Battery Runtime Calculator

Estimate hours of operation at a constant load. Nominal energy = capacity (Ah) × nominal voltage (V), in Wh. Delivered usable energy = nominal energy × usable capacity (%) ÷ 100 × conversion efficiency (%) ÷ 100. Runtime in hours = delivered usable energy (Wh) ÷ load power (W). Edit the fields, then select Calculate.

Assumptions & limits

• Capacity in Ah, voltage in V and continuous load in W must be finite and greater than zero. Both percentage inputs must be greater than 0 and at most 100. There is no fixed upper limit on capacity, voltage or load, but unrepresentable or zero-underflow energy/runtime results are rejected, including overflow when converting hours to minutes.
• Use Ah, not mAh: divide a mAh value by 1000 before entering it. Load is output power in W, not current in A. Keep the voltage and capacity on the same battery-pack basis.
• Usable capacity is the fraction of nominal capacity you choose to use; efficiency is the fraction of that energy reaching the load. The page starts at 85% usable capacity and 100% efficiency. These are example settings, not battery-chemistry recommendations. The engine assumes 100% usable capacity if that optional value is omitted.
• Voltage, load, usable fraction and efficiency are held constant. The model has no chemistry, discharge-curve, temperature, age or changing-load inputs. It does not predict a shutdown voltage or select a discharge limit for your battery.

120 Ah, 12 V, a 96 W continuous load, 80% usable capacity and 90% conversion efficiency.

1. Nominal energy = 120 × 12 = 1440 Wh.
2. Available before conversion = 1440 × 0.80 = 1152 Wh. Delivered usable energy = 1152 × 0.90 = 1036.8 Wh.
3. Runtime = 1036.8 ÷ 96 = 10.8 hours. The fractional hour is 0.8 × 60 = 48 minutes.

10.8 hours, displayed as 10h 48m; nominal energy 1440 Wh and delivered usable energy 1036.8 Wh.

A 0 W load is rejected: dividing by zero cannot produce a finite runtime. This tool does not model no-load storage or self-discharge. A usable-capacity or efficiency value of 0% or above 100% is also rejected; exactly 100% is accepted.

Runtime is the energy budget divided by your assumed continuous load, not a measured endurance guarantee. The hours-and-minutes display rounds to the nearest minute; decimal hours remain in the trace. Usable energy here is energy delivered after both percentage reductions. Do not apply the same loss in both fields, and do not assume 100% means a battery may be fully discharged.

• Unit basis: electric charge in A·s, energy in joules and power in J/s; with 3600 seconds per hour, Ah × V gives Wh and Wh ÷ W gives hours. The milli prefix gives the mAh-to-Ah conversion. The International System of Units (SI), 9th edition

The two percentage factors, defaults, input ranges and excluded effects describe this calculator model. The example is arithmetic with chosen inputs; the cited unit definitions do not prescribe battery chemistry limits or actual runtime.