Estimate backup energy
Capacity and power are different
Watt-hours tell you how much energy a battery stores. Watts tell you how much power it can deliver at once. A refrigerator’s compressor can require much more startup power than its average energy use suggests. A battery may have enough energy for an appliance but still be unable to start it.
How sizing works
The tool assumes 90% inverter efficiency and an editable usable fraction. A 100 W load over eight hours needs 800 Wh at the appliance, or about 1,111 Wh nominal at 90% efficiency and 80% usable capacity. Product labels may quote nominal or usable energy; check which is being compared.
Use actual appliance specifications
The listed wattages are examples, not specifications for your devices. Fridges cycle, laptops change load and CPAP machines differ with humidifiers and heated tubing. For essential medical equipment, follow the manufacturer’s backup requirements and have an appropriate contingency plan.
Solar does not automatically provide outage backup
A grid-connected inverter generally needs suitable isolation and backup equipment to supply loads during an outage. A battery alone is not proof that an existing plug-in system can safely energize household circuits. Confirm the complete approved configuration with a qualified installer.
Separate resilience from payback
If your purpose is to keep essential appliances running, evaluate usable capacity, surge ratings, service and battery life first. Bill savings depend on your tariff and the extra cost of storage. Add realistic replacement costs to the savings calculator.