# amp hour question

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**2,511**admin
i have a small solar panel setup that produces 2.5 to 3.0 amps and i want to use it to recharge a 12 volt deep cycle battery. here in arizona i get about 6 useable hours of sun per day. i plan on using the battery and a 400 watt, 12 volt dc to 120 volt ac inverter to run a small evaporative cooler. this cooler draws 1.05 amps at 120 volts. can you please tell me how large a battery do i need to run this cooler for about 8 hours? the guy at the local battery outlet tells me that this cooler will use 12 amps per hour at 12 volts and that i will need a large bank of batteries to run it. i'm confused! thanks for your help. mark in mesa

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## Comments

5,070Super Moderators adminIt will use ~ 11amps @12 volts. 24 hours that means ~264amp hours or in this case 3168 watt/hours. This would require a BIG battery bank, and way more panels!

3.5amp for 5 hours would give you ~17 amp/hours You can see you are significantly low not counting any losses, say by a factor of 20 or so. Consider some alternatives including increasing your PV and your battery bank, and or decreasing you load(s)

Tony

PS Remember that any given amperage at 120volts will increase by a factor of 10 for 12 volts. So for example 1 amp @ 120 volts equals 120 watts.

The same 120 watts from 12 volts requires 10 amps. VoltsXAmps=Watts. Watts/Volts=Amps

25,034Super Moderators adminPeople don't realize how much power (energy) they use vs how much they can collect from solar panels...

A few things you can do to figure out how much power you will need, and how much battery storage and solar panels you will need to produce the power you want...

For example, how much power do you want?

Watt*Hours = P * Time = V*I*Time=120 VAC * 1.05 amps * 8 hours = 1,008 W*H

That is about $0.10 of electricity ($0.10 per kWhr) from the utility.

Now, there are some losses, such as your inverter is ~85% efficient and your flooded cell batteries are 80% efficient... So the amount of power you use per day is:

1,008 W*H / (0.85*0.80)= 1,482 W*Hours per day (1.482 kWhrs per day)

So, usually, for lots of reasons we can discuss later--we want the battery to last you 3 days without sun, and not drop below 50% capacity (basically, battery will last longer, and charge properly, and not draw too heavy of current for its size)... So the battery will be 6x your daily load:

1,482 WH * 6 = 8,892 WH

Now, you will see most batteries are defined by their "20 Hour Rate" in Amp*Hours.... Amp*Hours is simply:

AH=Watt*Hours/battery-bank-voltage

--lets assume 12 volts in this case (although, for a large system, you should use 24 or 48 volt battery bank voltage):

8,892 WH / 12 volts = 741 Amp*Hours--or roughly 8-10 "car sized" storage batteries...

Your batteries can be 2v, 6v, or 12v--You would add 4x 2 volt, or 2x 6v batteries in series to get a 12 volt bank... And you would add 12 volt strings/batteries in parallel to get the total amp*hour storage capacity you need...

Then back to calculating the amount of solar power you will need.

To do this, we can use this US government website and find the city nearest you... Mesa Az=>Phenoix Az... Using all the defaults except using Derating=0.52 because you are off grid (battery and inverter losses), and use 1kW of solar panels as the default (scale to your needs in a bit):

We figured out you need about 1,000 Watt*Hours per day, or 1 kWhr*30 days = 30kWhr/month...

If we assume 90kWhrs per month is minimum required for cooling days (9 months out of the year), then you need 1/3 the amount generated by a 1,000W (1kW) solar array--so about:

30kWh per month * 1,000 watts panels / 90 kWhr per month = 333 watts of solar panels.

From your question, it appears that your system is a 12 volt bus system with a Panel Rating of around:

3 amp * 15 volt = 45 watts

Or, if I had to guess more accurately, dividing out a derating factor of 0.77:

45W/0.77=58 watt

So, you are somewhere around a 50-60 watt solar panel, and you need somewhere around 330 watts (+/-) of solar panels to run the system 9 months out of the year...

In the end, there is no right or wrong answer--just sizing the system to your needs.

And a battery based system may not be the best answer for you... Notice there are lots of inefficiencies and additional costs in building a battery backed AC system... You could also use a DC evaporative cooler that only runs when the sun is up (if that is acceptable to you) and use almost 1/3-1/2 the number of solar panels and save the costs of charge controller, inverter, and battery bank...

Although, you probably would like the solar electricity for other uses too--and a full battery based off grid system would be a better choice...

In the end, the above numbers can be changed around if your system requirments are different (say you only stay on weekends--so battery bank and solar panels can be smaller and charge back during the week you are not there). Or you only need cooling 3 months out of the year, etc.

And, generally with solar, it is most cost effective when you are there 24x7--if this is a weekend summer place--then solar becomes pretty costly as most of your power is thrown away because you are not there to use it--then a small Honda eu1000i or eu2000i (or similar) generator starts to make sense--you still may want to have some batteries too--For example, run the generator in the morning to charge during the day and supply heavy loads--like water pumping--and use the batteries+small inverter at night for lights and radio...

Hope that helps.

-Bill

10,311Solar Expert ✭✭✭✭tony,

you have some errors in your post as 264ahx12v=3168wh. also, 3.5ax5hrs=17.5ah.

5,070Super Moderators adminThats what happens when I get in a hurry! Feel free to delete the post so as to not confuse anyone, and since Bill has (as usual) done a better job!

Sorry for any confusion!

Tony

PS I had it right in the beginning but somehow got confused with my watt/hours vs amp hours.