Sizing Inverter Power Systems, Part 2

The first video covered situations where you might be running an inverter off a vehicle alternator. In this video, we’ll assume the inverter is being run off a lead acid battery bank and figure out what size battery bank and charger is needed.

So the first step is to determine what sort of load you want to run for what duration of time. In this example, 50 watts AC power for 16 hours. This is assuming that in the winter, there are 8 hours of decent sun light where I can both run the load and charge the batteries off the solar panels.

So 50 x 16 = 800 watt hours and recalling the “divide by 10″ rule from the first video, that gives us 80 amp-hours (AH) of battery discharge capacity needed. Or you could look at is as a 5 amp (DC) load on the batteries for 16 hours, 5 x 16 = 80AH.
Different battery chemistries will have different numbers and specifications. For this example, I’ll assume regular lead acid battery technology.

So the next step is you need to pick a state of discharge you would be happy with for your battery bank. If this is a one-time emergency situation, you could probably drop to 50% state of charge. But for an on-going situation, something like 80% state of charge will give a longer battery life.

So if you only want to drop the batteries from 100% down to 80%, you are only using 20% of the capacity, or 0.2C. To get C, multiply by 5 (or divide by 0.2) to get C = 400AH. So you need a 400AH battery bank.

OK, you got your 400AH battery bank, now how to charge it. Again, 10 is the magic number, lead acid batteries typically like to be charged at around C/10, or in this case, 400/10 = 40 amps. So you need a 40 amp battery charger.

That might be a charger that plugs into the wall or it might be a solar or wind charge controller. In any event, it needs to be sized to handle 40 amps of charging. And a lead acid battery will also take a float charge in the range of C/100, so that 400AH bank will need about 4 amps of float charge to hold it once it has been recharged, going by the 1% rule of thumb for float charge rate.

So there you go, you now know to size the battery bank and charge controller to supply a given AC load for a given time.

The PVWATTS web page is an excellent resource for determining solar power potential in your area:

Now of course this example ignores cloudy days (for solar) of calm days (for wind). As noted, there are many excellent web pages available which can go into the sorted details of making an off-grid system that can run in any probably weather conditions, etc. If you need to design something like that, you need to account for all those details that have been ignored here for simplicity. My system is designed to work most of the time in my local weather conditions and if the weather does not cooperate, I just flip back to grid power.

If I wanted to run my 50 watt load overnight plus allow for 3 cloudy days, that would be 88 hours w/o a charge. 50 x 88 = 4400AH of discharge, then factor in some state of discharge factor and I would have a whopping battery bank and need a massive charge controller.

Other battery chemistries may have different formulas for depth of discharge and rate of charge vs. capacity, so adjust your numbers accordingly. The math is the same, but the actual numbers may be different.

This exercise was roughly how I selected my 40 amp charge controller and interestingly enough, my inverter/charger also can provide up to 40 amps of charging (the charger is built into the inverter). So that comes back to a balanced system. Either by solar charge controller or the back up charger could charge up to a 400AH battery bank.

I’ve heard of systems where someone might have snagged a 1000+AH battery bank and then has a couple of 15 watt Harbor Freight panels and a 3-5 amp charge controller. No way is that charge controller going to be able to handle that battery bank. It may not even be able to trickle charge it. Using the 1% float charge figure, a 1000AH battery bank will need about 10 amps of current just to hold in in float charge.

You can always have a larger charge controller than your battery bank would dictate. I only have about half the battery bank my controller could handle and it is working fine. I am usually running an inverter off the battery bank on sunny days, so that is sucking up excess energy. But without that load, the charge controller seems to read the battery Amp-Hour setting I entered and keeps the charging to a reasonable value. And I can always add more or larger batteries to the system as my needs dictate.

In the next video segment, we’ll look at how to size the power source for the charge controller. I’ll focus on photo-voltaic (solar) panels for this example, so stay tuned for that video:

If there are any questions, ask up in the comments section below.

As always, thanks for watching!

Post time: Apr-01-2018
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