solar charge controller, inverter, circuit breakers

What size charge controller for a 100 watt solar panel? Or 200w, 300w, or 400 watt solar power system? Or more than that?

(A charge controller installs between the solar panel array and a battery bank)
(description below)

There is a simple way to figure out the size (its rating).

Ready? Lets go…

Take the number of panels x watts to get the total watts of the solar array.

You then divide it by the voltage of your battery bank to get amps.

Add 25% to allow for margin (e.g. cold temperatures) and as always, round up.


1 x 100 watt solar panel / 12 VDC battery bank + 25% = 10 amps.

2 x 100 watt solar panels in series = 200 watts / 12 VDC battery bank + 25% = 21 amps.

3 x 100 watt solar panels in series = 300 watts / 12 VDC battery bank + 25% = 36 amps.

4 x 300 watt solar panels in series = 1200 watts / 48 VDC battery bank + 25% = 31 amps.

10 x 300 watt solar panels in series = 3000 watts / 48 VDC battery bank + 25% = 78 amps.

Okay, you get the idea… it’s just simple math.

Note that the most common solar pv charge controllers come in 12, 24 and 48 volts. High power systems will utilize 48 volt battery banks to advantage smaller cables and losses than otherwise on lesser voltage battery systems.

There’s more to it than what I’ve described (what size charge controller). However the calculation above is the most determining factor for the rating.

Other factors include:

  • input specifications (voltage & current from the solar panels)
  • MPPT (Maximum power point tracking – the most common) or PWM (Pulse width modulation)
  • temperature compensation
  • ability to charge varying types of batteries
  • user adjust-ability of charge parameters

Most manufacturers have sizing tools to help the decision process.


What is a charge controller?

MidNite Solar Classic Charge Controller

It is a device which most importantly properly charges a battery or battery bank, enabling as long a life as possible.

A solar charge controller is designed to receive power from a solar panel or array of panels and use it to charge a battery or bank of batteries.

The charge process is performed in a ‘smart’ or ‘intelligent’ way. This involves varying charge stages, voltages, and current — depending on the type of battery and other factors.

Not to be confused with an inverter, a charge controller is specifically designed to charge batteries in a controlled fashion.

Low power charge controllers are widely available, commonly used on 12 volt battery systems. Amzn has lots of them, for example.

However if you are considering a higher powered system, such as for an off-grid cabin (or whatever), I recommend that you contact Iron Edison in Denver. They will be happy to help determine your needs (they distribute charge controllers). They also specialize in battery banks, particularly Lithium Iron and Nickel Iron. Regardless, they are a sponsor here and will surely be glad to help.

Continue reading: The Four Essentials of Off-Grid Solar

How to measure power consumption – designing a solar system