Battery running low? Work out roughly how many hours of sun you actually need to top it back up, based on your real panel output and today's conditions.
Real-world solar output is well below the panel's rated wattage once you account for angle, heat, dust and charge-controller losses - this tool derates your panels to roughly 80% (good conditions), 60% (average/flat-mounted or winter) or 40% (partly cloudy/shaded) of their rated output, which is the typical real-world range solar installers quote. It also assumes flat-mounted or fixed panels rather than sun-tracking, and doesn't account for other loads still drawing power while you charge. Treat this as a planning estimate, not a guarantee - actual charge time varies with your controller, panel angle and the weather on the day.