"How long will a 10kWh battery last?" has two answers: how many hours a charge runs your home, and how many years the battery stays useful. Both are worked out below from manufacturer datasheets, warranty documents and Ofgem's household figures.
Nominal or usable: check which kWh you are buying
The label figure is not always the energy you can draw, so compare usable figures:
| Battery (manufacturer document) | Nominal or total energy | Usable energy | Depth of discharge |
|---|---|---|---|
| Solax T-BAT H 5.8 V3 (one module) | 5.8 kWh | 5.5 kWh | 95% |
| Solax T-BAT H 11.5 V3 (two modules) | 11.5 kWh | 10.9 kWh | 95% |
| Sigenergy SigenStor BAT 5.0 | 5.38 kWh | 5.2 kWh | 100% |
| Sigenergy SigenStor BAT 8.0 | 8.06 kWh | 7.8 kWh | 100% |
| Sigenergy SigenStor BAT 10.0 | not stated in Sigenergy's technical note | 8.76 kWh | 100% |
Two points stand out. A Sigenergy module sold as "10.0" lists 8.76 kWh usable. And the documents and settings do not always agree: Solax's T-BAT H V3 datasheet quotes a 95% depth of discharge, but Solax's UK and Ireland warranty terms give a maximum specified depth of discharge of 90% and say the default minimum charge for Triple Power batteries on its hybrid inverters is 10%, so part of the battery is kept in reserve.
Working figure used below: a battery sold as 10 kWh, at the 95% depth of discharge in Solax's V3 datasheet, gives 10 × 0.95 = 9.5 kWh usable. Treat that as the upper end. With a 10% minimum charge the same battery gives 10 × 0.90 = 9.0 kWh, about 5% less time in every row below. A little less again reaches your sockets, because converting battery power to mains power loses energy: Solax's X1-Hybrid G4 manual gives a maximum battery-to-AC discharge efficiency of 97.0% at full load.
Hours of use, worked from Ofgem's typical household figures
From 1 July 2026 Ofgem uses these typical domestic consumption values for electricity. Divide each by 365 for an average day, then divide 9.5 kWh by that:
| Ofgem usage level | kWh per year | Average kWh per day | 9.5 kWh usable covers |
|---|---|---|---|
| Low (single-rate meter) | 1,600 | 4.4 | about 2.2 days (52 hours) |
| Medium (single-rate meter) | 2,500 | 6.8 | about 1.4 days (33 hours) |
| High (single-rate meter) | 3,800 | 10.4 | about 0.9 days (22 hours) |
| Medium (multi-rate meter) | 3,400 | 9.3 | about 1.0 day (24 hours) |
Treat these as ceilings. Homes do not use electricity evenly, and a battery normally covers the evening and overnight, then refills from solar or a cheap-rate tariff. For a steady load the sum is simpler: hours = usable kWh ÷ load in kW. As an illustration, 9.5 kWh runs a steady 0.5 kW for 19 hours, or a steady 2 kW for about 4.75 hours.
Capacity (kWh) decides how long; power (kW) decides how much at once. Solax rates one T-BAT H 5.8 V3 module at 2.9 kW charge and discharge (4 kW maximum), and Sigenergy rates a BAT 5.0 at 2,500 W maximum. If a household with low usage has no solar and is out all day, a large battery may sit partly unused, and a smaller one may do the same job.
What a battery can and cannot run in a power cut
Backup is not automatic; it depends on how the system is wired and set up. Two manufacturer examples:
- Solax hybrid inverters (X1-Hybrid G4 manual): during a power failure the inverter powers the EPS (off-grid) loads on a separate backup circuit from solar and the battery, with a typical switching time of under 10 milliseconds. The manual's full-load EPS version uses an externally attached X1-Matebox. Rated EPS output runs from 3,000 VA on the 3.0 kW model to 7,500 VA on the 7.5 kW model. In self-use mode, if the battery is already at its minimum charge when the grid fails, the inverter will not enter EPS mode; backup mode holds back a larger reserve (30% by default) so there is charge left for an outage.
- Loads to keep off the backup circuit: Solax's UK warranty terms warn that equipment with a high starting current, such as water pumps, fixed-frequency air conditioners and motors, can overload EPS, and recommend not connecting it to the EPS port. The G4 manual adds that a fridge or washing machine can draw 3 to 5 times its rated power when it starts.
- Sigenergy: Sigenergy says its Energy Gateway, integrated with SigenStor, detects grid outages and switches to backup power, and claims 0 ms load-side disruption. If backup matters to you, check the Gateway is itemised on your quote.
In EPS mode, Solax's manual says solar feeds the loads first and charges the battery with any surplus, so a daytime outage can last longer than the table suggests; an evening one gets no help from the panels.
Lifespan in years: what the warranties promise
| Warranty document | Capacity promised | Term (whichever comes first) |
|---|---|---|
| Solax UK & Ireland terms (V1.5), T-BAT H 5.8, 5.76 kWh nominal | At least 70% of nominal energy | 120 months from commissioning or 17.9 MWh throughput |
| Sigenergy factory warranty for Europe (November 2023), 5 kWh module, 5.2 kWh usable | 70% of usable energy | 10 years or 15.85 MWh throughput |
| Same document, 8 kWh module, 7.8 kWh usable | 70% of usable energy | 10 years or 23.77 MWh throughput |
Worked example: 17,900 kWh ÷ 5.76 kWh nominal is about 3,100 full cycles, or about 8.5 years at one full cycle every day (about 3,250 cycles, or 8.9 years, if each cycle delivers the 5.5 kWh usable listed in the V3 datasheet). For the Sigenergy 5 kWh module, 15,850 ÷ 5.2 is about 3,050 cycles, or about 8.4 years. A battery cycled fully every day can reach its throughput limit before the ten-year date. One that cycles less is more likely to reach the date first.
What 70% means in hours: 70% of the 9.5 kWh working figure is 6.65 kWh, which covers about 23 hours of Ofgem's medium single-rate average day. Sigenergy measures that 70% against usable energy; Solax measures it against nominal energy. The battery still works then; it just holds less. Solax's V3 datasheet quotes more than 6,000 cycles to 70% under test conditions (25°C, 0.5C), but the warranty is the figure you can rely on.
Warranty conditions can shorten the protection:
- Solax asks for products to be registered within 90 days of commissioning; otherwise the warranty counts from nine months after the manufacturing date.
- Solax's terms list the T-BAT H 5.8 at 5.76 kWh nominal, while the V3 datasheet says 5.8 kWh, so check which warranty terms are named on your own paperwork.
- Sigenergy's full ten-year cover relies on the system staying online for firmware updates. If it stays disconnected, Sigenergy says it will still provide five years from first installation.
- Sigenergy's European warranty excludes equipment installed within 500 metres of the coastline. If you live on the coast, ask for the current UK terms in writing.
Getting a battery sized for your home
National averages are a starting point; your own meter readings are better. If you would like a system sized locally, the battery storage Hull page explains the free battery assessment, where YEERS analyses your energy usage and gives a personalised recommendation. To compare the Sigenergy and Solax systems YEERS fits, see solar battery installation across Yorkshire. Already have panels? Read adding a battery to existing solar panels.
