Should I add a second battery?
Plan the battery you have first, and add a second only if your home uses a lot. In a year of 20 modelled homes, planning their 9.5 kWh battery added £260.14. Doubling it to 20 kWh added under £9 a year left to its own settings, and £61.62 planned.
What these figures rest on: 20 London households’ real smart meter readings (Low Carbon London) with modelled batteries of 5 to 20 kWh and 4 kWp of panels, on the prices Octopus published and the sun that came, planned on the forecasts as issued, 1 October 2025 to 30 September 2026. How Orza is tested.
A second battery left to itself added less than £26 a year.
Two second batteries: about 5 kWh added to a 5 kWh one, and about 10 kWh added to a 9.5 kWh one. Each figure is what the second battery added to a year’s saving, against the same home with no battery.
| Second battery | Agile with Outgoing Octopus, left to itself | Same tariff, planned | Go, no export payment, planned |
|---|---|---|---|
| 5 kWh to 9.5 kWh | +£25.87 | +£66.42 | +£153.37 |
| 9.5 kWh to 20 kWh | +£8.67 | +£61.62 | +£113.92 |
Added a year by the average of 20 homes, 1 October 2025 to 30 September 2026, each battery charging and emptying at up to 5 kW. Standing charges are left out, and no limit was put on export. The full table for each size is on the battery size guide.
- Left to itself, the second battery mostly sat unusedA battery on its own settings stores spare sun and gives it back to the house before the next sun comes, so a second one has little extra to store: across the 20 homes, doubling to 20 kWh this way added from £1.51 to £20.49 a year.
- Planned, it earned more, and more again on GoPlanned around the prices, a second battery also fills when power is cheap and sells or uses it when it is dear. On Go, with its cheap night, that was worth £113.92 a year for the second 10.5 kWh.
Orza plans the battery you have every half hour, before you buy a second.
Free, with no card: an email and a password, and you look round an example home first. Connect your battery through its maker’s cloud when you’re ready, or never. It starts in Monitor only, and nothing is sent to it until you choose.
Planning one battery beat a second left to itself in every home.
Set what planning the battery you already have added against what a second battery on its own settings added, home by home, on Agile with Outgoing Octopus:
A year, average of 20 homes
- Planning a 9.5 kWh battery: +£260.14From £170.99 left to itself to £431.13 planned. More than a second battery left to itself added in all 20 homes.
- Planning a 5 kWh battery: +£219.59From £145.12 to £364.71. More than a second battery left to itself added in all 20 homes on Agile, and in 16 of the 20 on Go.
- One 5 kWh battery planned saved twice what 20 kWh saved left to itself£364.71 against £179.66 a year, on the same tariff. Size counted for far less than what the battery was told to do.
Planned, a second battery paid most where a home used most.
Doubling from 9.5 to 20 kWh, planned, added very different amounts from home to home. Across the 20, a year:
9.5 to 20 kWh, planned, home by home
- On Agile with Outgoing Octopus: £40.53 to £177.74Half the homes gained more than £54.39. One in 20 gained more than £100.
- On Go: £7.93 to £478.68Half gained more than £72.90, and 8 of the 20 more than £100.
- By use, as the battery size guide foundHomes under 4,000 kWh a year gained £43 on Agile and £30 on Go; the five over 6,000 kWh, £97 and £260.
Set the installer’s price against what the second battery saves over its life.
A battery lasts about 10 to 12 years, by the Energy Saving Trust’s estimate. At this year’s prices, for the average home, a second battery saved this much over that time. If it costs more, it does not pay for itself.
The second battery over 10 to 12 years, average home
- 9.5 to 20 kWh, planned: £616 to £739 on Agile, £1,139 to £1,367 on Go
- 5 to 9.5 kWh, planned: £664 to £797 on Agile, £1,534 to £1,840 on Go
- 9.5 to 20 kWh, left to itself, on Agile: £87 to £104Planned, a home that used a lot gained more than this, and one that used little gained less. Prices change from year to year, and this is one year of them.
Before you add a battery.
A year of 20 modelled homes is strong evidence, not a quote for yours. These are the things that change the answer.
All our guidesGet more from your battery
How Orza is tested
Is a second battery worth it if mine runs on its own settings?
In our year, rarely. Doubling a 9.5 kWh battery left to itself added £8.67 a year on Agile with Outgoing Octopus for the average home, and no more than £20.49 in any of the 20. Planning the battery already there added £260.14. How the ways of running a battery compare.
Is a second battery worth more on Octopus Go?
Yes. A cheap night is worth filling a bigger battery for, so planned on Go the second 10.5 kWh added £113.92 a year, against £61.62 on Agile with Outgoing Octopus. Go, Agile or Flux with a battery.
Does a second battery need a second inverter?
Not always: many batteries are built from modules that share one inverter. Ask your installer what yours can take. Every battery here charged and emptied at up to 5 kW, the same for one battery or two, so the second added room to store, not power.
How much should a second battery cost?
Less than it saves over its life. For the average home here, a second battery doubling 9.5 kWh to 20 kWh saved £616 to £739 over 10 to 12 years planned on Agile, and £1,139 to £1,367 on Go. How long a battery takes to pay back.
What size should the second battery be?
Each extra kWh saved less than the one before. On Agile with Outgoing Octopus, planned, each kWh from 5 to 9.5 kWh was worth £14.76 a year, and each from 9.5 to 20 kWh £5.87. What size solar battery do I need?
What does Orza cost?
Nothing right now: everything is free while we spread the word, and no card is needed to join. Paid tiers will come later, and there will always be a free version. Whatever size your battery, Orza plans it every half hour for your own tariff and home.
Get the most from the battery you have.
Free, with no card. Look round an example home first, and connect your battery when you’re ready.