Hi. Further to my questions about 'just right' system for remote cabin where only electric appliances are a fridge (2 A) and a fireplace blower (1.5 A), during three summer months. What I've understood for sequence of decisions is:
You don't want to have lights, or watch TV, or maybe build something and need an electric drill? I would add a few hundred extra watthours for lights, entertainment, a laptop. At my own cabin, which has a frig running, I use about 2.5 to 3.0kWh per day in the summer. Based on what I'm already using, I'd suggest you'd need about the same, unless you are some sort of historical recreater.
- Next thing is the inverter..... make sure the capacity of the inverter can handle your future needs
Since your refrigerator has a compressor motor, I'd scale your inverter to at least 5X the running watts. So, 240W X 5X = 1200W. Samlex makes a 1500W sine-wave inverter that might be a good fit here. Schneider makes a nice 2500W inverter that's split-phase 120/240VAC.
- Next is the Solar charge controller. If you don't buy a large enough one, you will have to add a 2nd one when you add panels.
- Then consider the batteries. There is no one size fits all answer for batteries. I DONT YET HAVE A CLEAR PICTURE HERE.
Start with the battery size first, then fit the the panels to the battery, and fit the controller to the panels.
Let's go back to the example of my cabin, and say you need 2500Wh per day. I do think that is a realist recommendation. Let's assume you don't want to deplete your battery more than 25% per day. I say 25% because you could have two whole days without sun before battery depletion approached 50% (for lead-acid). So, 2500Wh/0.25 = 10,000Wh. At 24V that works out to be a 10000Wh/24V = 416Ah battery. Look at Trojan's L-16 battery as a good fit here.
Now, the panels and the controller. With a 420Ah Trojan battery at 24V you'd want about 1/8C for the charging amps. So that works out to be
420Ah X 0.125C (.125 is 1/8th) X 26V charging X 1.25lossfactor = 1706W. If you got a good deal on six 300W grid-tie panels that would fit well.
Lastly, the charge controller. You'll want MPPT so you'll minimize losses due to voltage drop. You can wire your 300W panels in series and let the controller transform the raw high-voltage solar current down to the lower battery charging current. Let's assume that you bought those 300W panels, so (1800W/26V) charging X 0.8 loss factor (0.8 is the reciprocal of 1.25) = 55Amps. I'd get a controller that could handle at least 60A. Look at Epever, Outback, Midnight, and MorningStar which all carry MPPT controllers that handle that amperage.
There's the math to help you decide what you need. If you know you won't need 2500Wh per day, change that value. If you want a bigger refrigerator, then change that value. If you select Li batteries, then change the C value. Although your needs may change, you simply plug in the alternative numbers. The math stays the same.
