Usable vs Nameplate kWh: Why Your Battery Delivers Less Than the Label
A 13.5 kWh battery does not deliver 13.5 kWh. Depth of discharge, round-trip efficiency and degradation cut usable capacity — here’s the math and how to size around it.
Every home battery is sold by nameplate capacity — the total energy the cells hold. What runs your house is usable capacity, which is smaller. Ignoring the difference is one of the most common sizing mistakes, and it shows up as a battery that runs out sooner than the brochure implied.
What eats the difference
- Depth of discharge (DoD). Some systems reserve 5–20% to protect longevity. Modern lithium batteries often allow 80–100%, but the spec sheet matters.
- Round-trip efficiency. About 90% of the energy you put in comes back out; the rest is lost in conversion and heat.
- Degradation. Capacity fades with cycles and age. Most warranties guarantee about 70% at year 10 — so a decade-old “13.5 kWh” battery may deliver closer to 9.5 kWh.
- Temperature. Cold garages and extreme heat reduce usable capacity exactly when outages are most likely.
The math on common sizes
| Nameplate | Usable at 90% DoD × 90% efficiency | Runtime at 1 kW | Runtime at 3 kW |
|---|---|---|---|
| 10 kWh | ~8.1 kWh | ~8 hours | ~2.7 hours |
| 13.5 kWh | ~10.9 kWh | ~11 hours | ~3.6 hours |
| 20 kWh | ~16.2 kWh | ~16 hours | ~5.4 hours |
Compare those runtimes with the label-based numbers. A 13.5 kWh battery marketed as “13.5 hours at 1 kW” is really about 11 hours at 1 kW once losses are counted — and less as it ages.
How to size around it
- Ask for usable capacity, not just nameplate kWh, and confirm the DoD the manufacturer allows.
- Add a buffer if you need guaranteed runtime: round up to the next standard size rather than designing to the exact number.
- Plan for year-10 capacity, not day-one capacity, if the battery is primarily outage insurance.
- Check surge ratings for motors — a well pump or A/C can momentarily draw several times its running load.
Enter your critical load and hours — the calculator rounds up to standard sizes so day-one sizing still covers year-ten reality.
Once you know the size, check how long it actually runs your loads in the battery runtime guide, and what it costs in the ROI calculator.
Sources
- Sun Supply — Batteries ranked by depth of discharge (2026)
- SAJ — How many batteries does a home need? (DoD explained, 2026)
- EIA — Battery storage capacity growth (2026)
- EnergySage — Battery capacity and cost benchmarks (2026)
All figures checked November 2026. Actual usable capacity varies by model and conditions.