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.
Battery pack sizing calculation
To determine what size battery pack you need for a target runtime, use the inverse calculation:
required energy (Wh) = load (W) × target hours (h) ÷ (usable % × efficiency %)
required capacity (Ah) = required energy (Wh) ÷ nominal voltage (V)
For example, running a 40 W LED light bar for 5 hours from a 12 V deep-cycle battery (80% usable depth of discharge, direct DC connection with 100% converter efficiency):
- Energy consumed: 40 W × 5 h = 200 Wh.
- Nominal energy needed with 80% discharge limit: 200 Wh ÷ 0.80 = 250 Wh.
- Required battery pack size in Ah at 12 V: 250 Wh ÷ 12 V ≈ 20.8 Ah.
In this example, choose a battery pack rated at least 21 Ah (or a standard 25–30 Ah pack for additional safety margin).
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.
Frequently asked questions
How do I calculate battery runtime for 12V LED lights?
Multiply battery capacity in amp-hours (Ah) by nominal voltage (12 V) to calculate watt-hours (Wh). Multiply by your battery’s usable percentage and converter efficiency, then divide by total load in watts: runtime (hours) = (Ah × V × usable% × efficiency%) ÷ load (W).
How do I calculate what battery pack size (Ah) I need for LED lights?
Multiply total LED load in watts by your target runtime hours to get required watt-hours. Divide by battery voltage and usable discharge percentage: Pack Ah = (Load Watts × Target Hours) ÷ (Voltage × Usable % × Efficiency %). For example, running 30 W for 6 hours on 12 V at 80% usable capacity requires (30 × 6) ÷ (12 × 0.80) = 18.75 Ah.
Why does usable capacity matter for battery runtime?
Different battery chemistries allow different safe depths of discharge (DoD). Discharging standard flooded lead-acid batteries past 50% can shorten their operating life, whereas AGM batteries commonly handle 50% to 80%, and LiFePO4 (lithium iron phosphate) batteries can often utilize 80% to 90% or more. Consult the battery manufacturer documentation.
Can I run 12V LED lights or light bars from a car starter battery?
A car starter battery is engineered for short high-current bursts to start an engine, not continuous deep cycling. Running LED lights with the engine off can deplete the battery below the level required to start the vehicle. Check your vehicle and battery manuals before relying on it for auxiliary lighting.
How does converter or inverter efficiency affect runtime?
DC-to-DC converters and 12V-to-120V/230V AC inverters lose 5% to 20% of power as heat during operation (operating at 80% to 95% efficiency). This loss increases the current drawn from the battery and shortens overall runtime.
Related guides
- Watts to Lumens: Practical Energy-Saving LED Lighting Guide
- LED Fixture Lumens Guide: Energy-Efficient Home Lighting