Continously running loads and array sizing?

Hope you can help clarify my thoughts.

Lets say hypothetically my Battery storage is 2000a.h and my continuously running daily load is 1000a.h., 200a.h continues to be drawn during the time the array produces power and the batteries take care of the rest overnight. (extreme numbers for ease of maths)

If my array matches the loads and puts out 1000a.h /day, then after one day of use my array can recharge the battery bank (800a.h used over night and 200a.h covered during the day, so 800a.h goes back into battery).

But with 2 days of no sun, then the batteries have to provide there full 100%DOD storage of 2000a.h (2 days use). Then the sun comes out, the array covers the 200a.h whilst the sun is out and puts in the remaining 800a.h to the battery. So my storage is back up to 800a.h. Overnight that 800a.h is drawn back out - storage back to 0amps. The next day has sun, so the array covers the 200a.h and feeds a further 800a.h into the batteries - storage up to 800a.h..... so if my array does not have surplus it can never catch up and recharge the batteries to their original 2000a.h and if there was a further day without sun my loads run out of power.

But if the array produces 1500a.h instead of 1000a.h (500 more than the load) then:
If my array puts out 1500a.h /day, then after one day of use my array can recharge the battery bank with some left over. But with 2 days of no sun, then the batteries have to provide there full storage of 2000a.h (2 days use). Then the sun comes out, the array covers the 200a.h whilst the sun is out and puts in a remaining 1300a.h to the battery. So my storage is back up to 1300a.h. Overnight 800a.h is drawn back out - storage down to 500amps. The next day has sun, so the array covers the 200a.h and feeds a further 1300a.h into the batteries - storage up to 1800a.h...

So I can't see how an equal wattage from the array to the daily loads can recharge this scenario when it is continuously running with no down time for the array to put any charge back into the batteries. To me it needs to be bigger than daily loads and have regard to the amount of amps to charge the batteries in quick time rather than watts.
Am I way off track here or what?
Thanks for your help.

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Continously running loads and array sizing?

    A couple of vital points here.

    1). No battery will survive 100% discharge. It'll croak.
    2). Efficiency losses. There's lots of 'em, starting with the fact that the batteries alone require about 10-15% more Amp/hrs to recharge than the actual 'used' Amp/hrs.
    3). Recharging is not simply a matter of putting back what's been used, but also of getting sufficient current to reduce sulphation and re-mix the electrolyte.
    4). You usually have about 4 hours of 'good sun' per day in which to replace all those 'used Amp/hrs'.

    If the array can not provide the running load and sufficient recharging your batteries die a premature death.

    We determine battery bank sizing according to loads and array sizing according to the battery bank.
    If you need 1000 Amp/hrs per day (that is MEGA power in the off-grid world!) then the battery bank must be at least 2000 Amp/hrs. To calculate the Wattage needed to recharge you also have to know the system Voltage:
    2000 Amp/hrs @ current rate of 10% = 200 Amps @ 14.2 Volts (for a '12 VDC' system) = 2840 usable Watts, or about 3550 rated Watts. That is a rough calculation. If you have to supply any additional Amps to run loads, then that must be included in the charge rate: Current from charge minus load current = effective charging rate.

    Incidentally, divide these whopping current figures by 10 and you have a fairly normal 12 Volt system (i.e. 100 Amp/hrs usage per day, etc.)

    When you look at it from a "days between charging" perspective, you see why off-gridders have back-up generators; it's more cost effective than massive banks and the panels/CC's needed to recharge them.
  • solarabcdsolarabcd Solar Expert Posts: 38
    Re: Continously running loads and array sizing?

    Thanks for the reply.
    Yep I understand the efficiency losses, dod limits etc and i had accounted for the available sun hrs. I just plucked some figures to put my thoughts out there.

    So the answer is 'yes', you need surplus amps (therefore wattage) to recharge the battery bank.

    Now the next question is (with no relevance to my previous figures):

    If you had an array that provided say 3000w and was wired so each string gave say 60 amps. If I had the same wattage array (same amount of modules) but wired more panels is series than parallel, so now the amps become say 40amps.

