Work out how long your battery can power a load, or how much capacity you need to get through the day.
Example estimateEnergy-based estimate
Estimated runtime
7h 12m
800 Wh available · 111.11 W average battery draw
Where the capacity goes
To your loads720 Wh
Losses & idle draw80 Wh
Outside usable fraction200 Wh
Usable energy over time
800 Wh available at the start
0 min1h 48m3h 36m5h 24m7h 12m
The estimate ends after 80% of nameplate energy is used. A BMS cutoff, starting surge, cold battery or worn cells can end a real run sooner.
Estimated runtime: 7h 12m
Optional: check voltage sag under load
This separate check uses the voltage and total resistance you enter. It estimates loaded voltage with every active load on at once. It does not change the energy-based runtime or predict when the battery will reach this voltage.
11.72 V loaded9.48 A · 0.28 V drop
Above your cutoff at this test voltage.
Test load: 111.11 W with all active rows on. Starting surges are not included. Defaults are examples; this check does not establish current ratings or hardware compatibility.
Watt-hours describe stored energy. Watts describe how quickly a load uses it. With a steady average draw, divide usable battery energy by the power taken from the battery.
Battery Wh = nominal V × Ah Average load W = sum of (watts on × time on ÷ 100) Battery draw W = average load W ÷ efficiency + idle W Runtime h = battery Wh × usable fraction ÷ battery draw W
For sizing, reverse the last calculation: required nameplate Wh = battery draw W × target hours ÷ usable fraction. Efficiency and usable fraction are decimals in these formulas.
A 1,000 Wh battery powering 100 W
At 80% usable energy, 90% efficiency and no extra idle draw, 720 Wh reaches the load. A constant 100 W load runs for an estimated 7 hours 12 minutes. To power that same load for 8 hours, you need about 1,111 Wh of nameplate capacity under those assumptions.
Reference and assumptions
NIST’s unit conversion reference covers electrical units. Victron’s Wiring Unlimited explains power, current and resistive voltage drop. The optional sag check combines P = V × I with loaded V = unloaded V − I × R for a constant-power load.
The defaults are planning assumptions, not manufacturer ratings. Estimates can span minutes, hours or multiple days. It does not model a cell-specific discharge curve, lead-acid rate effects, temperature changes, self-discharge or charging during use.
Battery runtime questions
Can I use a battery rated in Ah or mAh?
Yes. Choose the matching unit and enter the nominal voltage for that capacity rating. A 100 Ah, 12.8 V battery holds 1,280 Wh of nominal energy. Capacity ratings at different voltages are not directly comparable.
How do I estimate refrigerator or intermittent-load runtime?
Enter the running watts and the fraction of time the load is on. For example, 120 W at 25% time on averages 30 W. Actual cycling depends on conditions. For a short backup window, averaged duty cycle may not represent the real sequence of on and off periods. Starting surge is a separate equipment check.
What should I use for usable energy and efficiency?
Use measured or manufacturer-supported values for your battery and load path. If you enter a tested usable Wh figure, set usable energy to 100%. If you measure the load at the battery terminals, set efficiency to 100% and avoid counting the same idle draw twice.
Does this work for lithium-ion and LiFePO4 batteries?
The energy calculation works with either chemistry when the inputs describe your battery. It does not infer a discharge curve or cycle life from a chemistry label. Check your battery’s usable energy, current limits and cutoff behavior separately.
Why can the battery stop before the estimated runtime?
The estimate accounts for available energy. The load may still exceed a battery or inverter rating, or trigger a low-voltage cutoff. The optional sag check evaluates one operating point; it cannot predict a cutoff time without a suitable discharge model.
