"He Didn't Build That" I Did...(Part 4)

Three (3) 12-volt batteries connected in parallel

Solar Battery Sizing

Days of autonomy

First of all decide how many days worth of energy we want to store in our battery bank. Generally this is anywhere from two to five. Let’s assume we want the battery bank to last three (3) days without recharging.

Battery sizing/bank capacity

Then we can calculate the minimum battery capacity in terms of amp-hours (AH). Use the value of 2,880 watt-hours per day from part 3 of this blog article: http://engineer2poet.blogspot.com/2012/09/he-didnt-build-that-i-didpart-3.html, and multiply them by the number of days you decided upon which is 3 days. This should represent a 50% depth of discharge on your batteries. Therefore multiply by 2 and convert the KWH result into amp hours (AH). This is done by dividing by the battery voltage.

Therefore, solving for the energy we need from the batteries:

Since there are 1,000 watts per kilowatt (KW), we divide 2,880 watt-hours per day by 1,000 to convert it to kilowatt-hour (KWH) = 2.8 KWH/day

E (batt.) = 2.8 KWH/day x 3 days x 2 = 16.8 KWH

Converting this to AH we have to divide by the voltage of the system. This can be 12V, 24Vor 48V for commercial application. In our project we will choose to use 12V which is common for household use. 

The minimum AH capacity is then 16,800/12 = 1400 AH

Now if we divide by our battery's rating, we find the number of batteries we must use. Using 250 AH (recommended size) battery rating:

Number of batteries needed = 1400/250 = 5.6 = 6

Therefore we need to use six (6) 12-volt batteries. If we prefer just one day of power storage instead of three days, then two (2) batteries would be just fine. 

Since solar grade type of batteries are still pretty much expensive nowadays, I recommend using the marine grade deep cycle lead-acid batteries usually used in boats and recreational vehicles.

The cost of batteries and other components of our solar project will be discussed in the last part of this series  of blogs, when I discuss about the rate of return of our investment.