.28 hours of sun per day = 1,925 Watt Solar Array Minimum

That's a bit higher than your example in http://forum.solar-electric.com/showthread.php?16431-Sizing-your-backup-how-to-judge-risk-vs-cost-tradeoffs&p=123297#post123297)

Next, sizing the panel based on the battery bank... Basically, we suggest 1-3 days of "no sun" and 50% maximum discharge. Use 2 days as a "nominal" setup:
[*]3,300 WH per day * 1/0.85 inverter eff * 2 days no sun * 1/0.50 max discharge * 1/48 volt battery bank = 324 AH battery bank @ 48 volts

Now, assuming a C/8 typical maximum continuous discharge, the AC inverter rating would be:
324 AH * 48 volts * C/8 discharge rate * 0.85 inverter efficiency = 1,652 watt rated inverter "nominal max continuous power"
a flooded cell battery bank that can support C/2.5 rate of discharge:
324 AH * 48 volts * C/2.5 discharge rate * 0.85 inverter efficiency = 5,288 watt rated inverter (max surge for flooded cell)
and a surge at 2x of rated power (Watts or Volt*Amps -- VA):
5,288 Watt surge / 2 surge rating = 2,644 Watt max inverter rating based on battery surge capabilities
...

Some questions:
Say we cut the above by 1/2 (so 200 AH battery). If there is a small chance of a 4500w surge (say, cooking dinner with the mwave and toaster oven, with a few too many computers etc are running, and the fridge kicks in) ... what happens? Can the battery pack support this short term (several seconds at most) draw, with perhaps a larger then draw loss in charge.
Does the battery get a little bit damaged? Or does the battery not able to provide this charge, causing a shutdown or fault by the inverter? And in the latter case, might this damage inverter or household equipment, or require a lengthy restart procedure?

Now we need to check the Rate of Charge that we would suggest for such a battery bank--5% to 13% rate of charge:
324 AH * 59 volts charging * 1/0.77 panel+charge controller derating * 0.05 rate of charge = 1,241 Watt Array "minimum"

324 AH * 59 volts charging * 1/0.77 panel+charge controller derating * 0.10 rate of charge = 2,483 Watt Array "healthy nominal"

324 AH * 59 volts charging * 1/0.77 panel+charge controller derating * 0.13 rate of charge = 3,227 Watt Array "max cost effective array" (my humble opinion)

So, combined with the minimum array to supply 3.3kWH for 9 months of the year of 1,925 Watt Solar Array, and the above 1,200 to 3,200 watt array based on battery capacity... The recommend range for "your system" would be around 1,925 watt to 3,227 watt solar array...

Notice the minimum array of 1,925 Watt and the "healthy" nominal array based on battery size is 2,483 watts--That is a "balanced" system...

Anyway--I hope the above makes some sense...

And, this is a starting point... Suggestion/questions/changes are always good for another few posts.

It mostly does, but the unknown unknowns are unkown.

And since you don't seem bored yet, welll by gosh I do have suggestions/questions/changes!

Why does carib say (in http://forum.solar-electric.com/showthread.php?16431-Sizing-your-backup-how-to-judge-risk-vs-cost-tradeoffs&p=123346#post123346)

Assuming that battery bank is 200 Amp hours @ 48 Volts you won't need 2kW of array to recharge it. 1200 Watts is more likely. Putting too large of an array on a battery system is a waste of money.

Another question: why not have more PV power than these charging requirements; with the goal of providing PV to loads as they need them.
I understand that during battery-fully-charged states of nature, most of the time the pv output will be much more than household demand. So it is going to be
wasted.

Let me broach a philosophical point: that the rules of the off-grid world may not fully apply to the "backup+ups" world.
In the off-grid world the battery is use a lot. And it is very expensive. And you are using it in a day-to-day mindset.
Thus, it is wise to have a regular pattern of use that preserves battery life, such as limiting DOD, avoiding microcycling and ripples,etc).

