How to use the calculator
- Enter the capacity and nominal voltage from the battery label. If the capacity is given in watt-hours, choose Wh.
- Enter the total power of the connected lights or accessories in watts, or their current in amps at the battery voltage.
- Set the usable capacity to the depth of discharge the battery maker allows, and the efficiency of any inverter or DC-DC converter in between.
Formula
energy (Wh) = capacity (Ah) × voltage (V) | runtime (h) = energy × usable % × efficiency % ÷ load (W)
mAh are divided by 1,000 to get Ah. A load given in amps is turned into watts with load (W) = current (A) × battery voltage (V). The current drawn from the battery is load ÷ efficiency ÷ voltage.
Units
Capacity in amp-hours (Ah), milliamp-hours (mAh) or watt-hours (Wh); voltage in volts (V); load in watts (W) or amperes (A); runtime in hours and minutes.
Source
The basic relations between charge, energy and power (E = Q × V, P = V × I, t = E ÷ P). No battery or product data is built into the tool; all figures come from what you enter.
Limits and safety
- The result is an estimate from label values. Real capacity is lower in the cold, as a battery ages and at high discharge currents, so expect a shorter runtime.
- Different battery chemistries allow different depths of discharge. The usable percentage starts at 100% so the tool does not assume a value for your battery; use the figure from the battery maker’s documentation.
- A car’s starter battery has to keep enough charge to start the engine. Running accessories with the engine off can leave it unable to do so; check the vehicle and battery manuals before relying on it.
- Lights that dim, flash or change mode draw different power over time. Use an average or the highest setting you will use.
- Whether extra lights may be used on a vehicle on public roads depends on official rules where you drive; this tool does not check that.