Plug-in solar and time-of-use tariffs

Why the tariff decides the saving

Start with the arithmetic, because it is simpler than it sounds. A plug-in kit does not earn you money. It stops you buying some electricity. The value of a kilowatt hour it generates is:

• The unit rate you would otherwise have paid at that moment — if you use the electricity as it is generated; or
• Nothing, or close to it — if you are out and the surplus goes to the grid, because a self-installed kit almost certainly cannot get an export tariff. See the Smart Export Guarantee.

Panels generate in the middle of the day, in a curve peaking somewhere around solar noon. That is the hard constraint. You cannot move generation to the evening without a battery, and batteries sit outside the plug-in route — see plug-in solar with battery storage.

So the question for any tariff is simply: what do you pay per kWh between about 10am and 4pm? That number, multiplied by the kWh you actually use at home in those hours, is the saving. Everything below is a different answer to that one question.

Flat-rate tariffs, including the price cap

The simplest case. One unit rate all day, so every kWh your kit generates and you use is worth exactly the same.

From 1 October to 31 December 2026, Ofgem’s cap puts the average electricity unit rate at 26.32p per kWh. On a capped tariff, every kWh you generate and use at home saves you 26.32p, whether it happens at 11am or 3pm.

This is what our calculator assumes, because it is the position most households are in — Ofgem says over 60% of domestic customers are protected by the cap. If you are on a flat-rate fixed deal instead, substitute your own unit rate; the method is identical.

Economy 7 and other two-rate tariffs

Economy 7 gives you about seven cheap hours overnight and a more expensive rate for the rest of the day. For a plug-in owner this has a counter-intuitive effect, and it is good news.

Your kit generates nothing during the cheap window. The cheap rate is in the middle of the night. Every kilowatt hour your panels produce is displacing electricity you would have bought at the day rate — which on a two-rate tariff is typically higher than a single flat rate, because the cheap night hours are paid for by dearer daytime ones.

So a kit on Economy 7 is usually worth more per generated kWh than the same kit on a flat tariff, not less. The catch is that the household probably chose Economy 7 for storage heating or an EV, and those loads are at night, when the panels are doing nothing. Look at your daytime consumption specifically — see your home’s daytime base load.

Smart time-of-use tariffs

This is where a plug-in kit can be worth noticeably less, and almost nobody mentions it.

Smart tariffs price electricity by half-hour or by band, following wholesale costs. On a sunny, windy day, those costs are lowest in the middle of the day — because of all the solar and wind on the grid. Some tariffs now have deliberately cheap daytime or “sunny hours” windows to encourage people to shift usage into them.

Your kit and the cheap window are competing for the same hours. If your tariff charges, say, an illustrative 12p per kWh between 11am and 3pm, then each kWh your panels save you is worth 12p rather than 26.32p — less than half. The panels have not changed. The value of what they displace has.

We have used 400 kWh of self-used generation purely to make the arithmetic visible — it is not a prediction for any kit. Put your own postcode and system into the calculator for a generation estimate, then multiply by your own daytime rate.

None of this makes a smart tariff a bad choice. It may be saving you far more on an EV or a heat pump than the kit loses. It does mean the two decisions interact, and a payback figure calculated at the cap rate will be optimistic if your daytime electricity is cheap. See the payback period.

What may change: Ofgem’s October 2026 call for input

On 12 October 2026 Ofgem opened a call for input, “Energy price cap: updating the time-of-use methodology”. It is open until 20 November 2026. Nothing has changed yet, and a call for input is a long way from a decision.

Ofgem’s reason is that the cap was designed for a different market: it says the cap “was set at a time when there were few time-of-use (ToU) offerings apart from Economy 7 (E7), no half-hourly settlement, and when wholesale prices were less influenced by short-term drivers like weather”. Market-wide half-hourly settlement changes that, and Ofgem points to the “10-12 GW of consumer-led flexibility capacity by 2030” the Clean Flexibility Roadmap estimates is needed.

Today, a multi-rate tariff is capped on the average: suppliers “are able to set their rates over different time bands, provided the average of those rates (weighted by demand in each time band) is equal to or below the multi-register cap level”. Ofgem sets out three broad options for changing that, in its own words:

• Option 1 — Simple ToU: “We set a two- or three-band ToU tariff with prescribed unit rates for each time band. This would be the only option for a customer defaulting onto a ToU tariff.”
• Option 2 — Type of use caps: “We set multiple ToU variants based on differentiated demand profiles relating to different LCTs (eg a general ToU cap and a separate EV variant).”
• Option 3 — Tariff specific assessment: Ofgem would set the pricing parameters, and “suppliers could then apply their own demand profile that reflects the group of customers they are designing a tariff for to generate a cap level specific to that group”.

For a plug-in owner the thing to watch is whether any of these ends up putting a prescribed price on midday electricity. A capped midday band would make the value of a plug-in kit more predictable; a low one would make it smaller. We will update this page when Ofgem responds, and we are not going to guess which way it goes.

What to do with this

1. Find your daytime unit rate, not your annual bill. It is on your statement or in your supplier’s app, and on a time-of-use tariff you need the rate for roughly 10am to 4pm.
2. Estimate what you would actually use in those hours, not what the panels make. A kit that generates while the house is empty saves very little — see self-consumption.
3. Multiply the two. That is your annual saving, near enough, and it is the number that should drive the decision.
4. Re-check if you switch tariff. Moving to a smart tariff with cheap daytime hours can quietly halve what your kit is worth.

We do not recommend tariffs, and nothing here is advice about which supplier to use.