When solar panels are used, batteries should be able to power the site for how long without wind or solar?

• A. 1-2 Days
• B. 3-4 Days
• C. 5-10 Days
• D. 2 Weeks

Answer: (C)

The idea being tested is days of autonomy—the amount of time a solar-powered site with battery storage can run without any generation. In other words, how long the battery bank must keep the critical loads powered when there’s no sun or wind.

Five to ten days is chosen because it provides a practical buffer against outages caused by bad weather, extended cloudy periods, or delays in service and maintenance. It balances reliability with cost: long enough to ride out multi-day gaps in generation, but not so long and expensive that the battery bank becomes impractically large for typical site loads. In design terms, you size the battery to supply the expected energy use of the critical equipment for that duration, while accounting for factors like depth of discharge limits, inverter efficiency, and temperature effects.

Shorter backup periods (like one to two days) may leave the site vulnerable during multi-day outages, while a much longer backup (like two weeks) could be unnecessarily costly for most installations.