# what size batteries

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Registered Users Posts: 17
Hi guys/gals,

My brother in law wants to set up his system, he has 8 kyocera 120 watt panels and a whisper 200 wind generator. the question is, what size of batteries would be compatible with this setup? He has a medium size cabin, xantrex 2500 watt inverter charger, and a morningstar 60c charge controler, i know that does not matter, but i figure that to give the best response, we always need the full scenario. i appreciate any help you can give,

Thanx,

Mike

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Re: what size batteries

Assuming 12 volt battery bank (what is the inverter input voltage?):
• 8x 120 watts of panels = 960 watts
Assuming 5-13% rule of thumb for battery charge rate (based on 20 Hour capacity numbers) and 0.77 derating for solar panels/charge controller, the "recommended" range of battery capacity would be:
• 960 Watts * 0.77 derating * 1/14.5 volts charging * 0.05 = 1,020 AH # 12 volt bank maximum
• 960 Watts * 0.77 derating * 1/14.5 volts charging * 0.13 = 392 AH # 12 volt bank minimum
Now, there is also the issue of how much power is used by the loads at the cabin. Normally, we size the battery for 3 days of no sun and 50% maximum discharge or 6x daily load (for a large bank).

For a smaller bank, you could go down to 2x daily load (1 day of no sun + 50% maximum discharge).

Do you know the estimated loads? Is there a generator/AC battery charger available for powering heavy loads/during periods of bad weather?

If this is a cabin used 9+ months of the year, then planning for a large battery bank would probably be very nice.

If this is a summer weekend cabin, then a smaller bank and relying on a little Honda eu1000i/eu2000i for backup power is probably more cost effective (the batteries will probably "age out" and/or fail from maintenance issues vs simply hitting cycling life). In those cases, a large battery bank (5% charge rate, 3 days of no sun) is probably hard to justify financially.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 10,300 ✭✭✭✭
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Re: what size batteries

normally you should charge batteries between 5% and 13% of the batteries' ah ratings. pv ptc ratings are usually about 10% lower than stc and is a better reliable figure to go by during full sun periods. i don't remember the stc imp figure off hand for your pv, but i figure it should be about 6.9a so ptc would be about 6.2a. many will deduct further from that, but mppt has gains so we won't deduct anymore here.
now you did not specify the battery voltage, but i'll assume 12v. that would mean 8 pvs in parallel and the current will be 8x6.2a=49.6a. from here we can determine the range. 49.6a/.05=992ah. 49.6/.13=381.5ah. now if going with trojan batteries they prefer about a 10% rate and that would be 496ah noting that it may be difficult to get an exact match on the batteries for they come in certain capacities and you'd want to be close to that figure of 496ah. some batteries like agms are more efficient and can be charged at much higher rates, but when dealing with solar or wind it is easier to figure keeping it in that same 5-13% range as pvs are expensive just for quick charges. if you elect a quick charge with an agm find out what the manufacturer says the batteries rate of charge can go up to. sun extenders are able to go several times their ah rating and are not typical figures for other manufacturers.
if both solar and wind charge at the same time you don't want the batteries to see a rate of charge that will exceed the batteries' abilities. usually wind will not be high or consistent so you might guesstimate what you may typically see and be comfortable with in using as the current from wind. you don't want the pvs to go under that 5% in considering the wind and pv currents together.
i see you specify an inverter/charger at your cabin. if you have ac then you may want to lean a bit on the lower side of capacity for the grid could charge those batteries too meaning it is not as necessary for you to have as much battery capacity to run things should you exceed your normal draws you designed for. for generators rather than grid power you may not want to reduce as much as you would for grid power as generators use gasoline and make lots of noise, but how far you would want to design for may be determined in how much you might want to run the generator for. also keep in mind that it is better for something like solar to put the final 20-30% of charge in the batteries rather than the generator as generators are better at bulk charging. now don't quote me on that as gospel for inverter chargers may be good for this circumstance of the final stages of charging too. i'll let the guys comment more so on that aspect.
of course if you are using a different battery voltage it means the pv voltage will be downconverted accordingly and the battery bank is still figured for the current delivered to them as a charge rate, but just with a higher voltage battery bank to match the voltage of the pv system. it also is a given that the pvs can be used at a much higher voltage than the batteries by using mppt,. but iot would still go by the current outputted by the controller which will be higher than the inputted current depending on the voltage ratios.