Case Study  ·  Issue 7  ·  Tuesday 8 September 2026

Watts Happening

The UK energy newsletter that doesn't work for the industry

The Spark

On this customer's tariff, a unit of electricity costs 13.75p in the cheap windows and 42.03p at teatime. Three times more.

The whole point of a home battery is to buy power when it's cheap and use it when it's dear, so you never pay the 42p. Simple in theory.

Whether it works in practice comes down to a spec most buyers never think to check, and to whether the system was engineered properly or just sized to a sales target. This week, a real 2026 install, and how we made the calls.

This Week in Watts

How we actually design a heat pump and battery system, and why the tariff comes last

A little over 300 square metres, on the seafront in North Wales, built well enough that the heat-loss survey came back at 30 watts per square metre. That is low, and it matters: the house holds its heat.

Full disclosure: this was one of ours. I own the firm that fitted it, so weigh what follows accordingly.

Here is what most people get backwards. You do not design a system around a tariff. Tariffs change with little notice, and the customer often cannot even join one until the install is certified. So we design on the system's own merits first, from what hundreds of installs have taught us: the right heat pump for the measured heat loss, a battery sized to carry the house through the expensive hours, an inverter matched to what the property will actually draw. Then, once it is in and a tariff is chosen, we tune the settings to it. The design has to stand up whatever they land on.

Take the inverter. We fitted 3.68kW, where the trade reflex is to go bigger. Three reasons we didn't. Cost. Staying at 3.68kW keeps the job to a simple connection notification, where bigger means a full network application that can sit for weeks. And the one that matters most: we checked the manufacturer's figures for what the heat pumps draw in normal running, not the headline maximum, and that number sits comfortably inside what a 3.68kW unit delivers. It was sized to the house and the kit, not to a tariff. The biggest inverter is the easiest sale. It is rarely the question the maths is asking.

The tariff is the layer on top. This customer landed on Octopus Cosy: cheap power in three daily windows, a spike to 42p from four to seven. So we set the system to do its heavy lifting in the cheap windows, filling the battery and pushing the house past target, then coast through the peak on stored heat. On a house this slow to lose heat, the warmth is still there at seven. A dynamic, half-hourly tariff would be tuned differently again. Same hardware, different settings.

On 5 September the house drew 25.47 kWh, the battery took in 17.43 and gave back 15.15, and every large slug of grid import fell in a cheap window, not the dear one. That was a mild day, mind. The real test is a January evening, and we have told the customer as much. Change the tariff tomorrow and we change the settings, not the system.

The Bill Buster

Watts on the Market

Quick Watts

Plug-in solar

Plug-in solar became legal to buy and self-install in Great Britain on 27 August, capped at 800W. The catch buried under the headlines: one device per household, and the 3.68kW limit is cumulative, so an existing roof array can rule you out. The government legalised it without putting a savings figure on it.

(DESNZ plug-in solar spec · SI 2026/848 · in force 27 Aug 2026)

Price cap

The energy price cap rises 4% on 1 October to £1,723 a year for a typical dual-fuel direct-debit home, up £60. Only standard variable tariffs move; fixed deals are protected.

(Ofgem · price cap Oct–Dec 2026)

Heat pump grants

The Boiler Upgrade Scheme committed £166.2m of its £400m annual budget in the scheme year's first four months, on 17,073 applications in the quarter to July. The £9,000 off-grid uplift is driving it, and sitting on your hands risks the pot thinning before spring.

(Ofgem BUS Quarterly Report, Issue 17)