    Will this charge at the same rate just because there is enough wattage or is the fact that I now have 20 less amps mean it will charge slower? Or does the extra voltage (hence wattage) from each string mean this 40 amps is equivalent to the 60 amps?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Continously running loads and array sizing?

    How you arrange your array makes no real difference to its over-all Wattage output, except for decreasing line loss through the use of higher Voltage and avoiding problems with shading. It's the Watts of the panels converted to charging current by the Charge Controller. An MPPT type controller can take higher input and 'downconvert' it to the most efficient Amperage for the system Voltage. There are some efficiency 'tricks' here too, as controllers prefer to be run at an 'ideal' percentage of their maximum capacity. The closer to max, the more heat and the less efficiency. People have various results on just what that 'ideal' is.

    The main reason for re-arranging panels in various series/parallel configurations is to get the right Voltage/Amperage for the wiring & controller limits. If everything is parallel, then the Amps go up and you need heavier wiring. If everything is in series then the Voltage goes up and may exceed the controller's input maximum. Of course controllers have maximum current handling capacity too (on the output side), and that can't be exceeded.
  • solarabcdsolarabcd Solar Expert Posts: 38
    Re: Continously running loads and array sizing?

    Ok. So If I just stick with sizing based on watts rather than looking at amps then the charge controller takes care of the rest. i.e. in my last post the 40 amps effectively becomes the 60 amps once it passes through the MPPT charge controller, or I mean it stays at the same wattage and that is all the battery cares about and will charge at the same rate?

    So (forgetting about de-rate factors etc for a minute) if I had a 1000 Watt / day demand then I need an array larger than that to have any chance of recharging my batteries if my loads run continuously. Hence why a generator works out cheaper as you may need a massive array to cover the the recharging as well as the daily loads....?

    Am I on the right track?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Continously running loads and array sizing?

    Yes, the panels of the array can be arranged in whatever way best suits the conditions of downlead current and maximum input Voltage for the controller. With really large arrays you need multiple controllers as the most output current they can handle is about 80 Amps at whatever system Voltage (as in: 80A @ 48V = 3840 W).

    If you have 1000 Watts per day use, that's averaged over 24 hours as 42 Watt/hours average. That's a typical load and easy to supply. If you have 1000 Watts continuous then that's 1000 * 24 or 24 kW/hrs. That is pretty massive stuff in the off-grid world. In either case the array has to large enough to supply the recharge for the batteries in your 'window of opportunity' (usually about 4 hours) plus any concurrent loads.

    Here's some trivia:
    There's 24 hours in a day.
    Batteries are rated @ a 20 hour rate.
    There's about 4 hours of good sun per day on average.
    Coincidence? :roll:

    If you need massive amounts of off-grid power generators are usually the best way to go in terms of $ spent. My system can supply about 2.4 kW/hrs per day and, with the back-up gen, cost about $10,000. That would buy a fairly good size industrial generator and still have money left over for fuel. The downside of generators is the need to shut down for maintenance, continued re-fueling, and constant noise.

    This is why the first step of any system design is to define the loads.
  • BB.BB. Super Moderators, Administrators Posts: 29,204 admin
    Re: Continously running loads and array sizing?

    Ignoring losses, an MPPT controller converts power as:
    • Power=V*I= Vpanel*Ipanel = Vbatt-charging*Ibatt-charging
    So, if you have a 1,000 watts of panels (100 volts * 10 amps or 20 volts * 50 amps) input--you will have:
    • 1,000 watts / 14.4 volts battery charging = 69.4 amps into battery bank
    In reality, your average peak power from the array will probably not be 1,000 watts, but closer to 770 watts into the battery bank after all of the losses/conservative power estimates are taken into account.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • solarabcdsolarabcd Solar Expert Posts: 38
    Re: Continously running loads and array sizing?
    In either case the array has to large enough to supply the recharge for the batteries in your 'window of opportunity' (usually about 4 hours) plus any concurrent loads.

