National Grid ESO awarded capacity market contracts for 47.7 GWh of battery storage last week, enough to power 1.6 million homes for two hours. But the headline number is not the capacity. It is the duration: several projects will discharge for 12 to 18 hours, a record for UK battery infrastructure.
As reported by Energía Estratégica, the contracts mark a step change from the typical 1–2 hour lithium-ion units that dominated the last auction round. Eighteen-hour storage changes the economics of the grid, and the household bill that sits on top of it.
How grid-scale batteries cut your electricity costs
Ofgem sets the price cap based on wholesale costs, network charges, and policy levies. Wholesale power makes up roughly 40% of the typical household bill, about £630 a year on a 12,000 kWh dual-fuel tariff at current rates. When the wind blows at 3am, wholesale prices can drop to zero or negative. When demand peaks at 5pm on a January weekday, prices spike to £200–£400 per MWh.
Grid-scale batteries arbitrage that gap. They charge when power is cheap (low-carbon and abundant) and discharge when it is expensive. The more duration they have, the more of the evening peak they can cover. Eighteen-hour batteries can start charging on Sunday night, discharge through Monday’s peak, and still have reserve for Tuesday morning. That flattens the price curve.
Ofgem’s own analysis, published in its 2024 wholesale cost review, estimates that every 1 GWh of additional storage capacity reduces annual wholesale costs by roughly £1.5 million. Scale that to 47.7 GWh and the effect is material, though it takes two to three years to flow through to the price cap.
What this means for your home battery and solar setup
The same logic applies at household level, but the economics are different. A typical 5 kWh home battery costs £4,000–£6,000 installed, according to Energy Saving Trust data. Paired with a 4 kW solar array (around £6,000), it can shift midday generation into the evening and reduce grid imports by 60–80%.
The catch: home batteries only make financial sense on a time-of-use tariff such as Octopus Flux or EDF GoElectric. These tariffs charge low rates (5–10p/kWh) overnight or at solar peak and high rates (30–40p/kWh) during the 4pm–7pm window. Without a smart tariff, the battery pays back in 10–15 years, too long for most households. With one, payback falls to 6–8 years.
Grid-scale storage does not directly change that maths. But it does make the grid greener and more stable, which reduces the risk that suppliers scrap time-of-use tariffs. It also lowers the wholesale price during peak hours, which narrows the spread between cheap and expensive periods. A narrower spread means less profit per cycle for the home battery owner.
EPC impact and the policy angle
Home batteries do not currently feature in the Standard Assessment Procedure (SAP) used for EPC ratings. Solar panels do, they add up to 10 points depending on orientation and size. A battery alone does not lift your EPC band from D to C. But a solar-plus-battery system can push a typical 1950s semi from EPC 58 (D) to 68 (C), saving £200–£400 a year on the bill and potentially adding 5% to the property’s value.
The government’s Clean Heat Market Mechanism, due to start in April 2025, will require boiler manufacturers to sell a rising share of heat pumps or buy credits. That will push up gas boiler prices and make electric heating, backed by cheap battery storage, more attractive. Grid-scale batteries are the backbone that makes that switch viable without crashing the grid.
What you should do now
Households considering solar and storage should wait for the October 2025 price cap announcement, which will incorporate the first savings from these new battery contracts. In the meantime, check if your supplier offers a smart export guarantee (SEG) rate above 15p/kWh, Octopus, EDF, and Ovo all do. If you already have solar, a battery can be retrofitted for £4,000–£5,000. If you are starting from scratch, a combined system costs £10,000–£12,000 and breaks even in 7–9 years on a good tariff.
Applications for the next capacity market auction open in March 2026. Homeowners should monitor Ofgem’s website for changes to time-of-use tariff rules, which could improve battery payback further. The 18-hour battery is not a household product, yet. But the grid it enables is the one your heat pump and EV will rely on.
Frequently Asked Questions
No. The savings from these contracts take 2–3 years to flow through to the price cap. But they reduce wholesale price volatility, which should lower the average tariff over time. Households on time-of-use tariffs will see benefits sooner, as the gap between peak and off-peak prices narrows.
Home batteries typically last 10–15 years and are sized for daily solar shifting, not multi-day backup. Current 5–10 kWh units are sufficient for most households. Longer-duration home batteries (12+ hours) exist but cost £12,000+ and are rarely cost-effective without a smart tariff. Buy now if you have solar and a time-of-use tariff; wait if you are on a flat rate.
