# How much battery capacity?

do I need to run 10 amps at 110 volts for 4 hours? through a conventional good inverter?

Re: How much battery capacity?

I am not sure of your question... Is it: "how much battery capacity do I need to run a 110 VAC inverter at 10amps AC for 4 hours?"

If so, then the answer for a 12 volt battery system would be (assuming that you only drain the battery to 50% of capacity for good life):

12v AmpHours = (1/50% of battery capacity) * 4h*10a*110vac /12 volts = 733 amphours

At 24 volts (probably more reasonable for a 1.1kWatt inverter load):

12v AmpHours = (1/50% of battery capacity) * 4h*10a*110vac /24 volts = 367 amphours

If you were purchasing 12 volt batteries rated at 100 amphrs (20 hour rate), then you would need:

733 amphrs/100 amphr per batt = 7.33 = 8 12 volt 100 amphour rated batteries (for either the 12v or 24v system)

If the inverter is taking 10amps DC of 12 volts to generate ~ 1 amp at 120 vac..., then:

12v AmpHours = (1/50% of battery capacity) * 4h*10a*12vdc /12 volts = 80 amphours rated 12 volt battery.

Or, are you trying to tell if the battery you have is any good?

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
Re: How much battery capacity?

Hey thanks for the quick reply:

really what I was getting at was a system design and battery capacity calculation.
I am putting together a design for a cabin system in arizona, 5000 feet elevation so a somewhat moderate climate
My needs are:

1 small under counter fridge
about 6 total any one time compact flourescent lighting
1 stereo and blender for the occaisonal Margarita
12volt, 5 amp volt water pump about 1 hour per day

Heres the kicker:

In the summer I want to run this AC Unit : (13 SEER 3/4 ton/9,000 Btu Harbor Point Wall Split air conditioner)

I think it pulls about 8 amps I would only run the ac a max of four hours per day, just the hottest part of the day.

I use my place two days per week, maybe three max, so I was thinking of going larger (more) on batteries and charging with a small array of maybe 240 watts because I have 5 days to charge batteries for 2 days use.

any help on the details around these general specs or suggestions would be most appreciated.
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: How much battery capacity?

it's not just how much battery capacity, but how much money do you have. this is much more of a draw than you think as you have a refrig going 24/7 if you're there or not. you have to know what the worst case draw per week would be for that. in fact you need to do this for everything listed. add up all of the watthours you'll need for the week worst case for every item you plan on running. once that is known divide it by 7 as that gives the watthours required for the day. you are in arizona with lots of sun, but there will be cloudy days sometimes and there's less sun in the winter so i'll assume about 5 full sun hours. if somebody else looks it up they may give a better idea of the full sun expectations. divide your watthours by 5hrs to get the power needed per hour in watts. multiply by 1.2 for losses and multiply that by 1.1 to get the total wattage rating of the pvs needed in stc.
from here you have to figure out if you will be running the ac at night or during the day because the daytime use means that extra power doesn't need to be stored in the batteries for the ac use at night. do know that you will need a good sized sinewave inverter and not a modified sinewave inverter to run these items. figure out what i said above first and come back to us and let us know what you came up with and we'll take it from there.
Re: How much battery capacity?

Me I'm made of money LOL, I'm sure I have the budget to build up a nice system shooting for 5K

I kind of figured on turning everything off when I leave after the weekend, store no food, run the ac during the day but from about 2-6 or so and pretty light loads other than that. In fact I have been getting along during construction for lights, sterio and water pump on a marine battery all weekend and pull it down to only 11.8 volts from 12.6 to start.
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: How much battery capacity?

ok very good, but you'll still need to accurately add up all of your wattages. the frig and ac will be big draws, but do the math with all of your watthours and we'll see where it takes us, ok?
• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
Re: How much battery capacity?
do I need to run 10 amps at 110 volts for 4 hours? through a conventional good inverter?

Assuming a nominal 12 V battery bank, a true sine wave 120 VAC inverter and 90% inverter efficiency and a maximum 50% battery bank discharge: 120 V x 10 A x 4 hours / 12 V x 90% x 50% = 889 Ah

Assuming a nominal 24 V battery bank, a true sine wave 120 VAC inverter and 92% inverter efficiency and a maximum 50% battery bank discharge: 120 V x 10 A x 4 hours / 24 V x 92% x 50% = 435 Ah

Assuming a nominal 48 V battery bank, a true sine wave 120 VAC inverter and 93% inverter efficiency and a maximum 50% battery bank discharge: 120 V x 10 A x 4 hours / 48 V x 93% x 50% = 215 Ah

Inverter efficiency specs are for Outback FX models.