But in a setting where battery usage is infrequent -- say 4 days of outage and 20 UPS events a year, 2 of which are at high loads -- then these concerns are lessened.
For example, if DOD to 70% reduces cycles to 1000 (from 5000), but one will cycle the battery 100 times (4 days a year * 2.5 cycles per day * 10 years), then so
what. Assuming even the most babied battery needs replacement in 10 years, having 900 vs 4900 cycles left is irrelevant (yes, I understand their are nonlinearities, but are they THAT big).

Similarly if a max load surge events draw a lot of power (let me make up a number: 5% of battery capacity is used in a 2 second surge that consumes 0.1% of battery capacity),
but recharge can typically occur right away, then so what. Or if battery life is degraded by 0.1%, then 20 such events means a 2% loss of lifespan. Again, so what.

Thus, using your batteries in ways that would be foolish in an off-grid setting might be efficient in an "outage & ups" world.

BTW: ac coupling with a physical transfer switch to connect the GT to the main cb panel or the output or the subpanel of the battery/charger might be
kind of elegant. Most of the time no PV is wasted, the battery/charger can provide healthy ups services without having to worry about dealing with
unneeded PV power, and plenty of power is available during outages to charge the battery and support local loads while the sun is shining.
Yes, carib doesn't trust it, and you have to deal with overcharging, and the 100Ah rules etc still apply when the grid is down-- but those might
be small headance than those from grid-interactive solutions (i.e. http://forum.solar-electric.com/showthread.php?15616-6048-reducing-sell-amps-because-of-a-small-battery-bank-will-it-work)

Cariboocoot · July 2012

Re: Sizing your backup: how to judge risk vs cost tradeoffs

Here's something you're missing completely: relying on GT panels or larger array on a battery bank to provide power only works when the sun shines.

That is why battery banks are sized to provide the 24 hour power need; so that the power is available on demand.
The solar array is sized to recharge that battery bank. This inevitably means you will have "extra" power available when the sun is shining. That's when off-gridders do "load shifting" and take advantage of this extra power potential with opportunity loads.

Trying to rely on a small battery bank and a large solar array (by whatever configuration) limits you to using the power only during daylight hours. Fine if the loads can be used then. A/C during daylight hours for instance. Not so fine if you need to use them after the sun is down.

Also relying on that small battery bank will always have the drawback of not being able to handle the sudden high power demand when things are switched on.

Here is the rule-of-thumb formula for sizing an array to recharge a battery bank:

Amp hours * charge rate (typically 10%) = peak charge current * nominal system Voltage (lowest it should go) = recharge Watts / panel & controller derating (typically 77%) = array size.

Applied to a 200 Amp hour 48 Volt battery bank:

200 * 10% = 20 Amps * 48 Volts = 960 Watts / 0.77 = 1246 Watt array.

Again that is rule-of-thumb. Every install is different. Allowances have to be made for ambient conditions (hot climate reduces Voltage of panels, high elevation increases insolation, partial shading reduces panel output, et cetera) and how the system is used.

Yes you can use a larger array and yes you can charge at a higher rate than 10%. This is why you are having trouble quantifying systems in economic terms: no two are alike. It's not as simple as "this is the one and only way of doing it and it costs $X".

BB. · July 2012

Re: Sizing your backup: how to judge risk vs cost tradeoffs

That post is getting complicated to reply to... So I will just quickly post (already lost a "full post"...):

I am guessing on where the system will be installed--So with PV Watts and your rough location, we can better estimate your amount of sun and array size.

Regarding 1,200 watt array from Cariboocoot--That was for a 200 AH vs 324 AH bank in my example.

Also, there is a big discussion about 5% to 13% rate of charge... My suggestion for systems with light loads (vs battery bank size) and/or weekend only use, 5% rate of charge seems to do well for those folks.

For people with heavy power usage (discharge the bank 25% or more per day), 10%-13% is probably going to help the bank last longer. Some vendors (like Trojan) recommend 10% rate of charge (we use the ~20 Hour discharge rate in all of our rules of thumb).

