# Calculator for batteries needed to support xxx watt-hours?

Solar Expert Posts: 56 ✭✭
I'm trying to figure out how many Trojan L16RE-2V (or similar) batteries you'd need to supply 50,000 / 75,000 watt hours for 12 / 24 hours.

50,000 watt hours is an estimate of winter usage, and 75,000 is for summer.

Thanks.

• Solar Expert Posts: 35
lots. you don't hgave watt hours for 12 hour you just have watt hours. divide by battery voltage like 48 and get amp hours then just times 2 for minimum so like 50000 / 48 is 1041.6 amp houyrs or mor than 2083 amo hour battery which is pretty huge.
• Solar Expert Posts: 56 ✭✭
My bad. Forgot that watt hours are for 24 hour periods, generally. Battery I mentioned is rated at "1100 AH @ 20 HR rate" or "1235 AH @ 100 HR rate".

I confess to not understanding how to interpret this.

So if you have 75000 watt hours / 24 volts (12 of these 2-volt batteries), you get 3125 (of something?), and then times two (forgot why) = 6,250 (of something else ...)

I thought these were monster amperage batteries, and that a dozen of them fully charged would run something a long time. I guess I need a link to a tutorial or calculator that will do the math for me.
• Solar Expert Posts: 175 ✭✭✭
voltage * amps = watts
Voltage * AH = watt hour
• Solar Expert Posts: 232 ✭✭
75000 / 2200 = 34 batteries / 0.50 DOD = 68.18 batteries.
Round up to 69 batteries.
Obviously you need to round up again to a multiple of the desired system voltage, whatever that is.
• Solar Expert Posts: 5,145 ✭✭✭✭
lasitter wrote: »
I'm trying to figure out how many Trojan L16RE-2V (or similar) batteries you'd need to supply 50,000 / 75,000 watt hours for 12 / 24 hours.

50,000 watt hours is an estimate of winter usage, and 75,000 is for summer.

This is such a large load as to be unpractical, but for the sake of calculating, lets look at the numbers.

The L16 2v is a 1100 amp hour 2 volt battery and is capable of 80% discharges (not optimal for long battery life, but very capable of this several times a year, It is often discussed as a poorly designed battery since it is 3 cells in parallel)

So It can deliver 1100 amps x 2 volts =2200 watts (at a 20 hour rate) so you can divide the 50,000 watt load by 2200 = @23 batteries or about 1 - 48 volt string. This would be fine in an emergency but in a system you would likely want capacity for a 2 days of cloudy weather so double that if this is for a stand alone off grid system. To recharge this massive bank you would want a solar array capable of delivering 10% minimum of the battery bank capacity of 4400 amps at 48 volts or 440 amps x 48 volts= 21,120 watts x panel derating to 75% about a 28,000 watt array.

You will also want a good size generator in most parts of the country, In the winter you typically won't have more that 4 hours of charging in winter, so you would need generator assist to top off the battery bank most days....
Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
• Solar Expert Posts: 35
JoshK wrote: »
75000 / 2200 = 34 batteries / 0.50 DOD = 68.18 batteries.
Round up to 69 batteries.

can't be 69 can't be an uneven number the cells are 2 volts. need 12 to make 24 volts so it would be more strings of 12 each or four of them forr 4400 amp hrs to make the same power as 2200 on 48 volts and thats a lot of batteries to put together.
• Solar Expert Posts: 56 ✭✭
Photowhit wrote: »
So It can deliver 1100 amps x 2 volts =2200 watts (at a 20 hour rate) so you can divide the 50,000 watt load by 2200 = @23 batteries or about 1 - 48 volt string. This would be fine in an emergency but in a system you would likely want capacity for a 2 days of cloudy weather so double that if this is for a stand alone off grid system. To recharge this massive bank you would want a solar array capable of delivering 10% minimum of the battery bank capacity of 4400 amps at 48 volts or 440 amps x 48 volts= 21,120 watts x panel derating to 75% about a 28,000 watt array.

So if you got 24 of these batteries, what would they be capable of for a 24 hour period? Would you want to arrange them into 48 volts or a parallel configuration of 24?