HTH,
Jim / crewzer
watt hours usage

My estimate is 6000 watt hours the days I am there, the days I am not=0, average 3 out of 7 days or 2600 seven days a week?

so you think 800 amp hours of batteries at 12 volts (4 troajan big boys) and about 600 watts of solar would do it?
thanks
John
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: How much battery capacity?

john,
for the batteries and solar to provide the total 18,200w your battery capacity must hold that much times 2 or 36,400w and at 12v this is 3033amphrs. this is because it can't be drained past the 50% mark. that's allot of current capacity and allot of batteries. to charge this from solar at 5hrs of full sun everyday you'll need in theory about 700w stc of pvs.
here's how i broke this down:
18,200w/7days/5hrs=watts ptc per hour
watts ptc per hour x1.2 for losses and efficiencies x1.1 =watts stc rated per hour.
the exact figure was 686.4w stc in pvs.
as to the controller to use an mppt type controller would be warranted for this system like the outback mx60. losses could be reduced in wires if configured as a 24v pv system and voltages are downconverted to the 12v battery system voltage.
there is a problem with this whole scenario in that the % of charge current is very low compared to the current rating needed for the batteries. minimumly we recommend a 3% charge being delivered to the batteries and 3% of 3033 is 91amps or more than twice that which a 700w pv system would deliver. something would have to give here either in getting more pvs for the system than the calculated need or downsize the loads to make a smaller system work. i don't have to tell you the ac and the refrig are the culprits here as these are huge power draws and cause much pv power to be needed as well as a huge battery bank. see what happens when you calculate everything without those 2 items and see the difference. doing a system without the ac and frig is the way you should go and use a generator to supplement the power for the frig and ac.
Try the PVWATTS On Line Calculator

John,

1) Have you read the web page on batteries?
http://www.windsun.com/Batteries/Battery_FAQ.htm#Battery%20Size%20Codes

You need to balance your system cost vs battery lifetime. Discharging the batteries down to 50% will eat them up a lot faster than discharging them down to 20% -- this is not a linear relationship. But the cost (and size) of  a "20%" system may be way beyond your budget.

2) You need to consider how the system performs at different times of the year.

The NREL has a good on line system for estimating the amount of power you can expect from a PV system.

Here's a link of Arizona: http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/US/Arizona/

I suspect that the Prescott AZ data base is a good one to use for your application.

If you run their calculator with a PV array size of 1.0 KW, you'll see that the average expected power from the system comes in December ( 116 Kilowatt Hours). That's about 3.9 KWH of energy per day. You'll get about 33% more power during the peak power time (May-June).

3) The amount of re-charing that your PV system will be lower than the PVWATTS numbers. Batteries don't accept charge in a linear kind of way. They accept large amounts of charge when they're deeply discharged, but they only want to see small amounts of charge after that.

Strickly speaking this isn't true for all types of Lead Acid batteries. There are some batteries that could work much better than others for your application. You may want to take a close look at Concorde batteries:
http://www.concordebattery.com/xtender_main.php

They may deal much better with the fact that your PV system will start at low power in the morning, ramp up to full power (for less than an hour) and then ramp back down during the afternoon.
Re: How much battery capacity?
... run the ac during the day but from about 2-6 or so and pretty light loads other than that.

You may want to consider managing your power production a little also.  Folks that use gennies for battery charging typically use the excess generator capacity for washing laundry, vacuuming etc. while the genny is producing.  If you use the panels' production as it's produced you will be able to take one of the inefficiencies out of the equation, charging and discarging the battery.  Today solar noon for PHX is at ~12:30:  HERE (AZ doesn't do Daylight Savings Time) .  If possible, you should use your high load devices spread across this time, or at least try to.  If not possible, perhaps you could compensate by orienting the array a little more west since you would prefer to use the AC from ~2:00 to 6:00.  You will lose the direct irradiance on the array at solar noon but will not lose the inefficiency of charging the battery.  It may take a little calculation but you may end up with a better performing system for lower cost.  The NREL and solar noon calculators can assist with that.

Have a Happy Independence Day!