How to find right circuit breaker

Thread Starter


Joined Jan 20, 2009
Have two 100 watts solar panels hookup in parallel using HQST MPPT SOLAR charge controller 40 amp negative ground controller connect to 100 AH lithium battery.
Question(1) what size circuit breaker or if use fuse what size ampere need solar panel?
Question(2) what size circuit breaker or if use fuse what size ampere need battery 100 AH


Joined Apr 3, 2014
Check the current rating on the solar panels. Set your wire size to handle the maximum current from the panel. Set your fuse or circuit breaker rating to limit the current in the wire. In other words, the fuse protects the wire.

Do the same for the wire from the charge controller to your battery. If you want to allow for expansion, then size the wire to handle 40 amps. Set the fuse or circuit breaker to handle that as well. If you will never use more than the two panels, you can size the wire smaller. Assuming a 12 volt battery, and without taking into account conversion losses, your current should never be more than about 17 amps. 12 AWG wire is suitable for that current. Since 12 AWG can handle 20 amps, set your fuse or circuit breaker rating to 20 amps.

Note that you will never get 200 watts out of the panels, even on the best of days. However, using that to calculate fuse and wire sizes allows a bit of oversizing.

Lastly, remember that any load you attach to the battery may draw far more current. The wire to your load will have to be sized to that load and, of course, the fuse or circuit breaker protecting it. For example, if you use an inverter to power a 500 watt device, it will draw more than 40 amps. This will require much larger wire.


Joined Aug 7, 2020
Remember that the solar panels can never blow the fuse, a 10Amp panel will never deliver enough current to blow a 10A fuse. The fuse protects against battery current going the “wrong” way down the solar panel cabling to a short on the panel circuit.