If you plan to build a battery backed power system, you need to decide about the voltage you want to use. The main consideration for your is that at higher voltages the current is less so that you can use smaller (cheaper) copper wires between your solar panel, inverter, and battery bank. When you decide on a battery bank voltage all components need to run on that same voltage. There is an exception to this rule which I will explain further down.
Above you find a list of wire sizes. The three factors (besides coast) for the size are:
The chart is for automotive applications – which assumes high current flow over a longer time period – and it applies for a solar backup systems as well. The obvious rule is: the shorter the cable the less size it needs to have and the higher the voltage the less size the cables need to be.
- 2×12 Volt battery in series
- 50 Ampere rating of the solar UPS controller
- 29 Volt, 240W Solar panel
A 24V System
The distance from the battery bank to the controller is mostly less than 3ft but to avoid voltage drop I recommend 8 AWG wires. The connection to the inverter should have the same size. Most of the big inverter manufacturers offer 12 and 24 volt models. Some of the more expensive brands offer 36 and 48 volt models.
With a single solar panel (10A) connected you only need 10 AWG for at 25ft cable.
You have to pay attention to the solar charge controllers. If you have a voltage difference between panel and battery bank of more than 10V you should buy a Maximum Power Point (MPPT) charge controller. This is the exception I made in the beginning: With a MPPT controller you can mix 12V panels with 24V battery bank and vice versa.
There another charge controller type called Puls Width Modulatiom (PWM). PWM is significantly cheaper than MPPT but you cant mix voltages.
If you want to build an inexpensive system you should go for 24 volts. Here is something tricky for solar panels that you can easily miss: there are 60 or 72-cell modules. You cannot mix them on a single charge controller and for the 72 you should use a MPPT controller because of the higher voltage. The 60-cell is also more efficient than the 72 (see link).