Why think of a battery as a savings rate?
Most battery pages talk about payback: how many years until the savings cover the cost. That is useful, but it is hard to weigh against the other things you could do with the money. If the £2,499 would otherwise sit in a savings account, the fairer question is: what interest rate would that account need to pay to match the battery?
The idea is simple. A savings account pays you interest each year on your balance. A battery pays you nothing, but your electricity bill is lower each year. If you treat that yearly saving as the "interest" on what you spent, you can put the two side by side.
The maths, step by step
Best case on Octopus Go 12M Fixed, a 5 kWh battery saves about £419.74 to £490.29 a year depending on region. Divided by £2,499, that is an equivalent rate of about 16.8% to 19.6% a year.
These are the same figures as the maths on our homepage. Rates are Octopus Go 12M Fixed (product GO-FIX-12M-26-09-29), direct debit, including VAT, checked 10 October 2026 (Octopus product data, Octopus Go):
- Night rate: 9.5p per kWh.
- Day rate: 33.555p per kWh (Yorkshire, the lowest) to 37.4209p per kWh (Merseyside and North Wales, the highest).
- Battery: 5 kWh EcoFlow STREAM (EcoFlow UK STREAM AC 5000).
- Round-trip efficiency: EcoFlow does not publish a figure for the STREAM, so we assume 90%.
To deliver 5 kWh at the day rate, you buy 5 ÷ 0.9 = 5.556 kWh at the night rate, which costs 52.78p.
| Step | Lowest region (Yorkshire) | Highest region (Merseyside and North Wales) |
|---|---|---|
| 5 kWh used at the day rate | 5 x 33.555p = 167.775p | 5 x 37.4209p = 187.105p |
| Less 5.556 kWh bought at 9.5p | 52.778p | 52.778p |
| Saving per day | 114.997p | 134.327p |
| Saving per year (x 365) | £419.74 | £490.29 |
| Equivalent rate on £2,499 | £419.74 ÷ £2,499 = about 16.8% | £490.29 ÷ £2,499 = about 19.6% |
| Best-case payback | £2,499 ÷ £419.74 = about 6.0 years | £2,499 ÷ £490.29 = about 5.1 years |
Put the other way round: for a savings account to pay you £419.74 a year on a £2,499 balance, it would need to pay about 16.8% a year. To pay £490.29, about 19.6%. Payback and equivalent rate are the same maths turned round, so a shorter payback means a higher rate. The 16.8% to 19.6% is the simple figure: yearly saving ÷ £2,499.
Because the £2,499 is spent and the battery wears out, unlike a savings balance you get back, the like-for-like best-case annual return is about 10.7% to 14.6% over EcoFlow's 10-year warranty, or 14.6% to 18.0% if it lasts 15 years, worked out as an internal rate of return on the yearly savings.
What rate does your saving work out at?
Most homes will not hit the ceiling every day. Here is how different yearly savings translate into an equivalent rate on a £2,499 install. The £200, £300 and £400 rows are hypothetical examples, not estimates for your home.
| If you saved this much a year | Installed cost | Equivalent interest rate |
|---|---|---|
| £200 a year (hypothetical) | £2,499 | about 8.0% |
| £300 a year (hypothetical) | £2,499 | about 12.0% |
| £400 a year (hypothetical) | £2,499 | about 16.0% |
| £419.74 a year (best case, lowest region) | £2,499 | about 16.8% |
| £490.29 a year (best case, highest region) | £2,499 | about 19.6% |
To find your own number, work out a realistic yearly saving in the savings calculator, then divide it by £2,499. You can also check your region to see which day rate applies to you.
The tax difference
This is where a bill saving and savings interest really part ways. A lower electricity bill is money you do not spend, not interest paid to you. Savings interest is income, and it can be taxed.