For systems with 13% to 25% rate of charge, many time the system will be "in float" by noon or earlier. No damage, but wasted energy unless you have need for afternoon power (water pumping, washing clothes, A/C, work in shop, etc.)... Personal choice (panels used to be $10 per watt, today they are under $2 per watt-"over paneling" a system is much less of a expen$e this days).

Also, when charging over ~13% rate of charge, I would highly suggest a Remote Battery Temperature Sensor (I recommend anyway--but especially here). Deeply discharged batteries with over ~13% rate of charge can get hot. Hot batteries need lower charging voltages. It is possible to get thermal run-away where the the controller sees "falling" battery voltage as needing more current/charging. Poor battery voltage is falling because it is getting too hot--causing its charging voltage to fall. A battery temperature sensor connected to the charge controller will stop this from happening.

Rules of thumb (such as C/2.5 maximum surge current) are there to design a generic system will work well over time/temperature and state of charge.

New batteries/fully charge battery bank can probably take over C/2.5 surge currents with out problems. Get 5 years on a battery bank and discharged down towards 50% state of charge--your battery bank voltage is more likely to collapse (can't say for sure--just a margin of safety).

Also, once you have a battery bus voltage collapse, most inverters will simply restart once the battery voltage is a few volts over cutoff. However, is the load is still there (say starting a well pump), the inverter/battery bank may again power cycle. Some inverters will shutdown at that point and need a reset/cycle DC power bus. It all depends.

If you use AGM batteries, they certainly do "much better" than the C/2.5 rule of thumb.

UPS systems/usage are certainly different vs pure off grid systems. Note that many UPS battery banks only last a couple years and some are only designed to support a few "deep cycles" before replacing the battery bank.

Different needs. If you want more of a UPS type design, then use AGM batteries and a smaller battery bank (AH wise). You may have some limitations (maximum recommended solar array do to possible battery bus over voltage during MPPT sweeps for large wattage array with "small" battery bank)--So you may need to use a genset and/or grid power for "fast recharge" (seem to offer more stable battery charging voltages).

Many UPS systems use GEL type lead acid batteries. GEL batteries also offer very high surge current and good "float" service. However, their maximum charging current is C/20 (5% rate of charge)--Too high of charging current will ruin the battery bank (and why GEL batteries are not recommended here where most people want/need to recharge their battery bank much quicker than a C/20 rate would allow).

Which gets us back into "backup" design. For the most part, Solar makes economic sense when power is used 9 months or more of the year.

For weekend/summer cabins, UPS applications, etc... A small battery bank + genset + fuel storage + propane appliances usually makes better economic sense.

So--very seldom will you have the "optimum" system that can do both UPS service and years of off grid power with the same hardware/cost points. The two applications are quite different. And the battery banks used have different design/requirements too.

AC Coupled Grid Tied and Off Grid inverters do work. A bit on the complex side and, over the last few years, more companies are supporting the setup. And can be very nice if you have Net Metered billing and/or long distances between the array and the battery shed/home.

On the down side, other than the SMA Sunny Island system (which is very nice and very expensive in the US), the GT + OG AC coupled inverter systems are an example of "bang-bang" control. From the battery bank's point of view, it is either receiving maximum charging current or under load... There is never the "steady state" operation where the solar array is carrying the loads and the battery bank is in absorb/float charging state (when the utility grid is down).

For short term, this probably is not a big issue (during power outages). However, for a long term/off grid system, these type of control systems (including wind turbine diversion/dump controllers) are "hard" on the battery bank (forcing cycling when no cycling is needed). And, most likely, battery banks will not last as long when operated 24x7x365 with this type of charging system.

-Bill

BB. · July 2012

Re: Sizing your backup: how to judge risk vs cost tradeoffs

Regarding Marc's rule:

200 * 10% = 20 Amps * 48 Volts = 960 Watts / 0.77 = 1246 Watt array.