    With reference to my scenario, it appears you have to supply enough to recharge the batteries, cover concurrent loads and also have a bit surplus to charge the batteries (eventually) up to their original state if there is a prolonged period of no sun... If you want to recharge quickly then that surplus capacity has to be larger I guess.

    Thanks for your help guys. It appears what others have told me and some calculators out there; assume that your loads are not on continuously 24/7 and there will be a day or two with zero loads so that an equally size array to loads can actually recharge the batteries.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Continously running loads and array sizing?
    solarabcd wrote: »
    Thanks for your help guys. It appears what others have told me and some calculators out there; assume that your loads are not on continuously 24/7 and there will be a day or two with zero loads so that an equally size array to loads can actually recharge the batteries.

    I've never heard that formula before. :confused:
    My array is 700 Watts and it keeps up with charging/usage of 300+ Amp/hr batteries & ~2 kW/hrs per day. This includes running a refrigerator with defrost, a water pump, and a digester pump as well as the computer/satellite set-up. When the sun doesn't shine a few hours with a small generator puts back the power much more cheaply than would having triple the battery capacity and the necessary array to charge it.

    That said, I'm planning on improving (increasing) both the array and the bank size because as soon as you have 100 Watts available you find you use 110! :p

    Getting everything 'in balance' is tricky.
  • PhilSPhilS Solar Expert Posts: 370 ✭✭✭
    Re: Continously running loads and array sizing?
    Getting everything 'in balance' is tricky.

    THAT is about the biggest understatement I've read here today.

    I am ALMOST in balance, but it's taken over 30 years, numerous configurations and additions/modifications. BUT...

    ...I'm ALMOST in balance now! Maybe just a few more years and some more additions and modifications. ;)

    Phil

    phil,
    take it easy here as you should not direct such talk to members, but focus on the subjective matters on hand.
    niel
  • solarabcdsolarabcd Solar Expert Posts: 38
    Re: Continously running loads and array sizing?
    I've never heard that formula before. :.
    I was just saying the loads run 24 hrs per day for 7 days per week.

    I was just stating that it appears most sites and calculators just size your array to the load demand therefore not allowing that surplus need to charge the batteries quickly after a day or two or more with no sun. Plus forgetting the fact that in some cases (like my scenario) the loads can be on forever with no down time.

    I guess, as we have discussed, most just size the system to the load demand and then get a generator or maybe a wind turbine to help with the recharging.

    Thanks for you help and humor lads. I appreciate your time.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Continously running loads and array sizing?

    That would be standard procedure for a grid-tie array; size the panels to suit the inverter's max potential output.

    There's a reason why off-grid systems are called "battery-based"! :p
  • solarabcdsolarabcd Solar Expert Posts: 38
    Re: Continously running loads and array sizing?

    The funny thing is the calculator was specific to battery based systems. It just keeps wanting more array current to cover my daily loads current, even though the actual wattage from the array is much higher than the load demand in watts...
  • BB.BB. Super Moderators, Administrators Posts: 29,204 admin
    Re: Continously running loads and array sizing?

    If you assume 4 hours of full sun--then only 1/6th of the day do you have charging current. And the other 5/6ths of the day, you are (more or less) drawing from the battery bank.

    So, if you have a 100 watt * 24 hour load:
    • 100 watt * 24 hours * 1/4 hours of sun * 1/0.52 system derating = 1,154 watts of solar panels
    So, for a 100 watt load in spring/fall time frames (~4 hours of sun)--your panels need to be 11x larger than your average inverter output.

    "Small" 24 hour a day loads are frequently much worse than running a microwave or electric coffee maker on an off-grid system.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • audredgeraudredger Solar Expert Posts: 272 ✭✭
    Re: Continously running loads and array sizing?

    Not all solar calculators are created equal.
  • icarusicarus Solar Expert Posts: 5,129 ✭✭✭✭
    Re: Continously running loads and array sizing?

    I confess that I haven't read every post on this thread, but it should also be noted that for every watt/hour or amp/hour taken out of a battery, it takes ~1.25 wh/ah put back in just to stay even. (Remember your mileage may vary depending on battery chemistry.

