How efficient is a home battery, really?
A real 27 kWh battery gave back 97.4% of what went into it. Measured at the inverter, 82.1% came back: about 18 kWh lost in every 100, mostly on the way in and out.
What these figures rest on: one real 27 kWh home’s own battery and inverter readings, 9 September to 27 October 2026, with the prices Octopus published and Ofgem’s price cap. How Orza is tested.
A kWh makes three steps, and the battery is the smallest loss.
Round trip efficiency is the share of the electricity put into a battery that comes back out. The rest becomes heat. A home battery sits behind an inverter, which turns the house’s power into charge and back again, so a kWh bought from the grid loses a little going in, a little stored, and a little coming out.
Lost at each step, 30 September to 27 October 2026
- Going in: 6.2%From the inverter into the battery. It charged at about 5.1 kW on average.
- In the battery: 2.6%The battery’s own round trip was 97.4%, measured from its own readings over 25 days.
- Coming out: 10.2%From the battery back out through the inverter. It mostly ran the house, at about 1.1 kW on average, a fifth of its charging rate. Each step loses a share of what is left, so the three together come to 17.9%.
Orza plans your battery with what it loses going in and out.
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The battery alone kept 96.4% to 97.4%. At the inverter, about 82%.
A real 27 kWh battery and 13 kWp of panels, read from its maker’s cloud every five minutes in two ways: the power at the battery itself, and the power at the inverter’s connection to the house. Each figure allows for the change in what the battery held, so a battery that ends fuller than it started is not counted as a loss.
| Measured | Days | Round trip |
|---|---|---|
| At the battery | 9 to 18 September 2026, 10 days | 96.4% |
| At the battery | 12 September to 5 October, 24 days | 97.4% |
| At the battery | 30 September to 27 October, 25 days | 97.4% |
| At the inverter | 12 September to 5 October, 24 days | 82.8% |
| At the inverter | 30 September to 27 October, 28 days | 82.1% |
Measured on 30 September, 7 October and 28 October 2026 from the home’s own readings. At the battery: each day’s charge in and out fitted to the change in what it held, with a few days of broken readings left out. At the inverter, 30 September to 27 October: the battery’s own 97.4% with the losses going in and coming out, from both readings of the same five minutes.
- The battery’s own figure held steady96.4% on its first clean ten days, 97.4% over each of the next two periods. The same battery, a month apart.
- Where it is measured moves it by 15 pointsA battery and its inverter work as one, and a figure for one is not a figure for both. A round trip quoted for a battery is worth checking: at the battery, or at the house.
- One battery is one batteryThese are one home’s readings from its maker’s cloud, not a laboratory test. Another battery, inverter or household will give its own figure.
A kWh bought at 25.00p came back out at 30.46p.
At 82.1%, every kWh the house gets from the battery took 1.22 kWh to put in. So a stored kWh costs its buying price divided by 0.821, and storing it pays when the price it replaces is higher than that.
On Octopus Agile’s own averages
- Charged overnight: 30.46p a kWhAgile averaged 25.00p from midnight to 6am, 5 September to 6 October 2026, in this home’s region.
- Used from 4pm to 7pm: 16.98p saved a kWhAgile averaged 47.44p in those hours, against 30.46p for the stored kWh.
- Against a single rate: under 21.60pTo beat the price cap’s 26.32p a kWh from 1 October, a kWh for the battery has to cost under 21.60p.
Arithmetic on the 82.1% above and the Agile averages in our Agile guide. Charging from the panels can take a different path through the inverter, so its loss going in may differ.
Before you compare round trip figures.
Seven weeks of one battery are evidence, not a promise. These are the questions that come next.
All our guidesGet more from your battery
How Orza is tested
What is a good round trip efficiency for a home battery?
It depends where it is measured. This battery kept 97.4% of its charge on its own and gave back 82.1% at the inverter. Compare figures measured at the same place: at the battery with at the battery, at the house with at the house.
Why does my app show a different figure?
Apps and their histories can read the battery at different points. Over the same 24 days, this home’s readings at the battery gave 97.4% and its readings at the inverter 82.8%. Neither was wrong; they measure different things.
Is charging from the grid overnight still worth it?
When the gap is wide enough. A kWh bought at Agile’s 25.00p overnight average came back out at 30.46p, and Agile averaged 47.44p from 4pm to 7pm, so each kWh used then saved 16.98p. How Agile’s prices ran over a month.
Should I store spare solar or export it?
The round trip is one side of that sum and the export price the other. Our store or export guide works it through on a real home’s prices, and the export tariff guide compares what selling paid.
Does a bigger battery lose more?
Losses go with what passes through it: more kWh through a battery means more kWh lost, alongside more saved. What each extra kWh of battery was worth in a year of modelled homes.
Does filling and emptying it every day wear it out?
A little, with every cycle. Planned to the prices, a 9.5 kWh battery did about 200 to 450 full cycles a year in our runs, against 124 left to itself. How that compares with what a warranty allows.
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.
Charge only when the kWh that comes back is worth it.
Free, with no card. Look round an example home first, and connect your battery when you’re ready.