I am a bit more conservative:

200 AH * 59 volts battery charging * 1/0.77 panel+charge derating * 0.10 = 1,532 watt array

Most of the time, the battery bank will be near ~59 volts (14.5 volts for 12 volt system) from ~80-90% state of charge until 100% state of charge.

If you deep cycle the battery or operate loads during float (54.4/13.6 volts), then you can use a different/lower voltage...

In any case, answers within 10% are pretty much the same.

-Bill

danielh · July 2012

Re: Sizing your backup: how to judge risk vs cost tradeoffs

ggunn wrote: »

Have you considered a Sunny Boy/Sunny Island system?

Higher cost makes it harder to justify.

danielh · July 2012

Re: Sizing your backup: how to judge risk vs cost tradeoffs

Cariboocoot wrote: »

Here's something you're missing completely: relying on GT panels or larger array on a battery bank to provide power only works when the sun shines.

Trying to rely on a small battery bank and a large solar array (by whatever configuration) limits you to using the power only during daylight hours. Fine if the loads can be used then. A/C during daylight hours for instance. Not so fine if you need to use them after the sun is down.
..
200 * 10% = 20 Amps * 48 Volts = 960 Watts / 0.77 = 1246 Watt array.

I did think of that (embedded in my overly long post to BB).
So 1246 is reasonable (or something like that, say because you might be consuming a few hundreds of W during charing (as Bil notes in http://forum.solar-electric.com/showthread.php?16431-Sizing-your-backup-how-to-judge-risk-vs-cost-tradeoffs&p=123427#post123427)

(rant) Can I nit pick about forum design. Timing out and losing ones work is a pain! Lost 20 minutes this morning haveing to rewrite that long post. I know now to copy and paste a long post (to an open text editor) before hitting the submit button! Also, I have yet to figure out what sort order is displayed. I just keep clicking until I get what I want. (/Rant)

Yes you can use a larger array and yes you can charge at a higher rate than 10%. This is why you are having trouble quantifying systems in economic terms: no two are alike. It's not as simple as "this is the one and only way of doing it and it costs $X".

Using economic principles to give insights into tradeoffs when costs and benefit may be hard to quantify, and the choice alternatives are diverse and imprecisely defined ... is kind of my day job (so it is an interesting exercise).

danielh · July 2012

Re: Sizing your backup: how to judge risk vs cost tradeoffs

As usual, an informative pleasure to read your posts.

BB. wrote: »

That post is getting complicated to reply to... So I will just quickly post (already lost a "full post"...):
I am guessing on where the system will be installed--So with PV Watts and your rough location, we can better estimate your amount of sun and array size.

Ugh, happened to me this morning :<
I am in Montgomery county MD (so probably same pv environment as you)

..... My suggestion for systems with light loads (vs battery bank size) and/or weekend only use, 5% rate of charge s
... with heavy power usage (discharge the bank 25% or more per day), 10%-13% is probably going to help the bank last longer.

I would envision I would be at the other vertex of the triangle. Near 0% discharge most of the time, and near 50% a few days a year.

. Personal choice (panels used to be $10 per watt, today they are under $2 per watt-"over paneling" a system is much less of a expen$e this days).

Was it you who said (in some other forum) about a generational difference in off-gridders. The old notion was "batteries cheap, panels expensive" , but now its the exact opposite

Also, when charging over ~13% rate of charge, I would highly suggest a Remote Battery Temperature Sensor (I recommend anyway--but especially here).

What about the fancier battery monitors?

Rules of thumb (such as C/2.5 maximum surge current) are there to design a generic system will work well over time/temperature and state of charge.
New batteries/fully charge battery bank can probably take over C/2.5 surge currents with out problems. Get 5 years on a battery bank and discharged down towards 50% state of charge--your battery bank voltage is more likely to collapse (can't say for sure--just a margin of safety).
Also, once you have a battery bus voltage collapse, most inverters will simply restart once the battery voltage is a few volts over cutoff. However, is the load is still there (say starting a well pump), the inverter/battery bank may again power cycle. Some inverters will shutdown at that point and need a reset/cycle DC power bus. It all depends.
If you use AGM batteries, they certainly do "much better" than the C/2.5 rule of thumb.

