Battery-only runtime starts with energy, not the light bar’s marketing size. You need its electrical input power, your battery’s rated amp-hours and voltage, starting charge, a chosen usable fraction and an efficiency assumption. The result is an estimate of engine-off energy use. It does not tell you whether the vehicle will restart or whether its charging system can support the accessory.
Watts to amps at 12 volts
Current = input watts / voltage. An illustrative 120 W load at 12 V is 10 A; 60 W is 5 A. Real vehicle voltage varies, and a regulated accessory’s draw can change. Use the product’s actual electrical specification rather than an “equivalent brightness” wattage. Add other simultaneous loads with the electrical load calculator.
A battery-only worked example
An illustrative 12 V, 30 Ah battery has 360 Wh nominal energy. Choose 50% usable capacity, 100% starting charge and 90% efficiency for a planning scenario: 12 × 30 × 0.50 × 1.00 × 0.90 = 162 Wh. A 60 W load would run about 2.7 hours in that simplified model. At 120 W it becomes 1.35 hours. These are assumed inputs, not the specifications of a listed machine.
Amp-hours are conditional capacity
An Ah rating depends on the battery maker’s test conditions. Temperature, age, discharge rate, battery chemistry and cutoff limits affect available energy. Starting batteries and deep-cycle batteries serve different applications. Our battery-type choices supply visible planning assumptions, not permissions to discharge to a universal level. Check the battery maker’s data and the machine’s requirements.
Usable capacity is not a restart guarantee
Holding a percentage of nominal capacity in reserve does not prove enough cranking performance. Cold weather, battery condition and starting loads matter. The model does not simulate voltage sag, a battery management cutoff, detailed discharge curves or the vehicle’s standby loads. See Trojan’s battery reference for why the actual battery and application conditions require their own guidance.
Can the alternator run the light bar indefinitely?
The battery calculation cannot answer that. Published charging output can depend on engine speed and conditions, while the machine’s standard equipment consumes part of it. Only enter an accessory budget that you have established for the exact machine. A positive calculated margin does not size wiring, fuses, relays or connectors. Follow approved installation guidance.
What should I calculate next?
First total peak accessory draw; then estimate battery-only runtime using the same load. Duty-cycle averages may help an energy scenario, but they do not reduce the peak wiring requirement. Use the battery runtime calculator to change assumptions, and return to the owner reference for verified battery information.