A year with a 5 kWh battery on a small solar array

We had been on the fence about adding a battery for two years. The arithmetic kept arriving at maybe. A small solar array, in a small house, on a smart tariff, with one EV: every spreadsheet we built came out with a payback figure somewhere between seven and twenty years depending on what assumptions we leaned on. That is a large range when the headline number on the brochure was a confident seven.

So a year ago we stopped arguing about it and put one in. A 5 kWh LFP unit, hybrid inverter, all wired up in an afternoon. Then we did what we should have done at the start: we just measured. What follows is what twelve months of half-hourly meter data actually showed, against the assumptions we had argued our way into.

The roof and the tariff

The system is four kWp of monocrystalline panels split east and west on a 1970s semi in the Surrey-Hampshire border. Two adults, one child, an EV, no gas. The tariff for the test year was Octopus Intelligent Go. The export rate was the standard SEG number rather than anything fancier. Nothing about the setup is unusual.

Before the battery, our self-consumption was about 28 percent. We were exporting most of what the panels generated during the day and buying it back at a higher rate in the evening. The battery's job, on paper, was to catch some of that exported energy and feed it back in the evening, plus shift cheap overnight energy into the day-rate window.

What actually happened over twelve months

The numbers came in roughly like this. Self-consumption went from 28 percent to 54 percent of generation, which was around 950 kWh more solar used directly. At the difference between import and export rate, that was around £215 across the year.

The bigger contribution was the tariff arbitrage. Charging the battery from cheap overnight electricity and running the house off it during the day-rate window added another £380 or so, depending on how disciplined we were about scheduling around weather forecasts. That is the bit that surprised us most.

The bit we were wrong about

We had spent ages modelling the self-consumption side of the argument. That is the part you can model from solar generation curves and a load profile, so it felt like the rigorous bit. The tariff arbitrage side we had treated as a fuzzy bonus.

It turned out to be the bigger lever. The tariff structure on Intelligent Go has a wide enough peak-to-trough that even a small battery cycled most days adds up to a real number. The maths only really works if the household is willing to schedule things sensibly, but once you are doing that for the EV anyway, adding the battery to the routine is not much extra effort.

What would have made it worse

A flat tariff would have halved the savings. A tariff with a narrower peak-to-trough delta would have dragged the payback well past ten years. And a household that does not bother to look at the schedule at all would have left a large fraction of the benefit on the table. The battery is not a fit-and-forget appliance even if the salesperson suggests it is.

Would we do it again

On these numbers, yes. The catch is that most of the value comes from the tariff side of the calculation. The panels were a smaller contributor than we had been assuming. If smart tariffs vanish or narrow over the coming years, the case for the battery weakens significantly. We will rerun the numbers in a year and update this piece.

If you are doing your own version of this, the calculator on the tools page will walk you through it with your own inputs. Worth doing before you take a quote at face value.