Another reason for AGM batteries for the setup I describe (better for providing possibly large wattage during transient events)

UPS systems/usage are certainly different vs pure off grid systems. Note that many UPS battery banks only last a couple years and some are only designed to support a few "deep cycles" before replacing the battery bank.

I figure 2-3 year life for a UPS, at $100 a pop. Which is one of the "economic" reasons for seriously considering some kind of "batteryization". In fact, up until our friendly neighborhood dereecho provided a refresher course in coping with extended outages, the bigger reason.

Different needs. If you want more of a UPS type design, then use AGM batteries and a smaller battery bank (AH wis...--So you may need to use a genset and/or grid power for "fast recharge" (seem to offer more stable battery charging voltages).

I figure I am going to need a genset -- and I got one already (though I might ditch it for a honda eu2000).

So--very seldom will you have the "optimum" system that can do both UPS service and years of off grid power with the same hardware/cost points. The two applications are quite different. And the battery banks used have different design/requirements too.
AC Coupled Grid Tied and Off Grid inverters do work. A bit on the complex side and, over the last few years, more companies are supporting the setup. And can be very nice if you have Net Metered billing and/or long distances between the array and the battery shed/home.

Definitely have net metering up here. And, quite importantly, SRECs. So value of electricity sold to the grid is > retail cost. Achieving optimality may not be possible for both functions, but reasonable efficiency should be achievable. I posit that will be cheaper & easier than two seperate optimal systems.

On the down side,, the GT + OG AC coupled inverter systems are an example of "bang-bang" control. ... For short term, this probably is not a big issue (during power outages). However, for a long term/off grid system, these type of control systems (including wind turbine diversion/dump controllers) are "hard" on the battery bank

The suggestion by stevendv (http://forum.solar-electric.com/showthread.php?16394-Cost-effective-grid-interactive-choices-Outback-vs-Xantrex-vs-magnm-vs&p=123056#post123056) to have a transfer switch to select GT > main_cb_panel or GT -> inverter/charger_subpanel is interesting; it might remove this concern (99% of the time the battery is being properly charged (if charged at all) by the inverter/charger.

mtdoc · July 2012

Re: Sizing your backup: how to judge risk vs cost tradeoffs

It seems to me that the rules for sizing PV/AH that are so crtical for a strictly off grid system become less so with a Grid-tied with battery back up type system:

If the system is primarily to provide UPS like functionality for short grid outages then the PV is almost irrelevant since the grid can be relied on to keep the batteries charged as long as the banks AHs are enough to last through the typical outage. (the caveat is the too much PV /AH - AC ripple when selling issue) Genset/charger can obviously extend this. Incorporating PV would enable a tax credit...

If the system is needed to supply power for extendeded grid outages (days to weeks) then PV/AH becomes more like an off grid scenario. This is my situation and why I will be doubling my PV soon.

The OP's situation stands all this on it''s head since he has plenty of PV but is trying to decide how much of it to devote to strictly grid tie and how much (and is it worth the cost) to devote to a seperate grid interactive system. It's further complicated since he has not precisely defined his "critical" loads or back up needs and because he wants to do this system for $6K or less (excluding PV)) which may not be possible in any high quality way.

