Power is the rate of energy use; energy is an amount accumulated over time. A watt rating alone cannot tell you a monthly bill or battery runtime.
Multiply power by actual use time
The appliance worksheet multiplies watts by daily hours, quantity and days, then divides by 1,000 for kWh. A 50 W fan running eight hours uses 0.4 kWh that day under constant-power assumptions. Cycling appliances may draw less than their nameplate rating on average.
Apply a matching tariff
Multiply modeled kWh by the entered price per kWh. Standing charges, time-of-use bands, taxes and tiers are excluded. Match usage and tariff periods. Use measured consumption when available rather than assuming every nameplate rating represents continuous draw.
Expose battery assumptions
Amp-hours times nominal voltage approximates Wh. The runtime tool applies your usable-capacity share and conversion efficiency, then divides by constant load watts. Chemistry, temperature, age, cutoff voltage and inverter idle losses can change actual performance. This is energy arithmetic, not charging, wiring or protection design.
Worked example
Scroll the table horizontally on small screens to see every column.
| Capacity | Voltage | Usable | Efficiency | Load | Runtime |
|---|---|---|---|---|---|
| 100 Ah | 12 V | 80% | 90% | 100 W | 8.64 hours |
100 × 12 × 0.80 × 0.90 = 864 Wh; 864 / 100 = 8.64 hours in this simplified model.
Sources and checks
Examples are illustrative and can be checked with the linked tools. Read each tool’s assumptions before applying its result. How we verify methods.