Solar + Battery 2026 — Should London Homeowners Get Both?

Solar-only (no battery): typical 4kW system on a south-facing London roof generates around 3,400 kWh/year. Self-consumption rate without a battery sits at 30–45% — the rest is exported via SEG. Annual financial benefit is around £700–£900 on a 30p/kWh import rate and 15p/kWh SEG rate. Install cost £6,500–£9,000. Payback: 8–11 years.

Solar + battery: same 4kW PV with a 10kWh battery. Self-consumption rises to 70–85% — most generation is used directly or stored. Plus the battery can be charged from the grid on cheap tariffs (Intelligent Octopus Go, Cosy) and used during peak windows, adding tariff arbitrage on top of solar self-consumption. Annual financial benefit £1,050–£1,500. Install cost £11,500–£15,500. Payback: 9–12 years.

Battery-only (no solar): a 10kWh battery charged overnight at 7p (Intelligent Octopus Go) or 13p (Cosy off-peak) and discharged across the daytime and peak window. Annual financial benefit £400–£700. Install cost £5,500–£8,500. Payback: 6–10 years.

On pure payback period, battery-only is sometimes the fastest — but it caps the total annual benefit at £400–£700 because there is no generation. Solar + battery has the longest payback but the highest absolute annual benefit, and the system has 25-year solar warranties carrying value well past the payback point.

Battery-only on Cosy or Intelligent Octopus Go is a genuinely overlooked option in 2026. The economics work because the differential between cheap overnight import (7p) and peak window import (35–41p) is wider than at any previous point in the UK market.

The ideal candidate for battery-only: a home with no realistic solar option (north-facing roof, shaded, conservation area block, leasehold flat without panel rights), high evening peak consumption (heating, cooking, EV charging), and willingness to switch to a time-of-use tariff.

A 10kWh battery cycled daily at the Intelligent Go rate displaces 10kWh of peak consumption at around 30p — saving about £1.10/day or £400/year purely on the arbitrage. With a Cosy tariff and active use of the three off-peak windows, savings can rise to £500–£700/year.

The 0% VAT relief applies to standalone batteries (battery without solar) when supplied under a single MCS-installer contract. This was a 2024 extension to the original solar-only relief and remains in force until 1 May 2027. The relief saves £1,000–£1,700 on a typical 10kWh standalone battery install.

0% VAT on residential solar PV, battery storage and heat pump installs runs until 1 May 2027. After that, the government has indicated a return to 5% VAT (the pre-2022 rate) but legislation has not yet been laid to confirm.

On a £12,500 solar + battery install, the saving versus 20% VAT is £2,500. Versus 5% VAT it is £2,000. Either way, an install commissioned before 1 May 2027 is materially cheaper than the same install post-2027.

The relief applies to the install package — panels, inverter, battery, cabling, scaffold, labour — when supplied under a single contract by an MCS-certified installer. Self-bought components and separate fit-only labour do NOT qualify.

Practical timing: MCS installer order books for Q1 and Q2 2027 are already filling in mid-2026 as homeowners try to lock in pre-deadline installs. To capture the relief, complete the survey and quote stage before the end of 2026; sign the contract no later than January 2027. Leaving it until Q1 2027 risks installation slots pushing past the deadline.

The relief also covers ancillary works carried out as part of the install — consumer unit upgrades, supply uprates, and the EV charger if installed alongside (where the charger is invoiced as part of the renewable install package). Confirm bundle invoicing with your installer at contract stage.

DC coupling: the battery is wired directly to the solar inverter on the DC side, before the inverter's AC conversion stage. Solar generation that exceeds the inverter's AC output is captured into the battery without an AC conversion loss. Higher efficiency, lower cost (one inverter rather than two), but requires a hybrid inverter and a battery selected to work with it.

AC coupling: the battery has its own inverter and connects to the home's AC bus alongside the solar inverter. Two inverters, two conversion losses, but flexible — any battery can be added to any existing solar system, and the battery can charge from the grid as well as from solar.

DC coupling is the default for new installs where solar and battery are being commissioned at the same time. Tesla Powerwall 3, GivEnergy AIO, and most modern hybrid systems run DC-coupled. Roundtrip efficiency is around 92–94%.

AC coupling is the default for retrofit — adding a battery to an existing solar system without replacing the existing solar inverter. Roundtrip efficiency is around 85–88%. The lower efficiency is real but the install cost is materially lower (no inverter replacement) and the retrofit can usually be done without scaffolding.

Practical rule: if you are installing solar and battery together, choose DC-coupled hybrid kit. If you have an existing solar system and want to add a battery, choose AC-coupled retrofit unless your existing inverter is at end-of-life anyway.

Oversizing solar (installing more solar capacity than the home's annual consumption can absorb) is profitable when SEG export rates are good. At 15p/kWh export and 30p/kWh import, every kWh exported is worth half what it would be worth self-consumed — but the marginal install cost of going from 4kW to 5kW or 6kW is small (£600–£1,500), and the export income compounds.

Oversizing battery (installing more storage than the daily generation can fill) is profitable when off-peak grid charging is cheap. A 13.5kWh battery charged from grid at 7p and discharged at 35p saves around £3.78/day if fully cycled — £1,380/year. The 'oversize' battery is paid for by tariff arbitrage even on days with no solar.

For homes with high daytime consumption (work-from-home, EV daytime charging, heat pump), oversizing solar is the better play — more direct consumption, fewer battery cycles.

For homes with high evening peak consumption (cooking, heating, EV evening charging) and a time-of-use tariff, oversizing battery is the better play — more arbitrage cycles per year, more peak displacement.

The simple sizing rule: solar capacity should roughly match annual consumption (4kW for a typical London 3-bed family at 3,800kWh/year); battery capacity should match evening peak consumption from peak window onset to next morning's off-peak window (typically 8–14kWh for a London family).

Edwardian/Victorian 2–3 bed terrace, south-facing roof, family of 3–4, EV: solar (4kW) + battery (10kWh). Payback 9–11 years; annual benefit £1,100–£1,400. The system pays back well within the panel warranty period and continues to deliver benefit for another 15+ years.

1930s semi-detached, west-facing roof, family with no EV: solar (4kW) only. Battery economics weaker without high evening peak consumption; add battery later when economics or EV purchase justify it. Payback 9–11 years.

1960s flat with limited or no roof rights: battery-only on Intelligent Octopus Go or Cosy. Payback 7–9 years. Apply for solar later if roof rights are negotiated with the freeholder.

Listed building in a conservation area: integrated all-black solar slate panels are sometimes permitted; verify with local planning before pursuing. If solar not viable, battery-only is the fallback; ASHP combined with battery delivers good heating and electricity arbitrage.

Across all configurations, the 0% VAT window to May 2027 means it is materially cheaper to install now than to wait. Where the homeowner is fundamentally interested in solar or battery, the 2026 install will be 5–20% cheaper than the same install in mid-2027 once VAT returns.

One overlooked variable for London landlords specifically: the 2030 MEES band C deadline. Solar adds 6–12 SAP points to the EPC; a battery alone adds no SAP points but improves running cost economics for the tenant (and reduces complaint risk under the new Property Portal). For a D-rated rental that needs 8–14 SAP points to reach band C, solar plus the £10,000 MEES cost cap math typically lands as the cheapest practical route. Standalone batteries do not bridge MEES on their own.