Solar plus battery in Somerset: what payback actually looks like

Most quotes you see online for solar plus battery quote a single payback figure. Eight years. Ten years. Twelve. The truth is, payback is a calculation with at least four moving inputs, and any installer who gives you one number without asking about your usage is either guessing or selling. Here is how the maths actually works on a Somerset house, and where the figure lands when you stop rounding everything in the customer's favour.

What payback actually means

Payback is the year your cumulative savings cross the upfront install cost. The savings come from three different mechanisms, and they each behave differently.

• Import offset: Every kWh your panels generate that you use directly is a kWh you do not buy from the grid. At a 27p import rate, that is 27p saved per kWh consumed at the moment of generation. This is the largest line on most quotes, but it relies on you being home to use the power.
• Export tariffs: Power you generate but do not use gets exported and paid out by your supplier. Smart Export Guarantee rates run roughly 4p to 15p per kWh depending on supplier, far below your import rate. Export income is real but secondary.
• Time-of-use savings: If you have a battery and a tariff like Octopus Go, you charge the battery overnight at 7p to 9p, then run the house off it during the day at the equivalent of avoiding 27p import. That arbitrage is the largest saving lever a battery adds, and it has nothing to do with how much sun you get.

The "ten-year payback" headline collapses all three into one figure and tends to assume you are home all day, that export rates stay flat for two decades, and that import rates do not move. None of those are reliable assumptions. A proper estimate breaks the maths apart and shows you each line.

A realistic Somerset example

Here is the rough shape of the numbers for a south-facing 4-bed in Taunton with a working couple home in the evenings, two adults, no electric car yet.

• System: 5 kW rooftop array, 10 kWh battery, full DNO and MCS paperwork. Indicative install cost £11,500 fitted.
• Generation: Around 4,400 kWh per year on a south-facing Somerset roof at this size, allowing for typical shading and weather.
• Annual savings, import-only tariff: Roughly £980 per year. Rough split: £540 from self-consumed daytime generation, £140 from exports, £300 from the battery shifting evening usage off-peak.
• Import-only payback: Around 12 years on those numbers. Inside the panel warranty, but not the headline figure most adverts lead with.
• Annual savings with Octopus Go enabled: Around £1,250 per year. The battery now charges off-peak at roughly 8p and discharges through the day at the equivalent of 27p, lifting the time-of-use line from £300 to roughly £570.
• Octopus Go payback: Around 9 years on the same install cost.

These numbers are honest mid-points, not best-case. Add an EV and the picture shifts further in your favour because the off-peak window now also pays for the car. Take the EV away, and the payback drifts back out.

What changes the payback

Four levers move the figure noticeably. None are surprises, but all of them matter.

• Roof orientation: A south-facing roof at the right pitch tops the generation table. East-west splits typically give up roughly 10 to 15 percent of annual generation, but they spread it across the day, which can actually help self-consumption if you are home morning and evening. North-facing roofs rarely make sense.
• Daytime occupancy: Two adults working away from home all day will self-consume far less of their generation than a household with someone home, kids in school holidays, or a home office. Lower self-consumption pushes you onto export rates and stretches payback.
• EV ownership: An electric car is a battery on wheels and the most powerful payback accelerator there is. A 30 mile commute uses around 9 kWh a day. Charging that off-peak from a tariff like Go saves more than the panels do on most rooftops.
• Time-of-use tariff: Without a battery and an off-peak tariff, the system is a generation-and-export piece of kit. With them, the battery becomes the savings engine, often outpacing the panels themselves on annual benefit.

What we tell customers honestly

We turn down a chunk of the surveys we go to, and we will tell you out loud when we do. There are three honest answers we give regularly.

• Sometimes the answer is no: A north-facing slate with heavy shading from a beech tree, a heritage property where panels are not permitted, or a household with very low daytime usage and no plans for an EV. The maths does not work, and we will not pretend it does to win the job.
• Sometimes the answer is "wait two years": Storage prices are still falling roughly 6 to 10 percent a year. If your roof is fine but your usage profile only just makes the numbers work, sitting tight a year or two often improves the deal materially. We will say so.
• Sometimes the answer is "panels first, battery later": If the budget is tight, a panels-only install with battery-ready inverter is a sensible halfway. Add the battery when you are ready, or when you put an EV on the drive. We design the inverter and consumer-unit headroom around that path so the upgrade is straightforward.

Honest payback is the one number you can actually plan against, and we would rather be the installer that gives it to you than the one that wins the quote on a fantasy.

Frequently asked questions