Any off-grid power system is usually only as good as the battery backup system it is connected to. You could have enough solar panels to power Chicago, but if you don’t have a way of storing the power they are producing, then you won’t have any electricity available to you at night. In other words, no electric lights, air conditioning, television or coffee maker.
The solution is to install a battery backup system. These typically operate at a nominal 12 volts and are most commonly made from 12 volt lead-acid “deep cycle” batteries, although they can be made from other types as well. The Tesla Wall is a battery backup system made of lithium-ion batteries.
Deep cycling of batteries refers to discharging the battery to less than 50%. This is common when you’re pulling power off at times the batteries aren’t being recharged, such as at night. The deep cycle batteries I’ve mentioned there are commonly referred to as “marine and RV batteries.” Those need to be able to be deep cycled, without destroying the battery. So they’re made differently than car batteries, even though they look pretty much the same.
It takes a lot of batteries to have enough storage for your home, so you’re probably going to want to put the system together with just a couple of batteries, and then add additional batteries later. That’s fine. As long as the batteries are connected in parallel (positive to positive and negative to negative) you can add as many together as you want, without problem.
The total system actually consists of three basic components, not counting the solar panels or wind turbines that are producing the power. Those three are a solar charge controller, the batteries and a voltage inverter.
The solar charge controller is a battery charger that is designed for a 12 volt input, rather than one that is designed for a 120 volt input, which might be kept in the garage in case the car’s battery goes out. These are rather simple devices, which range in price from under $20 up to a few hundred. Unless you need a digital readout, as best as I can tell, the inexpensive ones are just fine. The only precaution is to ensure that you buy one which will handle as many amps or watts of input as you’re going to produce.
This solar charge controller has an input from your solar panels and an output that goes to the batteries. Some will also have an additional output, allowing the power from the solar panels to be used directly as 12 volt power, perhaps to power some lighting or other devices.
At the other end (electrically speaking, not physically) of the batteries you’re going to need a voltage inverter. This device receives the 12 volts DC input from the batteries and produces an output of 120 volts AC to run your home electronics. When buying one of these, read the specifications carefully. There are two output ratings, continuous and peak. But the device can only provide peak power for about 10 minutes without damaging it. So make sure that you buy one with a continuous output that is high enough for your needs.
One important thing to understand is that for the voltage inverter to boost the voltage ten times, it has to get all that electricity from somewhere. It does that by taking in ten times the current than it needs. In other words, to provide an output of 4 amps at 12 volts AC, the voltage inverter needs to take in 40 amps at 12 volts DC. So, make sure you have enough batteries and large enough connecting wire to handle that amount of input.