Battery Backup Calculator (UPS / Inverter)

The calculation

Battery energy is simple: the load in watts multiplied by the hours you want it to run gives watt-hours. Two real-world factors then increase the size you need. The inverter wastes some energy as heat, usually 10 to 15%, and a battery should not be emptied completely. Lead-acid and tubular batteries are normally used to about 50% of their rated capacity to keep them healthy; lithium iron phosphate can use around 80%.

Divide the final watt-hours by the battery bank voltage to get amp-hours (Ah). A 12 V battery rated 150 Ah stores about 1.8 kWh in total, but only about 0.9 kWh of that is usable at 50% depth of discharge.

Series and parallel

Connecting batteries in series adds their voltage, and connecting them in parallel adds their capacity. Match your inverter's DC input: a 24 V inverter needs two 12 V batteries in series, and a second pair in parallel doubles the capacity. Always connect identical batteries of the same age and brand.

Choosing the load

For a normal load-shedding setup, add up only what you actually want during an outage, such as fans, lights, a router and a refrigerator, rather than the whole house. A refrigerator and a ceiling fan together use far less than an air conditioner, which will drain a battery bank within an hour or two. If the load includes an AC or a water pump, use the battery result together with the inverter surge rating.

Lead-acid or lithium

Lead-acid batteries cost less to buy, but they are heavy, need maintenance and wear out faster when often discharged deeply. Lithium costs more up front, allows deeper use and lasts for more cycles. Compare the cost per usable kilowatt-hour over the battery's warranted life instead of the sticker price alone.