According to GOV.UK: Tax on savings interest, your tax-free allowances for interest include the Personal Allowance, the starting rate for savings and the Personal Savings Allowance. GOV.UK lists the Personal Savings Allowance as follows (GOV.UK: How much is tax-free):
| Income Tax band | Personal Savings Allowance |
|---|---|
| Basic rate | £1,000 |
| Higher rate | £500 |
| Additional rate | £0 |
If your savings interest is more than your allowance, you usually pay Income Tax on the amount above it. So if you already use up your allowance on other savings, a pound of interest is worth less than a pound to you after tax, while a pound off your bill stays a pound. Interest inside an ISA is usually tax-free (GOV.UK: Individual Savings Accounts), which we cover in home battery vs cash ISA.
The honest catches
An equivalent rate is a handy way to compare, but a battery is not a savings account. Before you move money, weigh these up.
- You cannot get the £2,499 back. A savings account keeps your balance and pays interest on top. With a battery the money is spent. You get the yearly saving on your bill, not a balance you can withdraw.
- The battery wears out. Batteries lose some capacity as they age, so the yearly saving can drift down over time. EcoFlow lists a 10-year warranty for STREAM 5000, STREAM AC 5000 and the Expansion Battery 5000 (EcoFlow UK warranty policy). A savings balance does not wear out.
- The saving depends on you. It depends on how much electricity you use during the day, which region you are in, and which tariff you are on. If you are away a lot or use little power during the day, you will save less.
- Tariffs change. The figures here are for one Octopus Go 12M Fixed product, checked 10 October 2026. When the fix ends or rates move, your saving and your equivalent rate move too.
- Keep an emergency fund. Money you might need at short notice belongs somewhere you can get at it. Only think about a battery for money you are happy to commit.
Frequently asked questions
Divide the yearly saving on your bill by the installed cost. Best case, a 5 kWh EcoFlow STREAM at £2,499 fully installed saves about £419.74 to £490.29 a year on Octopus Go 12M Fixed (checked 10 October 2026), which works out at a simple equivalent rate of about 16.8% to 19.6% a year (yearly saving ÷ £2,499). Because the £2,499 is spent and the battery wears out, unlike a savings balance you get back, the like-for-like best-case annual return is about 10.7% to 14.6% over EcoFlow's 10-year warranty, or 14.6% to 18.0% if it lasts 15 years, worked out as an internal rate of return on the yearly savings. That assumes one full 5 kWh cycle every day, all of it used at the day rate, and an assumed 90% round-trip efficiency, so real savings and the real equivalent rate will be lower (see how we worked it out).
A lower electricity bill is money you do not spend, not interest paid to you. Savings interest is different: GOV.UK says that if your savings interest is more than your Personal Savings Allowance you usually pay Income Tax on the amount above it. GOV.UK lists the allowance as £1,000 for basic rate, £500 for higher rate and £0 for additional rate taxpayers (GOV.UK).
No. With a savings account you keep the balance and can withdraw it. With a battery the £2,499 is spent. You get the yearly bill saving, not the capital. The battery also loses some capacity as it ages, so the yearly saving can drift down over time.
It is the same maths turned round. Best-case payback on £2,499 is about 5.1 to 6.0 years. The equivalent interest rate is the yearly saving divided by the cost, about 16.8% to 19.6% best case. That is the simple figure. Because the £2,499 is spent and the battery wears out, unlike a savings balance you get back, the like-for-like best-case annual return is about 10.7% to 14.6% over EcoFlow's 10-year warranty, or 14.6% to 18.0% if it lasts 15 years, worked out as an internal rate of return on the yearly savings. Thinking of it as a rate makes it easier to compare with what the same money could earn in a savings account.
Treat that as a ceiling. It assumes a full 5 kWh cycle every day, all of it used at the Octopus Go day rate in your region. Most homes will save less. Your saving depends on your usage, your region and your tariff, and tariffs change. The savings calculator lets you put in your own numbers.
Put a rate on your bill savings
Join the waitlist for an electrician-wired 5 kWh EcoFlow STREAM from £2,499. Up to about £420 to £490 a year on Octopus Go, best case (see how we worked it out). Habo will try to serve anyone who registers in an area where we have an electrician.
Join the waitlist