Despite this - I'll give it my best shot anyways.. 8)

One proposal for a "balanced" system:

3X3 array of his 260 watt panels = 2340 x 0.77 derate = 1800 watts PV

Outback GVFX3648 with CC in Outback FP1 or Midnite Solar Epanel prewired system - about $4000
4 x Universal battery 12 volt 200 AH AGMs (http://www.solar-electric.com/unba200amagm.html) = $1372
(not as high quality as SunXtenders or others but would probably meet the needs of this system)
Meanwell PB1000-48 battery charger to run with existing genset when the sun don't shine = $270 (http://www.powergatellc.com/mean-well-pb-1000-48-power-supply.html)
Balance of system components $500-1000
Shipping - guestimate $500
Electrician install (excluding PV) - guestimate $1000

Total = $7500-8000 ( could possibly get it lower with some bargain hunting)

At 10 KWH of battery bank it would provide up to 5 Kwh of "no sun, no generator" back up to 50% DOD.
How long that lasts of course depends on the "critical loads". Sun hours and fuel supply would dictate length of extended back up capacity.

Does not meet his budget but I think it would serve the intended purpose of extended back up of critical loads for an "average" household" .

OK that's all I got - good luck :-)

BB. · July 2012

Re: Sizing your backup: how to judge risk vs cost tradeoffs

danielh wrote: »

As usual, an informative pleasure to read your posts.
Ugh, happened to me this morning :<
I am in Montgomery county MD (so probably same pv environment as you)

Nope--that was my typing/computer's location of touch pad (sometimes I turn the touch pad off when typing longs posts). Also, the cntrl/alt keys seem be pressed by a ghost...

I would envision I would be at the other vertex of the triangle. Near 0% discharge most of the time, and near 50% a few days a year.

That is pretty much the definition of a UPS system.

Was it you who said (in some other forum) about a generational difference in off-gridders. The old notion was "batteries cheap, panels expensive" , but now its the exact opposite

Not me--but probably true to a degree... Although, it is almost people are handicapped by old ideas. Leads them to a large battery bank with small charging resources (less than 5% even). And when they "ran out of power", the recommendation was a larger battery bank (yes, that was common advise not too many years ago, and probably still true in some areas of the Internet/off grid resources). Adding batteries instead of charging resources--was still a bad idea back then, and still is now.

What about the fancier battery monitors?

Presently battery monitors are (for the most part) not integrated with charge controllers... So, other than a programmable alarm (for limited type of events), the typical battery monitor is to inform the system owner about the present state of charge of the battery bank. I don't know of any Battery Monitor that can warn of "over charging events"... (not that I know much in details about monitors or what is available).

A battery monitor will not be a substitute for a remote battery temperature sensor controller charge controller.

Another reason for AGM batteries for the setup I describe (better for providing possibly large wattage during transient events)

I tend towards recommending small AC UPS for computers (UPS's have losses--I don't want to waste the power to provide a UPS for my fridge and microwave/toaster ovens so I don't have to reset the clock)--and a "balanced" off grid solar PV system for balance of home (with conservation/power management for motors via choice of motors/systems and/or VFD type controllers).

In the end, you will probably do several designs and compare the cost/benefits for your needs. And they probably will not be black and white type answers.

I figure 2-3 year life for a UPS, at $100 a pop. Which is one of the "economic" reasons for seriously considering some kind of "batteryization". In fact, up until our friendly neighborhood dereecho provided a refresher course in coping with extended outages, the bigger reason.

And how much planning/equipping do you do for a once in 100 year event? Our last 1 week power outage was in the Mid-1950's. The rest have been for a few hours every couple years.

We have "the big one" (earth quakes) every 100-150 years.... Last "big" one was in 1907. Earthquakes tend to have very scattered heavy damage among almost no damage. "Earthquake" proofing an older home is a much better idea here vs adding a entire off grid solar capable system.

I figure I am going to need a genset -- and I got one already (though I might ditch it for a honda eu2000).

Two is always better. Spare and a backup. Plus you can always "barter" fuel/unneeded genset/electricity if ever needed. Plus the larger genset can be useful for power tools/AC etc. when needed and convenient (fuel availablility, etc.).