I am considering a 12.5kw Perkins genset for the house.

The 75000 watt hour consumption for a day is based on two old 11 SEER AC systems. We are planning on some geothermal configuration (probably Waterfurnace Series 7, 3-5 ton) which is supposed to be way more efficient at 41 EER / 5.3 COP. I hope all that means the watt hour summer consumption would fall dramatically (assuming the system is designed properly, etc.)

We have a lot of roof space and I'm still trying to figure out if it would be more cost efficient to just cover the roof or to try and do something with trackers.

It is a grid tied system, but I'm planning for emergencies as well. I like the idea of being able to run the house overnight on energy stored up during a sunny day.

Our situation is a bit unique from the standpoint of the Alternative Minimum Tax brackets, and the Federal and State tax credits and loans for renewable energy / geothermal projects.

We've just now gotten our kwh rate down to 21 cents from 34 cents a couple months ago (Mass had an emergency rate increase ...)

The way it works out with the taxes is that if we do nothing and want to pay a standard energy bill of \$10,000, then we have to bring home \$14,500, pay the tax on that, and then pay the energy bill. Maxing out the credits and almost entirely eliminating energy payments with renewable energy is an enormous win.

The solution is not going to be all PV or all genset or all grid or all oil heat (now) or all natural gas (coming soon), but rather some combination of the above. It's all a bit dizzying, but I really appreciate the advice I'm getting here ...
• Solar Expert Posts: 232 ✭✭
lasitter wrote: »
So if you got 24 of these batteries, what would they be capable of for a 24 hour period?
Well for a few days until the batteries are ruined. The members here are very adamant that if you run a battery dead it is shot. So you could use the full 50000 for a few times, then no more. Instead you want to double that to 48 batteries so each one only gets run down half way. If you want it to keep yo powered for two days at a time, then double it again. But if you ever need more power, say the 75000 you mentioned before, then the numbers are higher for two days.
• Solar Expert Posts: 35
that's wrong as 70 batteries wouldn't be 24 or 48 volts. he should use 48 volts for this much power anyway and first try to get the usage down because its a lot of power for off grid and if you have grid then theres no paint in batteries at all.
• Solar Expert Posts: 5,145 ✭✭✭✭
lasitter wrote: »

So if you got 24 of these batteries, what would they be capable of for a 24 hour period? Would you want to arrange them into 48 volts or a parallel configuration of 24?

You will not be able to do better than the grid! I think I'm about as close as can be done and I've figured I'm around 26 cents a Kwh + cutting wood and feeding a wood stove in the winter. If you have the grid and 'net metering' that may well be cost effective way to go solar. Most of the best designed cheapest quality components with even energy use to the available sun (heavier use in the summer usually) Cost 50 cents or more per KWH!

If you want to do improvements, do that first! then see what your energy use is, Conservation of energy, learning to live using less energy, have an energy audit, Do you leave a tower computer on 24/7? 3 of them? switch to laptops and have them hibernate could use 1/10th of the energy and maybe even more! Insulate remove air leaks... Move and use less space!

I went and said you could draw down the battery bank to 20% SOC but didn't reduce the amount of energy in the same manner. So my use of 24 batteries would only provide 80% of your needs on a 1 time basis.

Those expensive batteries, you will need to replace them, I think those parallel 2v cells I wouldn't count on lasting more than 7 years. and drawn down to 20 % SOC regularly, you might get 3 years service if that... You would want double the battery bank as I eluded to in my original post and even then you would want to run the generator on days with any compromise in sun...

Off Grid require 3 to 4 times the array as a grid tied array to provide or off set the same load, roughly in most locations. So a 10,000 watt array grid tied might offset your electric bill.... You might look into that as an option. If there is some catastrophic failure of the grid, the government or some protective government trained solders protecting their families will take away your off grid solar... So the survivalist mentality doesn't work...

Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
• Solar Expert Posts: 232 ✭✭
froggersix wrote: »
70 batteries wouldn't be 24 or 48 volts

Yes I know, but he didn't specify a target, so I didn't have anything to round up to.