Relationships used: V = I×R, P = V×I, P = I²×R, P = V²/R. The 2 fields you most recently typed in are treated as the knowns.
Formula: Runtime (hr) = (Ah × V × efficiency) / Load (W). For multiple batteries, multiply Ah by the parallel count and V by the series count. Default values shown are for a typical 9 Ah / 12 V battery driving 100 W at 85% efficiency. Real-world results vary with battery age, temperature, and depth-of-discharge.