    As I have said before,, I think a derate factor of ~55% is way too optimistic. I think that a 30% rate is probably net/net closer to reality, by the time you take in load shifting, taper charging, panels that have capacity by no place to put the power as the batteries near full etc.

    It would be interesting to do a study to see what that real number might be.

    Tony
  • solarabcdsolarabcd Solar Expert Posts: 38
    Re: Continously running loads and array sizing?

    When you say de-rate at 30% your saying, as per bills equation, that you multiply the figure out by 3.33. i.e. make the system 3.3 x larger than the standard ratings.

    Does this cover all other efficiency losses etc or just from the charging perspective?
  • BB.BB. Super Moderators, Administrators Posts: 29,204 admin
    Re: Continously running loads and array sizing?

    Yes, you have it correct...

    It is hard enough to tell people that they need ~2x the panel rating for their off-grid system--let alone 3.3x...

    A lot of this also is "safety factors"... If you have no backup power (no generators, no wind power, etc.)--then you would want to have more solar panels.

    If you have backup power--that may mean that you need to run the genset a few hours extra a week or month when bad weather rolls in--and/or cut back on some of the optional loads.

    In the end, your daily generation may vary by 10-20% (depending on your local weather) on a monthly average. And, you may have the better part of a week during winter when the panels output less than 5% of their "rated" power due to stormy weather.

    The 52% derating is the point at which you will need to run your generator (based on predicted average output).

    You can make up more complex spread sheets (using power during the day from the panels vs drawing power at night from the battery bank... Using an inverter that has a low power standby mode vs one that does not, etc.)...

    In the end, there are so many variables, and the hardware being relatively expensive--it is usually the most cost effective to plan for "average" solar collection and use a genset to make up for the few times a month where bad weather limits your power collection...

    To make a system 100% solar (no fuel based backup generator) gets really expensive just to cover those couple of weeks a year when there is simply almost no sun for a week at a time.

    You can, to a degree, design your system to expand (choose 48 Volt battery bank, and your MPPT charge controller can handle 4x the number of panels vs running on a 12 volt bank--60 amps at 12 vs 48 volts).

    Adding to an old battery bank is not the best thing to do... Roughly the new new batteries will carry more of the load vs the older batteries, and eventually age out and die at (again roughly) the same time as the older battery bank. In general, it is best to operate batteries as matched sets (same brand/model/age).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • audredgeraudredger Solar Expert Posts: 272 ✭✭
    Re: Continously running loads and array sizing?

    Real life scenario; A off grid calculator say for my location, 6.5 KWh use per day and,2.5 day backup I need: 10 175 watt panels and 2181 amp hr battery bank @ 24 v. I have 18 175 watt panels and a 2250 amp hr battery bank.

    As you can see, the battery bank is not far off target but, the panels are 1.8 x larger than target. Thin overcast days, we almost hold our own but, with heavy cloud cover you can figure on nothing. As in most parts of the country, we can get a week of bad weather and we have occasionally had two weeks straight of bad weather. After a couple of days of bad weather it will take a couple of days of clear weather for the batteries to top off.

    If I increase the backup to 7 days the calculator says 6105 amp hrs @ 24 v and, it does not increase the panel requirement. So if I ran off that 6150 battery bank for a week, it would take a week of clear days to recharge it with my "over-sized" array. IMHO, a week is too long to leave the batteries discharged!

    BTW, another 6 panels are in the future
  • icarusicarus Solar Expert Posts: 5,129 ✭✭✭✭
    Re: Continously running loads and array sizing?
    solarabcd wrote: »
    When you say de-rate at 30% your saying, as per bills equation, that you multiply the figure out by 3.33. i.e. make the system 3.3 x larger than the standard ratings.

    Does this cover all other efficiency losses etc or just from the charging perspective?


    Perhaps I was not clear. The basic de-rating for off grid, battery systems is ~53%. That is, after taking in all system loses, if you have 100 watts of PV, you might expect to yiled net/net ~53 watts of power, multiplied by the number of hours of good sun.

