Why doesn't the battery just pay back on the export-vs-buy spread?

It does, but the spread isn't as large as it looks. Standard SEG tariffs are 5–15p/kWh export; standard import tariffs are 25–35p/kWh. That's a 10–30p/kWh spread per stored kWh. A 5 kWh battery cycled once daily stores ~1,800 kWh/year, so the maximum theoretical saving is £180–£540/year. Real savings come in at 70–85% of theoretical (battery round-trip efficiency, partial cycling, summer surplus that exceeds battery capacity). Payback on a £4,500 battery investment lands at 9–14 years on most UK homes.

How big a battery should I get?

Match it to your evening consumption pattern. A typical UK 3-bed home uses 4–6 kWh between 5pm and 11pm — a 5 kWh battery covers most evenings and is the sweet spot for payback. Homes with a heat pump add ~5–10 kWh of evening/overnight load in winter, which justifies 10 kWh capacity. Homes with an EV charged overnight don't benefit from extra battery capacity unless they also have V2G capability; the EV battery itself acts as storage.

Does the BUS grant or any UK subsidy cover batteries?

No. The Boiler Upgrade Scheme covers heat pumps only, not batteries or solar. There's no current UK government grant specifically for residential batteries. Some regional schemes (Welsh Government Nest, certain council-led initiatives) offer means-tested support for solar + battery; these change frequently and have small caps. Most UK households pay the full battery cost upfront.

I'm on Octopus Cosy / a smart tariff already — does that change the maths?

Yes, materially. Smart tariffs price overnight electricity at 7–18p/kWh during cheap windows. With a battery you can charge from the grid at the cheap rate AND store midday solar, so the battery cycles more than once daily. That doubles the annual storage throughput and roughly halves the payback period — 5–8 years rather than 9–14 — for homes that can take advantage of both arbitrage opportunities.

Solar + battery vs solar alone in 2026: when adding a battery pays back

Solar + battery vs solar alone — typical UK numbers in 2026
	Solar alone (4 kW)	Solar + 5 kWh battery	Solar + 10 kWh battery
Install cost	£5,000–£7,500	£8,500–£12,500	£11,500–£16,500
Annual generation	3,500–4,200 kWh	3,500–4,200 kWh	3,500–4,200 kWh
Self-consumption ratio	30–40%	60–75%	70–85%
Exported to grid	60–70%	25–40%	15–30%
Annual SEG income	£100–£300	£60–£200	£30–£150
Annual bill saving	£400–£700	£700–£1,100	£850–£1,300
Total annual benefit	£500–£900	£800–£1,250	£900–£1,400
Payback period	7–11 years	9–13 years	11–14 years
20-year net benefit	£8,000–£15,000	£10,000–£18,000	£11,000–£20,000
Smart-tariff arbitrage?	No	Yes	Yes
Power-cut backup?	No	Some inverters	Most inverters

Solar + battery vs solar alone — typical UK numbers in 2026— Ranges are typical for a 3-bed UK semi with a south or east/west facing roof. Specific numbers depend on roof orientation, shading, household consumption pattern, and SEG tariff choice.

The self-consumption story

A 4 kW UK solar install generates 3,500–4,200 kWh per year, most of it between 10am and 4pm. The problem: typical UK homes use most of their electricity between 5pm and 11pm (cooking, lighting, TV, hot water heating). Solar without a battery only directly serves 30–40% of generation — what’s used by the fridge, daytime occupants, always-on devices. The remaining 60–70% gets exported to the grid at the SEG tariff rate (3–15p/kWh depending on supplier) and the household re-buys electricity in the evening at the import tariff rate (25–35p/kWh).

Adding a 5 kWh battery flips this. Midday surplus charges the battery; evening demand draws from it. Self-consumption rises to 60–75%, exports fall, evening grid purchases roughly halve. On a 10 kWh battery, self-consumption can reach 80%+ but the incremental gain over 5 kWh is small unless you have a heat pump or EV adding evening load.

The cashflow maths

At 2026 UK prices, the economics for a typical 3-bed semi with a 4 kW solar install (no smart tariff):

Solar alone: £6,000 install. Self-consumption 1,200 kWh/year × ~30p saved = £360. Export 2,500 kWh/year × 10p SEG = £250. Total benefit £610/year. Payback ~10 years; 20-year net £6,200.
Solar + 5 kWh battery: £10,000 install. Self-consumption 2,500 kWh/year × ~30p = £750. Export 1,200 kWh × 10p = £120. Total £870/year. Payback ~11.5 years; 20-year net £7,400.
Solar + 10 kWh battery: £14,000 install. Self-consumption 2,800 kWh × ~30p = £840. Export 900 kWh × 10p = £90. Total £930/year. Payback ~15 years; 20-year net £4,600.

