Nominal capacity is the headline number a battery is sold under. Usable capacity is what you actually get out of it, and it is usually lower.
Three things sit between the two. The battery is not run down to empty. Some energy is lost every time it is charged and discharged. You may also choose to hold a reserve back for power cuts. This guide works through all three using the batteries on our published installations.
Key Points
- Depth of discharge is the share of the pack the battery management system will use. Both Fox ESS batteries we fit publish 90%.
- Round trip efficiency is what survives a full charge and discharge cycle. Both Fox ESS batteries we fit publish 95% or better.
- The inverter takes its own small cut on the way in and on the way out.
- A backup reserve is a setting, not a fault. If you hold 20% back for power cuts, that 20% is not there for the evening.
- Two different batteries with the same nominal number can behave differently. Compare usable figures, not headline figures.
- Manufacturers do not all publish the same rows. Where a figure is not on the datasheet, ask for it in writing.
Common Misconceptions
"10 kWh means 10 kWh of electricity in my house." It does not. Take off the depth of discharge, the round trip loss and any reserve.
"The losses mean the battery is faulty." No. The manufacturer publishes them.
"All 10 kWh batteries are the same." They are not. Two packs can differ in depth of discharge, efficiency, charge speed and warranty.
"Bigger is always better." Only if you empty it. A large battery you never run down is capacity you paid for and do not use.
"Usable capacity falls off a cliff." It does not. It declines slowly, which is why warranties are written over ten years.
Real-World Advice
Our published 10 panel installation in Eastbourne is a 4.6 kWp array with a Fox ESS EP6 Plus battery of 5.76 kWh. It runs on a Fox ESS H1 hybrid inverter. See the Eastbourne case study. The installer's note says the design showed the panels making close to twice what the household uses. So the battery is mostly there to store spare power.
Take the published figures for that battery and inverter and follow the energy through.
| Step | Figure | Source |
|---|---|---|
| Nominal energy | 5.76 kWh | Fox ESS EP6 datasheet |
| Depth of discharge | 90% | Fox ESS EP6 datasheet |
| Usable energy in the pack | about 5.18 kWh | 5.76 multiplied by 0.90 |
| Round trip efficiency | 95% or better | Fox ESS EP6 datasheet |
| Energy you must put in to store that | about 5.5 kWh | 5.18 divided by 0.95 |
| Inverter discharge efficiency | up to 97% | Fox ESS H1(G2) datasheet |
| Energy reaching the house | about 5.0 kWh | 5.18 multiplied by 0.97 |
So a 5.76 kWh battery delivers around 5 kWh into the house on a full cycle. That is not a defect. It is how every lithium battery works, and the figures are printed on the datasheet.
The batteries on our published work
| Battery | Nominal | Depth of discharge | Usable in the pack | Round trip efficiency |
|---|---|---|---|---|
| Fox ESS EP6 Plus | 5.76 kWh | 90% | about 5.18 kWh | 95% or better |
| Fox ESS ECS4800-H2 | 9.32 kWh | 90% | about 8.39 kWh | 95% or better |
| Anker SOLIX X1-B5-H, one module | 5 kWh | not stated on the datasheet | see note | not stated on the datasheet |
| Anker SOLIX X1-B5-H, two modules | 10 kWh | not stated on the datasheet | see note | not stated on the datasheet |
| Anker SOLIX X1-B5-H, four modules | 20 kWh | not stated on the datasheet | see note | not stated on the datasheet |
The Anker datasheets we hold give the module energy, the chemistry, the charge and discharge power and the ten year warranty. They do not publish a depth of discharge row or a round trip efficiency row.
UK-Specific Considerations
Both battery makers we fit publish a ten year warranty. That warranty is on the product. It is separate from any promise about how much capacity remains after ten years, which is a different clause in a different document.
Winter matters here. A battery covers the evening. It does not carry a UK home through a dark December, whatever its size. The published usable figure is the honest basis for an evening plan, not for a year round one.
If your tariff has cheap overnight power, usable capacity is also what you can buy cheaply and hold. Round trip efficiency then costs you real money, because you pay for every kWh you put in and get back slightly less.
Backup during a power cut is a separate setting. Both the Fox ESS H1 and the Anker SOLIX X1 have an emergency supply output. The Anker datasheet gives a switch time of under 10 milliseconds. If you want that, you hold a reserve, and the reserve is not available for normal evening use.
When This Isn't Right For You
Comparing on usable capacity alone is not enough if the two systems have different power ratings, different warranties or different backup ability. Capacity is one column of the comparison, not the whole table.
Before You Spend Money
Ask for the nominal capacity, the depth of discharge and the round trip efficiency in writing, for the exact model you are quoted.
Ask what the usable figure works out at. Do the sum yourself from the datasheet if you want to check.
Ask how much power the battery can deliver at once, not just how much it holds. A battery that stores plenty but delivers slowly will not cover a shower and an oven together.
Ask whether backup is included, and how much reserve it holds.
Ask what happens at year ten, and get the capacity retention clause in the warranty document rather than a number in a conversation.
Frequently Asked Questions
Q: Why can't a battery be run down to zero?
A: Running a lithium cell fully flat shortens its life. The battery management system keeps a floor, and the datasheet publishes it as depth of discharge.
Q: Is 90% depth of discharge good?
A: It is a normal published figure for the home batteries we fit. Both Fox ESS units state 90%.
Q: What does round trip efficiency actually cost me?
A: At 95%, storing about 5.5 kWh gives you about 5.18 kWh back out of the pack. The difference is lost as heat.
Q: Does the inverter add more loss?
A: Yes, a small amount. The Fox ESS H1(G2) datasheet gives up to 98.5% charging the battery and up to 97% discharging it.
Q: Should I size the battery on nominal or usable capacity?
A: Usable. That is the number that meets your evening.
Summary
Nominal capacity is the label. Usable capacity is the delivery. For the Fox ESS batteries on our published work, the published depth of discharge is 90%. The published round trip efficiency is 95% or better. So a 5.76 kWh battery gives about 5.18 kWh out of the pack and about 5 kWh into the house. The Anker datasheets we hold do not publish those two rows, so we have asked for them. Size a battery on the usable number, and remember that any backup reserve you set comes off the top.