    My point is, that 53% assumes average sun and average use. My number is closer to ~30% for the following reason(s).

    First, lets assume that your system runs 53% efficient, such that as the sun sets, your battery comes 100% charged. But now let's assume that for a variety of real world reasons, your battery comes charge 2 hours before the sun set. That two hours of potential power is essentially wasted by not having a place to "put" it.

    There are all sorts of scenarios where potential power is not captured as a result of basic system configuration. Most systems are over sized to allow full charging in a day or so after a number of days of poor sun. This works well, but when you have day after day of good sun, you will always be leaving some power on the table so to speak.

    So, as I said earlier, it is pretty hard to quantify on an ongoing basis, but if you take the 53% and then derate some more,, I thing ~30% is a fairly reasonable number.

    As I have also recorded, we use ~6-700 watt/hours out of the battery, and on a good day we can generate (according to the controller data log) ~2kw. So on the days when we have to only put 600 wh back in, and we have great sun, if we can't find a way to use the extra 1.4 kw,, it is essentially gone to waste. (This only gets worse later in the season when we have 18 hours of sun, 14 on the panels). So the derate factor only gets bigger.

    I hope this all makes sense,

    Tony
  • solarabcdsolarabcd Solar Expert Posts: 38
    Re: Continously running loads and array sizing?

    Yep I understand.

    So if I had calculated that I need a 2000w array with available sun light (before de-rating anything) than I need to multiply that by at least 1.88 (1/.53) and up to 3.33 (1/.3). So an array around 3760w to 6660w to have any hope of charging and accounting for real world losses. And be prepared to be potentially wasting some power when there are prolonged sunny periods. Choose the high figure if I want to keep the batteries topped-off and avoid potential long periods with the batteries depleted.
  • BB.BB. Super Moderators, Administrators Posts: 29,204 admin
    Re: Continously running loads and array sizing?

    And/or add a properly sized generator and battery charger for the "dark times" in life...

    In the end, you will probably need a small generator and charger anyway--so you are only adding fuel and maintenance costs to the additional run time.

    Very roughly, you can get between 2-5 kWHrs per gallon of gasoline from a relatively efficient gasoline setup... That is matching the generator + battery charger output to what the battery bank can accept.

    Many times, people want to purchase a good quality prime mover genset--but those are frequently in the 10-15 kW range--and most battery banks and the attached chargers are much smaller--leading to very high fuel consumption.

    Having a second, smaller genset frequently saves costs overall because of the much lower fuel consumption. You still have the larger genset for when you need to run your shop tools and when the small genset it out of service.

    If your battery bank is running low--usually your best bet is to start the generator in the morning and run bulk charging (try to have the genset around 50% loading) get the batteries back to 80-90% state of charge) and then let the solar finish off the rest of the day,

    Running the genset for the last 10-20% of the battery charging (low charge current wrt generator output) is usually a very expensive use of fuel.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • solarabcdsolarabcd Solar Expert Posts: 38
    Re: Continously running loads and array sizing?

    Cool.

    Avoiding maintenance is a big issue in this scenario. Filling gens etc is a pain and time consuming for this situation. Maybe a wind turbine will be the next best option as cost and space available is limiting the amount of solar there can be.

    Thanks for your time and knowledge guys.
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Continously running loads and array sizing?

    these derating figures are arbitrary as it could be better or worse depending on equipment used and steps taken. you could start lower, but be prepared to have the need to expand within a reasonable period of time if it turns out to be not enough.

    this may not be all that can be done or it may not apply to all installs, but to get a better handle on where you might stand in a design you may wish to;

    1 use mppt controllers to recover some of the power lost.

    2 keep resistive losses in the wires low as in under 2% voltage drop, with 3% voltage drop for the system as an outside possibility. note here that if you design the wires at 3% vd with minimal pv current and need to expand later that it will jump out of a good tolerable range. plan ahead for expansions.

    3 use agm batteries as they can be around 90% efficient on charging as opposed to 80% on fla types.
  • BB.BB. Super Moderators, Administrators Posts: 29,204 admin
    Re: Continously running loads and array sizing?

