If there's no load, solar panels won't produce any power no matter the weather, so it can be surprisingly difficult to tell when is a good time to run fridge0. Some approaches to deal with this:

• Have some other load, like a water heating element, that can load the solar panels when fridge0 is not.
• Use a photocell to detect when it's sunny.
• Look at the battery state of charge, eg if the battery is full assume it's sunny. Or use the battery charging as a load.
• Try running fridge0 and see if the solar panels start to produce enough power, otherwise turning it off to avoid using the battery for more than a minute.
• Query the charge controller for the PV voltage, to detect when it's found a maximum power point that will be sufficient to run fridge0.

Joey Hess's fridge0

My software mostly looks at the state of charge of the battery to tell when enough PV power is available to run fridge0. It also estimates power used by other parts of the house, to check if the solar panels are getting good power.

Since my charge controller does not report when the battery is in the absorb charge stage, I have experimented with a switchable dump load, which is sometimes useful to detect the absorb charge situation and eke out an extra 30 minutes of fridge0 runtime.

With some tuning I got my software to work well at finding appropriate times to run fridge0 in all conditions. Checking a photocell and PV voltage are two approaches I'd like to try as they could be simpler and need less tuning for the specific installation

others

Please share how you deal with this in your fridge0 build.