How to Calculate a Battery Bank’s Ah (Amp hours) and Wh (Watt hours)
For this example, I’ll use my own off-grid battery bank to calculate total Amp hours and Watt hours.
However, this will give you a quick overview to help you figure it out. I’m not going to get into the finer points or caveats. Perhaps it will give you some ideas for your own alternative energy battery bank in the future.
Let’s get started…
I use a charge controller and a DC > AC inverter system to recharge a solar panel array that is approximately 4000 watts. My goal was to build a 48-volt battery bank (we’re talking DC volts here) with enough Watt hours (kWh – kilowatt hours) of energy storage to run my essential systems for a few days before needing to be recharged.
Because it’s more efficient (than, say, 12 volts), has less loss, and consumes less current, and most off-grid inverters prefer 48 volts… What is the significance of 48 volts?
Instead, I’ll use 12-volt batteries in this example because that’s what I had on hand when I built this battery bank. As a result, there are numerous battery types and voltage configurations to choose from. I’m not going to go into all of that right now.
Simply put, if this battery is drained to the point of being “dead,” it will provide 1 amp for 100 hours. Alternatively, 20 amps for 5 hours. How about the amperage hour rating? Do you see what I mean? My batteries are each 12 volts and 100 Ah (amp hours) each. It all depends on how you look at it…
But with this general principle, I’m not going into that much detail.) (Well, technically, a fully charged 12-volt battery would be 12.7 volts at rest with no load.)
[ For more information, see Battery State of Charge ]
Watts are calculated by multiplying Volts by Amps. Alternatively, 1.2 kWh (kilowatt hours) of power. As a result, 12 x 100 = 1200 Watt hours will be provided by this single battery.
I strung (connected) four batteries in series to get to 48 volts. As a result, that string produces 48 x 100 = 4800 kWh of energy.
I connected six strings of four batteries in a series. Then I connected all six of those strings in a parallel fashion. As a result, 6 x 4800 = 28800, or 28.8 kWh of energy was stored.
To illustrate my battery bank, I drew a simple block diagram:
They’re AGM (Absorbent (or Absorbed) Glass Mat (or Material), which means I can keep them indoors without them off-gassing. These batteries are of the lead-acid variety.
But I just wanted to let you know. These parameters differ significantly depending on the battery type / chemistry, which I won’t go into here. The faster they die off, the deeper you discharge them on a regular basis. A cycle life versus depth-of-discharge curve exists for all batteries.
In my case, this significantly extends the battery bank’s overall cycle life. I avoid discharging my batteries more than 30% off the top (30% DOD, or depth of discharge) because of the type of battery I have.
So, before recharging, I’ll try not to use more than 30% of 28.8 kWh, or about 9 kWh of energy for my particular battery bank.
That’s enough to power my essentials for a few days, including my furnace (which is also tied to my hot water), refrigerator, chest freezers, and well pump in the winter.
Because I have a lot of energy in the summer, I use the system to power my entire house (including the air conditioners). Despite the fact that the air conditioners are turned off at night (power hogs). It works out well because we have a lot of cloudy winter days here.
Okay, that was a quick lesson on calculating battery bank Amp Hours and Watt Hours.
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