To estimate how long a 12V lithium battery will run a load, multiply its rated watt-hours by the fraction you plan to use and by conversion efficiency, then divide by the load’s average watts. For example, using 80% of a 2,688Wh battery through a 90%-efficient converter gives about 19.4 hours at 100W; replace those assumptions with your own battery and equipment values.
Use this formula for a quick estimate
Estimated runtime (hours) = battery energy (Wh) × usable fraction × conversion efficiency ÷ average load (W)
If the battery label gives amp-hours instead of watt-hours, estimate its energy first:
Battery energy (Wh) ≈ nominal voltage (V) × capacity (Ah)
Use the nominal voltage shown for your battery, not an assumed value based only on its “12V” label. Enter percentages as decimals: 80% becomes 0.80, and 90% becomes 0.90. The result is an estimate, not a guaranteed runtime.
Choose the values that match your setup
Use the battery’s watt-hour rating when it is available. Otherwise, multiply its nominal voltage by its amp-hour capacity. The usable fraction is the portion of that energy you plan or are allowed to draw. Use a value appropriate to your battery and intended use; the examples below do not set a recommended discharge limit.
For an inverter or DC-to-DC converter, use its stated or measured efficiency. If you do not know the efficiency, calculate more than one clearly labeled assumption rather than presenting a guessed value as exact. For a direct-DC load with no conversion stage, an efficiency of 1.00 is an idealized estimate; it assumes no conversion loss.
Use the load’s average power in watts. If you measure power at the battery input, conversion losses are already reflected in that measurement, so do not apply them again. If you use the appliance’s power on the output side of an inverter, account for inverter efficiency separately.
Worked example: a 210Ah battery at 100W
The Vipboss 12V 210Ah Bluetooth LiFePO4 battery has a stated 12.8V rating, 210Ah capacity, and 2,688Wh of stored energy. It is designed for RV, marine, solar, off-grid, and backup power use.
For an illustrative calculation, assume 80% usable energy and 90% conversion efficiency. These are example inputs, not recommended settings or a measured result:
2,688Wh × 0.80 × 0.90 = 1,935Wh available to the load
At a steady 100W load, 1,935Wh ÷ 100W ≈ 19.4 hours. With the same assumptions, a 50W load gives about 38.7 hours, while a 200W load gives about 9.7 hours. Change the usable fraction, efficiency, or load to fit your equipment.
A 210Ah battery may suit an RV, marine, solar, off-grid, or backup setup. Check whether this battery’s 2,688Wh capacity and stated output limits suit your load before choosing it.
Use average watts when a device cycles
A device that switches on and off does not draw its full rated power all the time. Calculate its average load over a representative period:
Average load (W) = total energy used (Wh) ÷ elapsed time (hours)
For example, an illustrative 60W device that draws 60W for 15 minutes in each hour uses 15Wh per hour on average, or 15W. With the example’s 1,935Wh available, the estimate is about 129 hours. This assumes the device draws no power while off. If an inverter stays on between cycles, include its idle use in the average load.
Check power limits as well as runtime
Energy determines the estimated hours, but a battery also has limits on how much power it can supply at once. This model supports a stated maximum continuous discharge current of 200A and a stated maximum load power of 2,560W. Compare the load’s continuous and startup demands with the applicable battery and equipment ratings. If a load exceeds a required limit, a positive runtime estimate does not make that setup suitable.
Frequently asked questions
What is the basic battery runtime formula?
Estimate runtime by dividing usable battery energy in watt-hours by the average load in watts.
Should I use the appliance’s maximum watts?
Use average watts over the operating cycle when the device switches on and off. Include inverter idle consumption if the inverter stays on between cycles.
Can a runtime estimate ignore surge power?
No. Runtime and power capability are separate checks; startup or continuous current can exceed a battery or inverter limit even when stored energy is sufficient.
