A deep dive: How one home’s ZEB shifted 94% of heating to off-peak electricity

In 2025/26, a tepeo ZEB heat battery boiler used 8,776kWh of electricity to heat one rural UK home. Around 94% of the ZEBs recorded electricity consumption occurred during  off-peak tariff periods. The homeowner estimated a £638 electricity cost for space-heating over the period. 

That is a compelling result, but the detail is what makes this story really interesting. The homeowner has analysed almost 3 years of ZEB data while changing the heating system, adding solar panels and installing home battery storage. Here’s what the numbers show and where comparisons need care.

The home: An 85 m² chalet bungalow built in 1980, with insulated cavity walls, double glazing and 100 mm of roof insulation.

The switch: After the gas boiler failed, a tepeo ZEB was installed for space heating in October 2023. A Sunamp Thermino 210e and myenergi Eddi provide domestic hot water separately. 

The setup: The ZEB is installed in a lean-to attached to the house, rather than inside the heated living space. The home’s gas supply was disconnected in early 2024.

What did the ZEB cost to run?

The homeowner matched half-hourly ZEB electricity data to their Intelligent Octopus Go tariff periods to estimate these space-heating costs:

The first period begins with the ZEB’s October 2023 installation, so it is not a full July-to-July season. The home and tariff also changed over time; the rows are a record of this home’s experience, rather than a controlled comparison. These figures cover the ZEB’s space heating, not hot water, EV charging of the total household electricity bill.

How much electricity was shifted off-peak?

During the winter of 2025/26, the ZEB used 8,235 kWh during off-peak hours and 541 kWh during on-peak hours. That means about 94% of its electricity consumption occurred in cheaper tariff periods. Meanwhile, the homeowner’s data shows that around 83% of the heat was delivered during on-peak hours.

That’s the practical value of thermal storage: the ZEB can consume electricity when the tariff is cheaper and supply heat later, when the house needs it. In January 2026, the month with the highest heating demandat 1,689 kWh, the homeowner recorded around 11% of electricity use on-peak.

The average tariff price assigned to ZEB electricity was 7.8p/kWh in 2025/26, down from 11p/kWh in 2023/24. Tariff changes, the home’s other technologies and the way the system was used all matter here, so the reduction should not be attributed to a single upgrade.

What does the home’s heat loss tell us?

The homeowner compared delivered heat with local weather using heating degree days, calculated against a 15.5°C baseline over 1,806 days. Their figures suggest that, below about 15.5°C, the home needs roughly 3.6 kWh more heat per day for every 1°C drop in average outdoor temperature. This helps them estimate when a full off-peak ZEB charge will be enough to heat the home.

Using this figure, they calculated that a full off-peak charge should usually cover a day when the average outdoor temperature is above roughly 4.5°C. Colder days may require some on-peak charging. This is an estimate for this particular home, rather than a rule for every ZEB.
How does that compare with other electric heating?

The homeowner modelled the same 2025/26 space-heating requirement with a direct electric boiler. They estimated a cost of £1,522 on the same tariff, assuming electricity was used when heat was needed, or £1,675 at the price-cap unit rates used in their analysis. Compared with the ZEB’s estimated £683, the same-tariff model was £839 higher. The separate price-cap scenario was £992 higher; that comparison reflects both the heating setup and the different tariff assumptions.

They also explored an air-to-water heat pump. On their tariff and usage assumptions, a seasonal performance factor of around 2.2 would match the ZEB’s cost. At an assumed factor of approximately 2.8, they modelled £541 for the year; at 3.2, approximately £475. The Electrification of Heat demonstration project reported a median seasonal performance factor of 2.80 for monitored air-source heat pumps, but that does not predict what a heat pump would achieve in this particular home.

These are illustrative running cost comparisons, not installed quotes or measured performance from alternative systems. Upfront cost, available space, installation work and tariff choice would all shape a real decision.

What does this mean for other homeowners?

This home shows why heat battery running costs depend on more than total electricity consumption. When the ZEB charges can make a substantial difference, because stored heat can be delivered after the cheaper electricity period has ended.

The tariff is a big part of that story. The homeowner uses Intelligent Octopus Go, which gives the whole home access to cheaper electricity during its off-peak hours. For someone also electrifying their car, an eligible EV and smart tariff could make those hours useful for both driving and heating. Solar panels and a home battery add further options: generating electricity during the day, storing it, or charging the battery when grid electricity is cheaper.

For this homeowner, the ZEB’s estimated space-heating electricity cost was £683 in 2025/26. It’s a useful example of what can happen when heating is planned alongside the rest of a home’s energy use, while the exact cost will depend on the home, its equipment and tariff.

Considering a ZEB for your own home?

The upfront cost matters alongside the running costs. Eligible customers can currently get £2,500 off the ZEB through tepeo’s Early Grant Match, reducing its product price from £6,000 to £3,500. Installation and any additional work are quoted separately.

Complete our Home Check for an initial indication of whether a ZEB could suit your home. If it looks like a good fit, book a conversation with a tepeo expert to discuss your heating needs and next steps.