How we check the rules and work out the savings
Two things on this site need explaining: how we decide a rule is true, and how we get from a kit size to a pounds-or-dollars figure.
Page last changed 13 Oct 2026
How we verify a rule
Plug-in solar law is new and moving. A bill that was not law last month may be law now, and news stories do not always keep up. So we do not publish a rule from one source.
- First read, from the primary source. That means the bill page on the legislature's own site, the statute, the statutory instrument on legislation.gov.uk, or the regulator's decision. We note the address, the exact wording or value, and the day we read it.
- Second read, by a different route. Another official page, a separate bill tracker, a governor's press release, or a news report that names the bill number. Reading the same page twice does not count.
- Compare. If both reads agree, the claim is marked verified. If they disagree, we go back to the primary source to find out which is right, then check again. If we cannot finish both reads, the claim stays off the site, or we label it as unconfirmed.
Every field gets this treatment: the bill number, whether it has been signed, the effective date, the watt limit, whether you have to tell the utility, and what certification the device needs. Not a sample of them. All of them.
When sources disagree
It happens more than you would expect. Trackers have given us different vote counts for the same Utah and Virginia bills, and a search snippet once gave Maryland the wrong start date. When the disagreement is about something that does not change what you can do, such as a vote tally, we leave the tally off the page and note the conflict in our log. When it does change what you can do, we do not publish the claim until the primary source settles it.
The status labels
Each state on the plug-in solar laws by state page gets one of five labels, and each label comes with an icon and a word, never just a colour.
| Label | What it means here |
|---|---|
| Legal | A law is in force and covers plug-in devices. |
| Passed, rules pending | A law has been signed but has not taken effect yet, or it is waiting on a regulator to set the details. |
| Bill in progress | A bill exists and has not become law. Passing one chamber does not move it out of this group. |
| No bill | We have not found a bill for plug-in solar in that state. |
| Failed | A bill was voted down or has died for the session. |
How fresh is it?
Every rule carries a "last verified" date. Our aim is to re-check every legal status at least once a week, and prices at least once a month. If you see a date older than that on a page, take it as a sign that we are behind, not that nothing has changed.
How the savings calculator works
The sum has three steps, and you can see all of them on the UK plug-in solar savings calculator.
- How much does the kit make in a year? We take the yearly output for your location, panel angle and direction from a public solar database, then scale it to your kit size.
- How much of that do you use? We have not found a scheme that pays you for power you do not use, so we only count the share your home actually consumes. This is the number you should argue with most. See below.
- What is that worth? The kWh you use, multiplied by what you pay per kWh.
In short: saving = yearly kWh × share you use × price per kWh, and payback = kit price ÷ yearly saving.
Where the numbers come from
| Input | Source | Notes |
|---|---|---|
| UK yearly output | PVGIS v5.3 from the European Commission's Joint Research Centre, PVGIS-SARAH3 data | Pulled on 21 Sep 2026 for 12 UK cities, with 14% system losses, in a 0.8 kW system and scaled from there. |
| UK electricity price | Ofgem price cap, 1 Oct to 31 Dec 2026 | 26.32p per kWh for electricity on Direct Debit, England, Scotland and Wales average. You can change it to your own tariff. |
| US yearly output | NREL PVWatts | The US calculator is not live yet. This is what it will use. |
| US electricity price | EIA average residential price by state | Same: the US calculator is not live yet. |
The parts that are our assumptions
These are not facts. They are round numbers we chose, and the calculator shows them so you can change them.
- You use 80% of what the panel makes. This is a guess. Someone who works from home might use nearly all of it. Someone out from 7am to 6pm might use half. We know of no measured average for plug-in kits, so we do not pretend to have one.
- Light shade costs 10% of output, heavy shade costs 25%. Real shading varies hour to hour. These are rough allowances.
- Prices are a snapshot. The kit price in the calculator is what you type in. Where we show prices on a page, we give the date we saw them.
- No payment for exported power. We have not found a scheme that pays for it, so the calculator values nothing that leaves your home. If we find one, the sum changes and we will say so.
What the calculator leaves out
It does not include mounting costs beyond what you type into the price field, the cost of an electrician if you need one, panel ageing, or changes in future electricity prices. It also does not estimate carbon savings yet, because we have not verified a grid emissions figure we are happy to publish.
Limits of all this
Two reads are better than one, but they are not a legal opinion. Official pages sometimes lag behind the actual law. Bill text can differ from the version that was signed. If your decision involves real money or your own wiring, use our pages to find the source and then read it yourself. And if you think we have got something wrong, tell us through the contact page. Fixes are listed on the corrections log.