The 5 kWh battery is usually the sweet spot. Adding the second 5 kWh ladders the install cost up by £3,500–£4,000 but only adds ~£60/year of benefit — marginal saving at the upper end of capacity. Exception: homes with a heat pump AND an EV may justify 10 kWh because the evening / overnight load is genuinely 8–12 kWh/day.

When the battery pays back fastest

Three home profiles where the battery payback shortens to 5–9 years:

Heat pump in the home. Heat pumps add ~10–15 kWh/day of electricity demand in winter, concentrated in evening and overnight hours. A battery storing midday solar generation directly offsets this load at the highest import rate of the day. Saving widens to £150–£300/year on top of the headline number.
Smart tariff arbitrage (Octopus Cosy + similar). Cheap-rate overnight electricity (7–18p/kWh) lets the battery cycle twice per day — once on solar surplus in the day, once on cheap-rate import overnight. Annual throughput doubles, payback halves to 5–8 years on a 5 kWh battery.
High-evening-consumption households. Families with school-age kids, gaming/streaming setups, or work-from-home households with heavy evening computing benefit more than averages assume. Self-consumption can hit 75–85% on a 5 kWh battery alone.

When the battery doesn’t pay back well

Out-all-day households on Economy 7. If the home is empty 7am–6pm and most consumption is overnight via Economy 7, the battery has limited daytime demand to cycle against and the cheap-rate overnight already removes most arbitrage. Payback stretches to 15+ years.
Excellent SEG tariff. A few UK suppliers offer SEG export rates of 13–15p/kWh. At that level, the benefit of exporting vs storing narrows substantially. Solar-only is sometimes the better economic call on those tariffs.
Planning to move within 7 years. Property listings price in solar but only weakly price batteries. The £4–£6k battery investment is hard to recover in resale if you move before payback.

Smart-tariff arbitrage — the underappreciated kicker

The big shift in 2026 economics is that batteries pair with smart tariffs in a way solar-only doesn’t. Octopus Cosy, Octopus Agile, EDF GoElectric and similar tariffs price electricity at 7–18p/kWh during off-peak windows and 25–35p during peak. A battery owner can grid-charge cheaply AND store solar — two arbitrage opportunities daily. On Cosy specifically, the typical saving is £200–£400/year beyond the solar-only baseline. This is what shortens battery payback from “11 years standalone” to “6–8 years on a smart tariff”.

Power-cut backup — a real but secondary feature

Some inverter + battery combos can island during a grid outage, powering essential circuits (fridge, router, a few lights). This is a UPS-grade feature, not full home backup — the heat pump, kettle and electric shower typically aren’t on the backed-up circuit. UK grid outages are rare enough (~50 minutes per customer per year on average) that this shouldn’t be the primary purchase driver, but for rural homes with longer typical outages it shifts the decision.

What to ask your installer

What’s the projected self-consumption for MY consumption pattern? Good installers will ask for half-hourly smart-meter data and model the actual fit. Bad ones quote 70% self-consumption as a default for everyone.
Which SEG tariff are you assuming? The payback maths is highly tariff-dependent. The installer should show working at 5p, 10p and 15p export rates.
What’s the battery’s warranty period and cycle limit? 10-year warranties are standard in 2026; look for ≥6,000 cycles as that comfortably exceeds 20 years of daily cycling. Avoid batteries with 3,000-cycle warranties — they degrade before payback completes.
Is the inverter sized for future heat-pump or EV load? A 5 kW hybrid inverter is the floor; 8 kW is sensible if you plan to add a heat pump or charge an EV from solar.

Switching pathway

Run a free pre-survey at propertoasty.com/check to get the solar roof fit + indicative system size for your property.
Get 2–3 quotes for both options (solar-only AND solar + battery) from MCS-certified installers. Compare the payback assumptions, not just the headline cost.
Check your half-hourly smart-meter usage data before deciding. The battery’s value is entirely about when you use electricity vs when solar generates; modelling this against your actual data is worth more than installer averages.

The takeaway

For most UK homes, a 5 kWh battery alongside a 4 kW solar install pays back in 9–13 years on standard tariffs and 5–8 years on smart tariffs. The single biggest payback accelerator is a heat pump or EV that adds evening / overnight electrical load. The single biggest payback drag is an out-all-day Economy 7 household where overnight cheap-rate already does the arbitrage the battery would otherwise capture. Sizing matters: 5 kWh is the sweet spot for most homes, 10 kWh only justifies the extra cost if you’ve also got heat pump + EV evening load.

Solar + battery vs solar alone in 2026: when adding a battery actually pays back