That's an example. Feel free to substitute bread machine, coffee maker, hair dryer, crock pot, or a host of other things—given the choice. This is something that all too often is ignored and the result can be either needing more PV panels or running your generator.

The answer, assuming your primary energy source is solar, is "when the sun is out". Easy answer but you need to understand why.

A battery is not 100% efficient. That is, the amount of energy you have to put into a battery to recharge it is more than the amount of energy you took out. Besides these cycle losses, batteries also become less efficient when you put a heavier load on them. For example, a battery that might put out 10 amperes for 20 hours (200 ampere hours total) might only put out 50 amperes for 3 hours (150 ampere hours).

Thus, if you can use the power from your PV panels (or any other source for that matter) directly to run your rice cooker your energy consumption will be lower than if you run it off the batteries and then have to recharge them.

The next question is how big will the savings be. That, unfortunately, is very difficult to answer. It depends on all too many variables including type of batteries, state of charge, age and current drain as a percentage of battery capacity. The best I can do is offer a SWAG.

A realistic high-end number is that you will need 20% more energy if discharging and recharging the batteries involved. A realistic low-end number is 50%. It could be better or worse than this range under certain conditions but they are a good guide. As you can see, the numbers are significant. To distort an old saying, "make rice when the sun shines".