What happens to the power you do not use
Panels feed the house first. Whatever the fridge, the router, the kettle and anything else switched on are drawing at that moment comes from the roof, and only the surplus passes through the meter and out to Northern Powergrid's local network. Nobody flicks a switch; the flow sorts itself out second by second. Every kWh used in the house as it is made is a kWh you do not have to buy. Every kWh that leaves is paid for at whatever rate your export tariff sets.
In a Leeds home those two streams move in opposite directions through the year. A south-facing kW in the city centre produces about 118 kWh in May and just 32 in December (PVGIS). Summer middays produce far more than most houses can use there and then, so a large part leaves for the grid. On short winter days the house usually swallows almost everything the roof makes, and from November to February a south-facing kW produces only 157 kWh in total.
The rules every export offer works within
The framework is set by Ofgem. Larger electricity suppliers have to offer an export tariff, and the rate must be above zero, but each supplier chooses its own rate and terms (Ofgem). To qualify, a home system has to be MCS-certified and no bigger than 50 kW, with its exported electricity measured on a meter, which in practice usually means a smart meter (GOV.UK). You are not tied to the company that supplies your electricity: you can choose another supplier's export offer, subject to its terms.
On Ofgem's pages and in suppliers' small print, the scheme appears under its formal title, the Smart Export Guarantee, often shortened to SEG. On this site we simply call it the export rules or the export tariff.
Smaller suppliers may offer export tariffs too, though the requirement to do so falls on the larger ones. Because the rate is set by the market rather than by the rules, it can differ widely between companies and change over time, which is why this page gives no rates at all. Any figure quoted to you should carry the date it applies from. Nothing in the rules depends on where in Leeds you live, and Northern Powergrid, which runs the local network, plays no part in paying for exports.
Comparing export offers
Offers that look alike on the headline rate can differ a good deal underneath. Line these up side by side before choosing:
- The rate. Pence per kWh exported, and whether it is fixed for a period or can move with notice.
- The term. How long the offer runs, what happens when it ends, and how much notice the supplier gives before changing it.
- Any tie to an import tariff. Certain export rates come only as part of a package with that company's import tariff. In that case judge the two tariffs as a pair: a strong export rate is less use if the import price that comes with it is high.
- The admin. How the supplier reads the export meter, how often it settles the account, and what it needs from you to start.
Nothing about the first choice has to be permanent. Subject to the terms of the offer you are on, you can move your exports to another supplier later, much as you would switch an import tariff, so read the exit conditions before signing as well as the rate.
A battery changes the comparison. If you store daytime surplus for the evening, less leaves the house, so the export rate matters a little less and the import price a little more. The battery guide covers sizes and how the network counts them, and the funding page sets out the other help open to Leeds households.
A worked example in kilowatt-hours
Take a 3 kW array on a south-facing roof at 35° in central Leeds. PVGIS models one kW there at 952 kWh a year, so 3 kW give about 2,856 kWh (PVGIS; our arithmetic). Ofgem's typical figure for a household's yearly electricity use is 2,500 kWh (Ofgem). On paper the roof makes about 356 kWh more than the house needs. In practice the two never line up hour by hour, and how much ends up exported depends on how much of the output the household uses as it is generated.
That share differs from one household to the next, so the table shows two illustrations: one where a quarter of the output is used in the house and one where half is. They are assumptions for the arithmetic, not measured figures. A household that is out all day will sit nearer the first; one with someone at home, or appliances set to run in daylight, nearer the second. Roof direction nudges the share as well: an east roof peaks while a household is getting up and a west roof as people come home, so either can line up with home use better than its yearly total suggests.
The seasons pull in different directions too. Spread evenly, Ofgem's typical yearly figure comes to about 208 kWh a month on our arithmetic, though real use tends to run higher in winter, with longer evenings of lighting. In May the 3 kW array makes about 354 kWh, well above that monthly average; in December it makes about 96, less than half of it.
| Roof faces | Output a year | Exported if a quarter is used at home | Exported if half is used at home |
|---|---|---|---|
| South | 2,856 kWh | 2,142 kWh | 1,428 kWh |
| South-east | 2,694 kWh | 2,021 kWh | 1,347 kWh |
| South-west | 2,664 kWh | 1,998 kWh | 1,332 kWh |
| East | 2,259 kWh | 1,694 kWh | 1,130 kWh |
| West | 2,220 kWh | 1,665 kWh | 1,110 kWh |
Two things stand out. Even on a west roof, a 3 kW array makes about 2,220 kWh a year, and under either illustration more than a thousand kWh of it leaves the house. And on a south roof the swing between the two illustrations is larger than the whole difference in yearly output between a south roof and a west one, which is why daily habits can matter as much as orientation to how much is exported. The roof picker gives the yearly figure for other directions and pitches.
Using more of it in daylight
Because a kWh used at home is one not bought, the sensible order is simple: use it first, store it second, export what is left. The practical changes are unglamorous. Run the washing machine or dishwasher on a timer around midday, charge laptops, tools and e-bikes in the afternoon, and, where the system allows it, send surplus to the hot water cylinder. None of that needs new equipment beyond a timer or an app setting.
Bigger loads shift the picture further. An electric car charged at home in the middle of the day, or a heat pump running through the afternoon, can soak up a large part of a summer surplus that would otherwise be exported. Neither is a reason to buy one, but where a household already has them, it makes sense to set their timings with the roof in mind.
In winter it matters much less. With the roof producing a fraction of its summer output, most of what it makes is likely to be used in the house anyway, and the export meter moves far less. The months to think about are April to September, when long Leeds evenings and early mornings stretch the generating day either side of noon.
What to have ready
Before applying for an export tariff, gather the MCS certificate for the system, which you can cross-check against the installer's entry on the MCS register, the date the system was commissioned, and details of the meter that records exports. The installer's notification to Northern Powergrid is a separate step; the grid connection guide explains it.
If you rent, settle in writing who holds the export tariff, the landlord or the account holder, before the system goes live; the renting guide covers the conversation. And if you are still deciding whether panels make sense at all, the cost of solar in Leeds sets out what systems of different sizes have cost in Yorkshire and the Humber, while the installer checklist covers the checks worth making before signing.