A full 13.5 kWh battery runs for 13.5 hours divided by the kW being drawn: 54 hours at 250 W, 13.5 hours at 1 kW, 4.5 hours at 3 kW and 1.2 hours at the Powerwall 3's 11.04 kW maximum. At Ofgem's typical household use of 2,500 kWh a year (6.8 kWh a day) it covers about two days, three days for low use and 1.3 days for high use. After ten years the warranty guarantees only 80%, so 10.8 kWh. Refilling from a 4 kWp array takes under a day in a London June and about three days in December, if the battery gets all the output.
Analysis · Published by SEO Dons Ltd · Updated 7 October 2026
The formula, and what it gives
Runtime in hours is the usable energy in kWh divided by the power in kW being drawn. A 13.5 kWh battery (the size people also search for as a 13kWh battery), such as a Tesla Powerwall 3 with 13.5 kWh of usable AC energy according to Tesla’s datasheet, therefore lasts:
| Power drawn | Run-time from full | What that might be |
|---|---|---|
| 0.25 kW | 54 hours | A fridge-freezer, router, a few LED lights and chargers |
| 0.5 kW | 27 hours | The same plus a television, laptop and boiler pump |
| 1 kW | 13.5 hours | A household using about 24 kWh a day |
| 2 kW | 6.8 hours | Several rooms of heating or a cooker ring |
| 3 kW | 4.5 hours | A 3 kW kettle or heater running continuously |
| 5 kW | 2.7 hours | Several large appliances at once |
| 11.04 kW | 1.2 hours | The Powerwall 3’s maximum output at 230 V |
The right-hand column is illustrative; check the rating plate of each appliance, because what matters is the kW it actually draws. Real run-times are slightly shorter when the battery has been set to keep a reserve, and the maximum power matters as much as the energy: the Powerwall 3’s output is configurable from 3.68 kW to 11.04 kW, so a home that wants the whole house on backup needs the setting at the top of that range, and an electric shower or several large loads at once can still exceed it.
Days of normal use
Ofgem’s typical domestic consumption values, revised from 1 July 2026, are 1,600 kWh a year for a low-use home, 2,500 for a medium one and 3,800 for a high one, on a single-rate meter. Dividing a full 13.5 kWh by the daily figure:
| Household | Use a year | Use a day | A full 13.5kWh battery covers |
|---|---|---|---|
| Ofgem low | 1,600 kWh | 4.4 kWh | 3.1 days |
| Ofgem medium | 2,500 kWh | 6.8 kWh | 2.0 days |
| Ofgem high | 3,800 kWh | 10.4 kWh | 1.3 days |
| Heat pump or electric heating | 6,000 kWh | 16.4 kWh | 0.8 days |
Two cautions. These are days of the whole house’s use, not days of independence: the battery only runs the house if it is installed with backup, and in normal operation it empties and refills every day. And a battery that covers two days of use is not saving two days of bills: the saving comes from emptying and refilling it every day. A battery that would last two days in normal use is bigger than most houses can empty daily, which is why the 13.5kWh battery page asks who can use all of it.
What the battery loses over time
Capacity falls with age. Tesla’s Powerwall 3 warranty guarantees 80% of the energy retained after 10 years, so a 13.5 kWh battery is guaranteed to hold at least 10.8 kWh at the end of the warranty. Every run-time above shrinks by up to a fifth over that period. This site’s savings model assumes a battery averages 90% of its new capacity over ten years, which corresponds to a fade to about 80% by year ten.
How long the sun takes to refill it
To put 13.5 kWh back into the battery takes about 15 kWh of generation, because the round trip loses 10% in this site’s model. Using PVGIS output for a 4 kWp south-facing array, and assuming the battery gets all of it:
| Month | London, kWh a day | London, days to refill | Glasgow, kWh a day | Glasgow, days to refill |
|---|---|---|---|---|
| December | 4.7 | 3.2 | 2.5 | 6.0 |
| March | 11.2 | 1.3 | 9.3 | 1.6 |
| June | 16.3 | 0.9 | 14.7 | 1.0 |
A bigger array refills proportionally faster: a 6 kWp roof takes two-thirds of those times. In practice the house uses some of the output as it is made, which stretches the times, and in winter the cheaper route is the grid. On Octopus Flux, with its 2am to 5am window, a battery can refill overnight from the grid regardless of the weather; the current Octopus Flux rates show what that costs in each region.
Why you cannot export all 13.5 kWh in one go
The Powerwall 3 can deliver 11.04 kW, but a normal G98 connection limits export to 3.68 kW on a single-phase supply. Over the three-hour 4pm to 7pm Flux peak that is at most 11.04 kWh, which is why this site’s model caps peak exports at 11.04 kWh a day. A bigger export limit needs a G99 application and the network operator’s agreement. The home’s own use in the same three hours comes off the top, so the practical export on an ordinary evening is less than the cap.
What to do with the numbers
- For backup, size by the kW of the things you must keep running and by how many hours you need, then check the battery’s power rating and that the quote includes the backup hardware. The backup power page explains what the Gateway does, and how long UK power cuts last shows how long cuts usually last.
- For savings, ignore the days-of-use figure and look at how many kWh the battery delivers in a year; the cost per kWh delivered post does that for three sizes.
- For a quote, ask for the usable energy, the continuous power in kW, the warranty’s retained-capacity guarantee and whether backup is included. Divide, and you can check any run-time claim yourself.
Questions people ask
How long will a 13.5 kWh battery power a house?
About two days for a home using Ofgem's medium 2,500 kWh a year, which is 6.8 kWh a day. A low-use home at 1,600 kWh a year gets about three days and a high-use home at 3,800 kWh about 1.3 days. In a power cut, with only a fridge, lights and a router running (a few hundred watts), it lasts more than a day; with everything running at 3 kW it lasts 4.5 hours.
How many solar panels do I need to charge a 13.5 kWh battery?
To put 15 kWh into the battery (13.5 kWh plus the round-trip loss) from solar alone, about 3.7 kWp in a London June, which is roughly eight 450 W panels, and about 12.8 kWp in December, which is roughly 28 panels, using PVGIS output for London and giving the battery all of it. In Glasgow the figures are about 4.1 kWp in June and 24 kWp in December. In practice the home uses some of the output and the battery charges from a cheap tariff in winter.
Can a 13.5 kWh battery run a whole house in a power cut?
For a while, if it is set up for backup. A Tesla Powerwall 3 needs the Backup Gateway 2 for that and can deliver up to 11.04 kW at 230 V, so a very heavy load such as an electric shower or several large appliances at once can exceed it. At 11.04 kW the battery would be empty in about 1.2 hours.
Does a 13.5 kWh battery lose capacity over time?
Yes. Tesla's warranty for the Powerwall 3 guarantees 80% of the energy retained after 10 years, which is 10.8 kWh, so the run-times in this post shrink by up to a fifth by the end of the warranty. This site's model assumes the battery averages 90% of its new capacity over a decade.