Definitely have net metering up here. And, quite importantly, SRECs. So value of electricity sold to the grid is > retail cost. Achieving optimality may not be possible for both functions, but reasonable efficiency should be achievable. I posit that will be cheaper & easier than two seperate optimal systems.

Assuming install a large GT system (or Hybrid GT system)--Yes, Net Metering and SRECs will help reduce overall system cost (plus tax breaks for "whole" solar system vs smaller UPS plus smaller solar system which is not 100% tax break compatible).

The suggestion by stevendv (http://forum.solar-electric.com/showthread.php?16394-Cost-effective-grid-interactive-choices-Outback-vs-Xantrex-vs-magnm-vs&p=123056#post123056) to have a transfer switch to select GT > main_cb_panel or GT -> inverter/charger_subpanel is interesting; it might remove this concern (99% of the time the battery is being properly charged (if charged at all) by the inverter/charger.

True, the amount of time you would spend on GT/OG "bang-bang" charging will be low... So battery life will be unaffected.

I am a big believer in looking at maintenance (costs, time, debugging, locating old/obsolete parts vs forced upgrades, etc.).

A whole home UPS--replace batteries every 7 years or so, repair/replace electronics after 10+ years (for the most part, 10+ year old "high-tec" electronics are not repairable as vendor has cut parts/service support and electronic compenents are not available on the open market, etc.

So--Figure on saving cash for new MPPT controller, new GT controller, new Off Grid inverter, every 10+ years... New battery bank every 6-10 years. And look at the energy costs (loses, battery charging, conversion losses for UPS's, etc.).

Not saying it is not worth it for you... Just that you will be replacing 2/3rds of your system every 10 years or so.

VS me--use laptops (have own battery), small $100 UPS every two years for one desktop system. And buy a new Honda eu2000i for $1,000 every ten years (if it still runs--I now have spare/something I can loan out for non-emergency use).

Buy 20 gallons of fuel and preservative once a year, recycle old fuel back into car (has been making me money lately with the rocketing price of fuel--better than my savings account).

Look for propane/natural gas version of gensets if you have natural gas/propane available.

-Bill

danielh · July 2012

Re: Sizing your backup: how to judge risk vs cost tradeoffs

mtdoc wrote: »

It seems to me that the rules for sizing PV/AH that are so crtical for a strictly off grid system become less so with a Grid-tied with battery back up type system:

If the system is primarily to provide UPS like functionality for short grid outages then the PV is almost irrelevant since the grid can be relied on to keep the batteries charged as long as the banks AHs are enough to last through the typical outage. (the caveat is the too much PV /AH - AC ripple when selling issue) Genset/charger can obviously extend this. Incorporating PV would enable a tax credit...

If the system is needed to supply power for extendeded grid outages (days to weeks) then PV/AH becomes more like an off grid scenario. This is my situation and why I will be doubling my PV soon.

The OP's situation stands all this on it''s head since he has plenty of PV but is trying to decide how much of it to devote to strictly grid tie and how much (and is it worth the cost) to devote to a seperate grid interactive system. It's further complicated since he has not precisely defined his "critical" loads or back up needs and because he wants to do this system for $6K or less (excluding PV)) which may not be possible in any high quality way.

Despite this - I'll give it my best shot anyways.. 8)

One proposal for a "balanced" system:

3X3 array of his 260 watt panels = 2340 x 0.77 derate = 1800 watts PV

Outback GVFX3648 with CC in Outback FP1 or Midnite Solar Epanel prewired system - about $4000
4 x Universal battery 12 volt 200 AH AGMs (http://www.solar-electric.com/unba200amagm.html) = $1372
(not as high quality as SunXtenders or others but would probably meet the needs of this system)
Meanwell PB1000-48 battery charger to run with existing genset when the sun don't shine = $270 (http://www.powergatellc.com/mean-well-pb-1000-48-power-supply.html)
Balance of system components $500-1000
Shipping - guestimate $500
Electrician install (excluding PV) - guestimate $1000

Total = $7500-8000 ( could possibly get it lower with some bargain hunting)

At 10 KWH of battery bank it would provide up to 5 Kwh of "no sun, no generator" back up to 50% DOD.
How long that lasts of course depends on the "critical loads". Sun hours and fuel supply would dictate length of extended back up capacity.

