SpecCalc Hub Guide
Battery runtime basics: Wh, Ah, DoD, and efficiency
How to translate battery labels into usable energy and runtime.
Wh is the common unit
Ah becomes Wh only after multiplying by voltage.
Usable energy
Depth of discharge and efficiency reduce nominal capacity.
Why it matters
Battery labels can be misleading when voltage, chemistry, discharge depth and inverter losses are ignored. Runtime estimates become more useful when nominal capacity and usable capacity are separated.
Formula or method
For Wh input, usable_Wh = capacity_Wh x depth_of_discharge x efficiency. For Ah input, first convert capacity_Wh = voltage x Ah. Runtime_hours = usable_Wh / load_W. If the battery is feeding an inverter and then an AC appliance, the practical runtime also depends on inverter losses, battery cutoff behavior and how hard the battery is being discharged.
Worked example
A 1200 Wh battery with 80% usable depth and 90% efficiency provides about 864 Wh of usable energy. A 200 W load would run for about 4.3 hours in the simplified model. If the same battery feeds a larger load, runtime falls quickly because the numerator stays fixed while the load term increases and because real batteries often lose effective capacity at higher discharge rates.
How to use in practice
Use this guide before comparing battery packs, portable power stations or backup scenarios. It is most useful when different products advertise Ah, Wh or optimistic runtime in a way that hides the actual usable energy and the effect of inverter or discharge losses.
Comparison table
| Topic | Value | Note |
|---|---|---|
| Wh label | Direct energy unit | Usually easier for runtime. |
| Ah label | Needs voltage | Ah alone is not energy. |
| DoD and efficiency | Reduce usable energy | Important for real planning. |
Checklist before using the result
- Check the units, equipment nameplate and real operating scenario first instead of trusting the nearest rounded number.
- Write down which factors the method models directly and which still need separate checking: losses, installation conditions, tariff structure or manufacturer behavior.
- Be explicit about the use case for the estimate: quick planning, option comparison, budgeting, team explanation or preliminary technical review.
- Before practical use, compare the result with equipment documentation, local rules and the real limits of the site.
Common mistakes
- Comparing Ah ratings at different voltages.
- Ignoring inverter and battery efficiency.
- Assuming old batteries still deliver nameplate capacity.
Limitations
- Real runtime depends on battery age, temperature, discharge rate and equipment cutoff voltage.
- Manufacturer runtime curves should be used when available.
- The result is an estimate, not a guarantee.
- Battery chemistry, BMS limits and inverter low-voltage cutoff can materially shorten the simplified estimate.
What to check next
After reading the guide, open the related calculator and test at least two realistic scenarios: a normal case and an edge case. Then compare the result with equipment documentation, local rules, site conditions and the factors that this simplified method intentionally does not model.
When to use the calculator
Use the Battery Runtime Calculator for a transparent first estimate, and the UPS Runtime Calculator when VA, power factor and UPS-specific assumptions are involved.
FAQ
Why is Ah not enough?
Ah must be multiplied by voltage to become Wh. A 100 Ah 12 V battery is not the same energy as a 100 Ah 48 V battery.
Should I include inverter loss?
Yes. If the load is AC and the battery is DC, inverter efficiency reduces usable energy.
Why can two batteries with the same Ah rating give very different runtime?
Because Ah alone does not capture voltage, usable depth of discharge, battery condition, chemistry limits or the discharge-rate behavior that changes effective capacity under load.
Related guides
Last reviewed: 2026-05-29