The easiest way to estimate runtime is to start with the battery capacity in watt-hours and divide it by the load in watts. If you had a 1,000Wh battery and a constant 500W load, the simple math gives about 2 hours. In real use, though, you usually get less than that because of inverter losses, battery protection limits, and the fact that many solar generators do not let you use every last watt-hour in the pack.
A practical estimate is to assume about 80% to 90% usable capacity after losses, depending on the system. For example, with a 1,000Wh solar generator, 85% efficiency gives roughly 850Wh usable. Divide 850Wh by 500W, and you get about 1.7 hours, or around 1 hour 40 minutes. If the unit is especially efficient, you might see closer to 1 hour 50 minutes. If it is a cheaper or heavily loaded inverter, runtime could dip below that.
If your battery is rated in amp-hours instead of watt-hours, convert first. Multiply volts by amp-hours to get watt-hours. A 12V 100Ah battery is about 1,200Wh on paper. At 500W, that sounds like 2.4 hours, but after losses and reserve capacity you may end up nearer 2 hours or a bit less. That’s why watt-hours are the better number to use when you’re estimating runtime.
It also matters whether the 500W load is truly constant. A heater or resistive load may stay close to 500W, but a fridge, pump, or tool can cycle or spike. A device that averages 500W but has startup surges can drain the battery faster than expected or even trip the inverter if the surge is too high. Always check both the continuous rating and the surge rating of the solar generator.
Temperature, battery age, and state of charge also affect runtime. Cold batteries deliver less usable energy, and older batteries usually hold less than their original rating. If you want a realistic number for planning, I’d use this rule of thumb: runtime in hours = battery watt-hours × 0.8 ÷ load watts. Then adjust up or down depending on the quality of the inverter and the type of appliance.
If you want the most accurate estimate, test it once with your actual load and start from a full charge. That usually tells you more than the marketing specs ever will.