    A few things that would be helpful for us to help you...

    First, what is the battery bank voltage you are asking about... 2,000AH at 12 volts or at 48 volts--a 4x difference in power (Power=Voltage*Current). Also, note that while peak power is important to sizing wiring and inverters, actual power*time (watts*hours) is needed to size the solar panels/battery bank/backup generator system. 1 kW for 20 minutes a day is different than 100 watts * 24 hours per day (300 Watt*Hours vs 2,400 Watt*Hours per day).

    What are your real loads--Watts Peak and Watt*Hours per day (by season if applicable).

    Where will the system be installed--Bakersfield CA area or some place else?

    Off grid solar systems (and even Grid Tied) are very "scale" sensitive... If you need a "small amount" of power--the hardware, battery bank voltage, etc. is different than a large scale system...

    Also, a system that is monitored by a live body with variable loads (don't do wash on cloudy days, start the genset when bank is low, etc.) vs a fully automated site that needs power 24x7 and uses a helicopter to ferry fuel and supplies in will be much more expensive/sophisticated.

    A smallish/reasonable sized system would be under ~100 kWHrs per month (or 3.,3 kWHrs per day (3,300 watt*hours per day). That is a good number for a full time home that is off-grid but wants all of the standard electrical comforts (assuming that heating/cooking/etc. is done with propane/fuel--probably either no or very small amount of Air Conditioning). You would be looking at a 48 volt battery bank system most likely.

    A very small system would be 1,500 watts inverter (run a fridge, a few lights, laptop computer, pump to pressurize sink/shower water)... A 12 volt battery system is do-able.

    Regarding wind vs genset--Unless getting fuel to the site is too expensive--I would go with the appropriately sized generator first. You will need one any way--and once you are on site, you can monitor your system/genset usage and decide where to add more power next (conservation, more solar panels, a wind turbine, etc.).

    My guess is that a small genset + charger + a few gallons of fuel per week/month is going to be a better use of time and funds than adding a wind turbine.... But, again, not knowing the size of your loads and your location--it is hard to be accurate.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarusicarus Solar Expert Posts: 5,129 ✭✭✭✭
    Re: Continously running loads and array sizing?
    BB. wrote: »
    A few things that would be helpful for us to help you...

    First, what is the battery bank voltage you are asking about... 2,000AH at 12 volts or at 48 volts--a 4x difference in power (Power=Voltage*Current). Also, note that while peak power is important to sizing wiring and inverters, actual power*time (watts*hours) is needed to size the solar panels/battery bank/backup generator system. 1 kW for 20 minutes a day is different than 100 watts * 24 hours per day (300 Watt*Hours vs 2,400 Watt*Hours per day).

    What are your real loads--Watts Peak and Watt*Hours per day (by season if applicable).

    Where will the system be installed--Bakersfield CA area or some place else?

    Off grid solar systems (and even Grid Tied) are very "scale" sensitive... If you need a "small amount" of power--the hardware, battery bank voltage, etc. is different than a large scale system...

    Also, a system that is monitored by a live body with variable loads (don't do wash on cloudy days, start the genset when bank is low, etc.) vs a fully automated site that needs power 24x7 and uses a helicopter to ferry fuel and supplies in will be much more expensive/sophisticated.

    A smallish/reasonable sized system would be under ~100 kWHrs per month (or 3.,3 kWHrs per day (3,300 watt*hours per day). That is a good number for a full time home that is off-grid but wants all of the standard electrical comforts (assuming that heating/cooking/etc. is done with propane/fuel--probably either no or very small amount of Air Conditioning). You would be looking at a 48 volt battery bank system most likely.

    A very small system would be 1,500 watts inverter (run a fridge, a few lights, laptop computer, pump to pressurize sink/shower water)... A 12 volt battery system is do-able.

    Regarding wind vs genset--Unless getting fuel to the site is too expensive--I would go with the appropriately sized generator first. You will need one any way--and once you are on site, you can monitor your system/genset usage and decide where to add more power next (conservation, more solar panels, a wind turbine, etc.).

