While solar panels don't work at night, a fridge usually does. The way you will feed it depends on the type of the solar system
- With a grid-tied system you can rely on the utility grid. The night rate is usually lower, so it won't cost you much.
- With an off-grid system the batteries will do all the work.
- With a hybrid system you can choose either the grid or the batteries to power your fridge, whichever is cheaper.
Using the utility grid is pretty straightforward: you just pay for a fridge at nighttime in accordance with the established tariffs. What about batteries? Figuring out the exact battery capacity for running a fridge at night is tricky and even pointless. First of all, in the night-time a refrigerator consumes less electricity: we hardly ever cook or eat at night, don't we? So, it's easier to maintain low temperature then. At the same time, you probably wouldn't purchase a battery just for your refrigerator only. If you want exact numbers for your refrigerator's night consumption, use an electrical meter.
However, it makes sense to demonstrate how to size a battery not only for a night support, but also for an emergency situation. Let's calculate the size of a battery that would support the refrigerator on its own all day and night long. Assumed daily consumption of a refrigerator is still 1.5 kWh.
If the battery is lithium-ion, its capacity is measured in kWh as well. Its efficiency is close to 100%, so we won't take it into account. However, you don't want to discharge it further than 80% on a regular basis. Therefore you need at least 1.5 kWh ÷ 0.8 = 1.875 kWh
If you have a lead-acid battery, its capacity is measured in Amp/hours. Its depth of discharge is around 50%, so the capacity should double the needs of your fridge. Plus, efficiency of lead-batteries is considerably lower – at around 80%. Power = Voltage * Amps, so assuming we'll get a 12V battery, its capacity should be bigger than 1.5 kWh * 2 ÷ 0.8 ÷ 12V = 312.5 A/h
We've calculated the minimum capacity for two types of batteries to run a refrigerator without any support for 24 hours. The same principle can be used for other appliances. We hope that these calculations give you an idea of how to figure out the energy needs of your refrigerator, the number of panels to support it and the battery capacity to run it independently.