Fairly new to using solar. I have been reading up on volts vs. amps in terms of what makes the most efficient system. I am considering a 24v battery bank with 4 12v batteries connected together. These are the Kirkland 115a starting/marine batteries. I am also considering a Victron 100/50 MPPT charger. This is the math I have so far.

Mobil RA 220-8B solar panels 1991

rated at: 230w. 10.55volts. 30.48 amps.

kirkland batteries= 115 amp hours * 4 quantity * 24 volts = 11,040 watt hours to charge %100

6 ra-220 panels in parallel = 54v * 30a Isc = 1620w

11040w / 1620w = ~7 hours of full sun to charge from 0% to 100% *keeping in mind partial current from shade

is this math correct? Will this charger work well with this setup? Do you see a better way to set it up?


2 Answers 2


When dealing in watts and watt-hours, it helps to get your math right if you compute a single unit's capacity, then simply multiply by the number of units. E.G. Go 115AH x 12 (not 24) volts to get 1380 watt-hours per battery, then multiply by the number of batteries, 1380WH x 4 = 5520WH whole pack.

I have no idea where you got 54V for 6 ra-220 panels in series (not parallel). You seem to be assuming 9V per panel, at 30A, = 270 watts per panel. See how much nicer "per unit" works? It's now obvious something is wrong with the math, since these panels are only rated at 220-230W. 225W x 6 = 1350W, but that's perfect conditions.

For general design, focus on the watts. The MPPT controller will sort out the volts and amps. However, you do need to make sure the "open circuit" voltage of the panels is appropriate for the MPPT controller.

Also, don't plan to bottom out the batteries if you're buying lead-acid. They are happiest when they never drop below 70% charge (i.e. only using the top 30% of charge in the battery). Dropping below 50% should be uncommon, and draining dead should be rare. Doing so prematurely ages the battery.

If you want a battery you can frequently deep discharge and otherwise abuse, look at Nickel-Iron or Nickel-Cadmium (not NiMH).


Your calculations are a bit off since each battery is 12V, the total energy capacity is 12 * 115 = 1280 Watt-hour each, or 5520 Watt-Hour total for 4 of them.

I couldn't find specs online for your solar panels, but am going to assume that the 10.55volts. 30.48 amps spec is a little optimistic (that's 320 watts). Let's assume 230W each in peak sun, so that's 1380 Watts for the 6 of them.

So, based on those specs, the panels would need about 4 hours for full sunlight to fully charge the batteries if empty.

The charger is rated for 50A max power from the panels, so you'll have to wire the panels all in series to prevent overdriving the charger. Depending on the panels, this limits your ability to handle shade, one panel in partial shade can reduce the power output of all of the panels.

But... you don't want to discharge the batteries all the way -- you'll want to go no lower than 75% - 50% charge level or battery life will be severely reduced. Also, you'll lose some power to inefficiency in charging/discharging the batteries, I'd figure around 20% loss for the whole system.

So instead of having 5520 Watt-Hours available from the system, you're closer to around 2200 Watt-Hours, so you may need to upsize your system if you were counting on having 5KWh/day.

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