    My guess is that a small genset + charger + a few gallons of fuel per week/month is going to be a better use of time and funds than adding a wind turbine.... But, again, not knowing the size of your loads and your location--it is hard to be accurate.

    -Bill

    This post is probably the best, most concise summary of small of grid systems. I might also add a few things,, but not to argue Bill's basic post.

    The single most important thing to keep in mind in designing any off grid system is first define and understand the loads, and then do everything possible to reduce them to a minimum, and then come up with alternatives to eliminate as many WH as possible.

    Using our simple system as an example. We live quite comfortably of grid with ~400 watts of PV. What we also do is use a generator for many large but occasional loads. Significantly, the shop loads (table saws, routers, grinders etc) all run off of the appropriate sized generator as needed. We certainly could have built a larger PV system to handle these loads, but that would have been very expensive, AND germane to this conversation, it would have (counter-intuitively) lead to a less efficient system, by being over powered 90% of the time. I will gladly pay for a few gallons of gasoline per year.

    We try very hard to keep both our peak loads (largest draw items) and total loads as small as possible. Like I have always said, the loads will and do grow with time. Look very carefully at the routine loads. For example, the difference between a TV that burns 150 watts, and a similar one that burns 125 watts, over 4 hours that is 100 WH/day potential savings. As you are designing and building a system, a Kill-A-Watt is your best friend. Using one to identify the most efficient hardware pays big dividends.

    Besides finding low power hardware is one thing, using alternatives to electricity is another. I have already mentioned the shop(s). Another place to look very carefully is the kitchen. For example, a stove top Coleman Mr Coffee machine makes coffee as well as the plug in variety, but uses no power at all, saving 1-300 wh/day maybe. Avoiding stoves with hot surface ignition, or even electronic ignition. Standing pilots use a bit of propane but use no electricity. Using a stove top toaster, (or in my case a special catalytic heater/toaster) saves both peak load and whs. They even make a propane fired hair drier if you really need one!

    The other thing that we do is some subtle time/load shifting. I have already mentioned pumping water whenever possible when the sun is out, or ideally when the batteries are nearly full and there is power to spare. We also charge the lap tops then as well. (not to mention the drill batteries, the saw batteries etc, other small batteries etc).

    Our system has now evolved to be pretty well balanced,, (as someone else suggested) after ~10 years. We have enough battery to run ~3 days without needing to sun a gennie (which is almost never, except in November!) and on the 4th day, we can get back to 100% if it is a good day.

    As I have also said often, given all that I know now, the major change I would make is I would have chosen a efficient conventional fridge over the LP Dometic. Alternatively I have considered running my LP fridge on 120vac when the battery is fully charged. The fridge management is pretty simple as the control board will choose 120 over gas if 120 vac is available. A simple relay controlling an inverter would be all that is required. The problem would be, the fridge would use nearly all the PV available, and would make charge control kind of tricky. (all to save a few cents worth of LP a day doesn't make much sense.).

    Sorry for the ramble.

    Tony
  • solarvicsolarvic Solar Expert Posts: 1,051 ✭✭✭✭
    Re: Continously running loads and array sizing?

    Icarus, I got the coleman coffee maker due to Bill and your suggestion. I bought it from Amaon. Got the Caraffe yesterday from Amazon. Surprized how well it works. I made a pot in the morning and11 hours it was still preety warm yet. Brand is Bodum, and I recomend. www.bodum.com. S:Dlarvic
    bodum chambord vacume bottle
  • icarusicarus Solar Expert Posts: 5,129 ✭✭✭✭
    Re: Continously running loads and array sizing?

    Pretty cool,, (hot) huh?

    If you pre-heat the carafe with boiling water, it will stay hot even longer. We used to make coffee the night before, put it in a Carafe, then wrapped the carafe in a foam "cozy" and then covered it with a blanket.


    Even with very cold room temps (in the old house) we had piping hot coffee. Don't do it much nowdays as the coleman works so well.

    Tony
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