How we check our figures

Where do Worthwatt's figures come from?

Prices, rates, rules and product details come from primary sources: government departments and official statistics, regulators such as Ofgem, official schemes such as MCS, and manufacturers' own datasheets and warranties. We open and read each one; no figure is taken from a search snippet, an AI-generated search summary or another website's write-up of it. Where nothing official exists, we may use a trade body, a survey or a company's own figures, and we say which it is in the sentence. Every guide and section page ends with its sources, each with the date it was read.

How do we read a figure before using it?

We record what the figure actually describes, because the same number can mean different things. The table shows the details we keep for one figure the site uses, the battery prices from the Department for Energy Security and Net Zero (DESNZ) [1].

Two figures on different bases are never joined into one range, and when we compare them we say how they differ.

Our own calculations

When we multiply, combine or model figures, the result is labelled as our calculation, with its inputs, so you can check it or redo it with your own numbers. Worked examples use a described, hypothetical home and say so; we do not invent customers, case studies or reviews. If a reasonable change in one assumption, such as the price you pay for electricity or the battery price, could change whether something looks worth buying, we show the range (for example a break-even price) instead of a single verdict.

How the battery planner works out its answer

The battery planner compares a full year of electricity costs for your home on its current tariff, with and without a battery, and against the few tariff switches it models (a tied export rate, Octopus Go with an electric car, and a time-of-use tariff you enter). It is not a market comparison, and it says when its answer depends on a switch whose terms it cannot confirm for you. The calculation runs in your browser, and the figures you enter are not sent to us. Its main inputs and methods:

• Solar output: PVGIS 5.3, the European Commission's solar tool, averaged for five UK places, for the roof direction you choose [3].
• How much solar a home uses, with and without a battery: a simplified Worthwatt model of representative dull, average and bright days in each month, fitted to examples published in MCS 032:2025, the standard for these estimates under MCS's redeveloped installer scheme [2].
• How close that model is to MCS: MCS publishes 60 no-battery values, all for a home using 3,500 kWh a year; fitted to 57 of them, our model is within 2.5 per cent on average (at most 6.7 per cent). For MCS's three published battery examples, which were also used in the fit, it puts the share of the solar output used in the home at 71, 83 and 81 per cent, against MCS's 69, 88 and 82 per cent. Those figures describe the examples, not a measured accuracy for your home's savings. MCS describes its estimates as an average for similar homes, not a prediction for one property [2], and the same is true of ours. MCS has not reviewed or endorsed the planner or Worthwatt, and we do not reproduce its tables.
• What a battery adds: against those three battery examples, the model's extra saving was 1 to 18 per cent lower than the one MCS's figures give, so the hopeful case adds 20 per cent to the part of the saving that comes from storing solar. The same examples were used to fit the model, so they cannot show how far it is out for other homes, in either direction. The check is kept with the planner's source files.
• Electricity prices: your own, or Ofgem's price cap for the period shown [4].
• Battery prices without a quote: the DESNZ medians above [1].
• Charging from cheap overnight electricity: limited by the battery's charge power and the number of cheap hours. In the hopeful case the battery is filled as if the next day's sunshine were known; in the cautious case, for an average day of the month, with any electricity bought that the real day does not need counted as wasted. In our checks that difference was up to 13 per cent of the saving.
• A range, not one number: a cautious case (85 per cent round-trip efficiency, more varied sunshine, a 10-year life, and capacity falling in a straight line to 70 per cent after 10 years) and a hopeful case (90 per cent, the AC figure on the Enphase IQ Battery 5P datasheet [8], less varied sunshine, 12 years, no capacity loss, plus the 20 per cent above on stored solar). The 85 per cent and how much the sunshine varies are our own choices for showing the spread; 10 to 12 years is the Energy Saving Trust's typical battery life [7]. The capacity figure is the lower of the two 10-year minimums in the battery warranties on our sources list: Enphase guarantees 70 per cent of the battery's nameplate capacity after 10 years [5], Tesla 80 per cent [6].

If your answers fall outside the range MCS's method covers (1,500 to 6,000 kWh a year of electricity use, and of solar generation), or if an answer you were not sure of could change the result, the planner says what it needs instead of giving a verdict. Its calculations are run against a set of automated tests every time the site is built.

What checks happen before a page goes live?

Each guide and section page goes through two rounds of checks before it is published. The first is the writer's own: house style, wording and a checklist of the mistakes we have made before. The second is a separate fact-check, done apart from the writing, which traces:

• every headline figure and calculation back to the exact source it came from;
• every chart and table against the figures behind it;
• every legal, tax and eligibility statement against the official text, with its conditions;
• every recommendation, including whether one changed assumption would reverse it.

Anything the check finds is corrected or removed, and checked again. Every guide and section page on the site went through this before publication. We also keep a log of the mistakes our checks and reviewers catch, each labelled by type, such as a figure taken from the wrong group of homes or a condition dropped from a rule, so the next page is written against them.

How do we keep figures up to date?

Each guide and section page shows the date its figures were last checked against their sources. We change that date only after a real check, never to make a page look fresh. The date we give search engines as a page's last update is a separate one: it moves only when the page's content changes. Figures that go out of date, such as prices, tariffs and grants, are given with the period they apply to or the date we read them, so you can see how current they are. The planner also warns you when its default rates are more than 60 days old, or once the price cap period they come from has ended, and asks you to enter your own.

Spotted a mistake?

Please tell us through our contact page, with the page and the figure. We check it against the source. If it is wrong, we correct it, check the correction, and change the page's checked date.