The calculation, in plain sight.
Daily energy = sum of quantity × average watts × hours per day, or measured Wh per day × quantity. Trip energy = daily energy × days.
Usable fraction = (starting charge − reserve) ÷ 100 × delivery efficiency ÷ 100. Required rated capacity = trip energy ÷ usable fraction. Runtime = rated capacity × usable fraction ÷ daily energy.
For example: 300 Wh/day for 2 days needs 600 Wh delivered. At 100% starting charge, 20% reserve and 85% efficiency, the required rated capacity is 882.35 Wh (883 Wh rounded up). A 1,000 Wh battery supplies 680 Wh, about 2.27 days at this average load.
Temperature, aging, standby draw and changing loads can shorten runtime. Output and surge power are not checked. Do not use this estimate to size life-critical backup.
Read the runtime guide →