Does not meet his budget but I think it would serve the intended purpose of extended back up of critical loads for an "average" household" .

OK that's all I got - good luck :-)

Thanks MT, that is useful.

Note that some fraction of the costs replicate costs that would of occurred anyways, so that $8000 could be a less; maybe a lot less (i.e.;
the outback replaces $1200 of microinverters). But can it get costs down to $4500 or so (0.70 x 4500 = close enough to my budget cap). We shall see.

danielh · July 2012

Re: Sizing your backup: how to judge risk vs cost tradeoffs

Another reasons for grid interactive?

http://www.washingtonpost.com/blogs/capital-weather-gang/post/are-we-ready-yet-for-potentially-disastrous-impacts-of-space-weather/2012/07/11/gJQAdbvJdW_blog.html

BB. · July 2012

Re: Sizing your backup: how to judge risk vs cost tradeoffs

danielh wrote: »

Another reasons for grid interactive?

http://www.washingtonpost.com/blogs/capital-weather-gang/post/are-we-ready-yet-for-potentially-disastrous-impacts-of-space-weather/2012/07/11/gJQAdbvJdW_blog.html

Are you saying another reason to have Off Grid capable solar? (panels+charge controller+batteries+inverter)?

For those new to the forum, pure Grid Tied inverters (solar panels + GT Inverter) can only run when there is AC power present... If the grid goes down, so will your GT Solar System (there are ways around the issue--but that involves more inverters, battery banks, etc.).

-Bill

vtmaps · July 2012

Re: Sizing your backup: how to judge risk vs cost tradeoffs

danielh wrote: »

Was it you who said (in some other forum) about a generational difference in off-gridders. The old notion was "batteries cheap, panels expensive" , but now its the exact opposite

I think you are referring to something I wrote to you on this forum a long, long time ago:
http://forum.solar-electric.com/showthread.php?p=122675#post122675

BTW, you might find it instructive to reread that old thread... it seems to me that this thread is recapitulating a lot of that old info.

--vtMaps

danielh · July 2012

Re: Sizing your backup: how to judge risk vs cost tradeoffs

BB. wrote: »

Are you saying another reason to have Off Grid capable solar? (panels+charge controller+batteries+inverter)?

For those new to the forum, pure Grid Tied inverters (solar panels + GT Inverter) can only run when there is AC power present... If the grid goes down, so will your GT Solar System (there are ways around the issue--but that involves more inverters, battery banks, etc.).

-Bill

Yes, that is what I meant. It was sort of a for-grins post, or at least I hope it is.

The one question is whether pv panels etc might be disabled by such a solar storm. Probably not (not massive enough)?

BB. · July 2012

Re: Sizing your backup: how to judge risk vs cost tradeoffs

In general, the size of the electrical network must be on the order of a mile or more to "receive" the magnetic flux from the sun... A "solar power system" is way to small to be affected (my humble opinion--if I am wrong, you can post back once the world is on-line again).

However, a GT and Hybrid GT system is directly connected to the utility grid, so it is possible a surge from the utility could take out anything connected.

-Bill

danielh · July 2012

Re: Sizing your backup: how to judge risk vs cost tradeoffs

BB. wrote: »

In general, the size of the electrical network must be on the order of a mile or more to "receive" the magnetic flux from the sun... A "solar power system" is way to small to be affected (my humble opinion--if I am wrong, you can post back once the world is on-line again).

However, a GT and Hybrid GT system is directly connected to the utility grid, so it is possible a surge from the utility could take out anything connected.

-Bill

Oh no, now I have to get a 3rd inverter that I only connect for use during systemic emergencies !!!

Sizing your backup: how to judge risk vs cost tradeoffs

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