Only the usable portion of the battery is counted, so the runtime reflects safe discharge limits.
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This runtime calculator answers the most common DIY solar question: how long will my battery last? Enter your battery's amp-hours, your system voltage, and the load you're running, and it estimates the hours of backup you'll get before the battery is depleted.
The calculation multiplies your battery's amp-hours by its voltage to get total watt-hours, then divides by your load in watts to get runtime in hours. It's the inverse of the battery sizing calculation — instead of asking how big a battery you need, you're checking what a battery you already own (or plan to buy) can deliver. Keep in mind this is a theoretical maximum; real runtime is a bit shorter because inverters draw a little overhead and you shouldn't discharge most batteries to absolute zero.
Use this to sanity-check a battery before you buy it, or to see how long your existing bank will keep the essentials running during a brownout.
Common questions about how long a battery will last.
Multiply battery amp-hours by voltage to get watt-hours, then divide by your load in watts. A 100Ah 12V battery (1,200Wh) running a 200W load lasts about 6 hours in theory — slightly less in practice due to inverter overhead and safe discharge limits.
Inverters consume a small amount of power themselves, and most batteries shouldn't be fully drained. Lead-acid in particular should not go below ~50%. Treat the result as a best-case ceiling and plan for a little less.
For the same amp-hours, yes — runtime scales with watt-hours (Ah × volts). A 100Ah 24V bank stores twice the energy of a 100Ah 12V bank, so it runs the same load roughly twice as long.
Use the Battery Required calculator — enter your load and the hours you want, and it returns the amp-hours you need.
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