My hangout

Re: My hangout

Welcome to the forum Harry.

300 Amp hours of battery will be a challenge for 280 Watts of panel. Twice that would be good, but those two panels will only put out about 15 Amps max which is pretty low for that much battery (assuming a 12 Volt system).

There really is no inexpensive way to get 12 VDC from 24 VDC and keep the battery bank balanced, especially if the power requirements are high.

35' from panels to batteries is okay, providing you use the right size wire. However, with two 12 Volt panels in parallel running 15 Amps that whole distance and keeping the Voltage drop below 3% you end up with something like 4 AWG wire, which is pretty expensive.

That's why going up in panels (four for example) and running them in series/parallel for higher Voltage and then using an MPPT type controller the 35 feet becomes much easier to deal with.

1500 Watt hours per day is not difficult on a 12 Volt system. Your 300 Amp hours of battery would do this (if everything goes well) but your 280 Watts of panel couldn't supply it: 280 * 4 * 0.52 = 582 Watt hours.

Mostly I think you should get more panels.

Re: My hangout

It would be more accurate to say you match your panels to the batteries.

The capacity of the battery bank is determined by how much power you need to supply. For example your 300 Amp hours @ 12 Volts taken to 50% SOC is roughly 1800 Watt hours DC (150 Amp hours * 12 Volt nominal).

Once you've got that, then you need to recharge it. Most manufacturers recommend a minimum peak charge current of 5%, but that is net (no loads being used at the same time). If you go for 10% it will work in most cases. There is also an upper limit set by the particular batteries chosen. But with the 'generic' 300 Amp hours as an example: 30 Amps * 12 Volts = 360 Watts. However, panels won't put out their nameplate rating in Watts. But they will put out their Imp rating. So trying to get close to that 30 Amp number with 140 Watt panels that produce slightly more than 7 Amps calls for four panels: 4 * 7 = 28.

And then there is the MPPT advantage. If you were to calculate the array size using that type of controller instead of the PWM type you get: 30 * 12 = 360 / 0.77 (efficiency of array + controller) = 468 Watt array. This is because the MPPT can convert any 'extra' Voltage above charging set point to additional current.

These numbers, btw, work best with only 25% DOD. This has to do with how many Watt hours you can harvest in a typical day. If you use four of those panels for a 560 Watt array on a typical battery system you get a result like this: 560 Watts * 4 hours equivalent good sun * 0.52 over-all system efficiency = 1164 Watt hours AC.

Now compare that to 25% DOD on 300 Amp hours @ 12 Volts: 75 Amp hours * 12 Volts = 900 Watt hours. It's always good to have the panels capable of more power than the batteries, as they will (hopefully) be recharging the batteries and supply loads at the same time.

Which is where you get into the "when do you use the power" situation. Once the batteries are full the panels are capable of supplying more than they will from just maintaining Float Voltage. If you can use loads at that time (usually midday) then you can improve the over-all efficiency of the system.

Is it starting to make sense?

Re: My hangout

Frankly you don't need to fuse the output of a 300 Watt inverter at all; it will fault first (consider that the circuit wiring is designed for 15 Amps, not 3).