Multiply the mAh by the voltage in volts and divide by 1,000. A 20,000 mAh power bank at 3.7 V holds 20,000 x 3.7 / 1,000 = 74 Wh. To go back, Wh x 1,000 / V.
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Find out how long a battery lasts from its capacity and load, convert mAh to Wh and Ah, work out the battery size you need and build cell packs in series and parallel.
The math is energy divided by power. A battery's energy in watt-hours is its capacity in amp-hours times its voltage: Wh = mAh x V / 1000. A 12 V, 100 Ah battery holds 1,200 Wh, and a 20,000 mAh power bank at 3.7 V holds 74 Wh. Runtime is the usable energy divided by the power the battery delivers. With 1,200 Wh and a 100 W load, that is 12 h in the ideal case. Two assumptions change this and they are your fields, with no ready-made value: the efficiency of the converter or inverter between battery and load (the battery must deliver more power than the load receives) and the share of the capacity you accept to use, since lead-acid batteries are often discharged only so far to last longer. With 85% efficiency and 50% discharge, the 1,200 Wh become 1,200 x 0.5 x 0.85 / 100 = 5.1 h. The tool does not model the Peukert effect (usable capacity drops at high currents), temperature, ageing or the voltage sag during discharge: the result is an energy estimate, not a guarantee. The nominal voltage by chemistry (3.7 V for LCO lithium-ion, 3.6 V for NMC, 3.2 V for LiFePO4, 1.2 V for NiMH, 1.5 V for alkaline, 2.1 V per cell for lead-acid) comes from the Comparison of commercial battery types table on Wikipedia and only serves to fill in the voltage field.
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Multiply the mAh by the voltage in volts and divide by 1,000. A 20,000 mAh power bank at 3.7 V holds 20,000 x 3.7 / 1,000 = 74 Wh. To go back, Wh x 1,000 / V.
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