Our Sources and Data Log: Every Dataset, Licence and Date

Every Dataset We Use, in One Table

This table lists datasets and documents behind figures in the calculator, with the owner, the use and the date we hold for each. Each estimate is built from this data and the other inputs named in our methodology, by the formulas set out there.

A row marked "Not used" sets no figure in the calculator. The rule documents we hold for grant and plug-in solar pages are in a second table further down.

Credits: Contains data from SmartMeter Energy Consumption Data in London Households, UK Power Networks (Low Carbon London), London Datastore, licensed under CC BY 4.0. The data was cleaned, grouped and summarised by us. Contains OS data © Crown copyright and database right 2026. Contains Royal Mail data © Royal Mail copyright and database right 2026. Contains National Statistics data © Crown copyright and database right 2026. PVGIS © European Communities, 2001-2026.

Figures That Changed This Month

On 11 November 2026 the calculator moved its installed cost to the DESNZ domestic retrofit mean, per kWp of panels fitted (total installed capacity), and began to price a battery on its label size. The same day it began to take its yield from our PVGIS runs for each postcode area and its self-use shares from our model on the London smart meter data. On 12 November 2026 the calculators began to open on a worked example of 9 panels of 475 W, in place of a 4 kWp example. On 14 November 2026 the battery verdict rule changed: a battery is a Good fit only when it pays back in 5 years or less.

Installed Cost Data: DESNZ Statistics and the MCS Data Dashboard

Our installed prices come from two DESNZ statistical series built from the MCS Installation Database: one for solar panels, one for home batteries. DESNZ publishes both. MCS is where the records start, because installers register each system there. MCS does not certify this site. A third source, the MCS Data Dashboard, is listed here because we do not use it.

DESNZ Solar PV Cost Data

Owner and title

Department for Energy Security and Net Zero, Solar PV cost data.

What it measures

The cost per kW of installed solar PV, by month and by size band, from the MCS Installation Database. It covers installs in England, Scotland and Wales.

Basis

We use the cost per kWp of panels fitted (total installed capacity). DESNZ also reports a declared net capacity (DNC) basis, and its figures on the total installed capacity (TIC) basis run lower than on the DNC basis.

Release cycle

Once a year. The latest release is dated 28 May 2026, and the next is due on 27 May 2027.

How we use it

The domestic retrofit mean cost per kWp of panels fitted (total installed capacity) for each size band prices a system. For a system of up to 4 kWp that mean is £2,109 per kWp for April 2025 to March 2026.

Licence

Open Government Licence v3.0, Crown copyright. Credit: DESNZ, Solar PV cost data, published 28 May 2026.

Limits

The data can be up to about 14 months old when we use it. It has no rows for Northern Ireland.

DESNZ Statistics on MCS-Certified Home Battery Installations

Owner and title

Department for Energy Security and Net Zero, MCS certified domestic battery installation statistics.

What it measures

Batteries registered with MCS and fitted in existing UK homes, from the MCS Installation Database. New builds and commercial sites are left out, and so is any battery not registered with MCS. DESNZ labels the series "official statistics in development".

Release cycle

First published on 29 May 2025 and last updated on 28 May 2026. The release covers April 2024 to March 2026.

How we use it

A battery is priced as the kWh on its label x the median price per kWh of nominal capacity for its size band, for April 2025 to March 2026, and never below a battery at the top edge of the band beneath. The calculator uses the three band medians from this release, checked at source.

Licence

Open Government Licence v3.0, Crown copyright

MCS Data Dashboard: Not Used Yet

We do not use the MCS Data Dashboard for any figure. The dashboard is open to anyone, is refreshed every 24 hours and offers exports. We have not read its reuse terms, and we have not tested an export. Until both are done, no "this month" cost from it appears on this site.

Generation Data: MCS Tables, PVGIS and Postcodes

We estimate generation with the form of the MCS method: system size in kWp x a yearly yield figure x a shade factor. The yield comes from our PVGIS runs for your postcode area; with no postcode, one stand-in figure is used. Your postcode sets Great Britain or Northern Ireland rates and the postcode area of the yield. The point we ran for each postcode area in Great Britain comes from OS Code-Point Open; Northern Ireland uses one Belfast point.

MCS Generation Tables (Kk Values)

The MCS method gives yearly output as kWp x Kk x a shade factor, where Kk depends on postcode zone, roof direction and pitch. It comes from MIS 3002, the MCS standard for solar PV systems, and the MCS PV guide. Our calculator uses the same form of sum. It does not use the Kk tables, which would need permission from MCS.

PVGIS (European Commission Joint Research Centre)

PVGIS is the solar output tool of the European Commission Joint Research Centre. Its SARAH3 and ERA5 databases cover the UK. We ran PVGIS 5.3 ourselves on 11 November 2026, and the calculator's yield by postcode area and its direction and pitch shares come from those runs.

Energy Prices, Use and Export Rates: Ofgem and Suppliers

Savings use the Ofgem price cap for the current quarter, Ofgem's typical use figures and an export rate worked out from Ofgem's own report, each with its date. A home in Northern Ireland gets Power NI rates, because the cap and the Smart Export Guarantee do not reach it.

Ofgem Energy Price Cap

Ofgem sets the energy price cap every three months, with changes on 1 January, 1 April, 1 July and 1 October. Our default unit rate is 26.32p per kWh and the standing charge is 54.83p a day, for 1 October to 31 December 2026. Both are Ofgem's Direct Debit averages from its page "Energy price cap unit rates and standing charges", which we checked on 8 November 2026. The cap covers England, Scotland and Wales. Licence: Crown copyright, Open Government Licence, under Ofgem's copyright page, which gives no version.

Ofgem Typical Domestic Consumption Values

When you do not enter your own use or the number of bedrooms, the calculator takes 2,500 kWh a year. That is Ofgem's medium value for electricity, in force since 1 July 2026. The low value is 1,600 kWh and the high value is 3,800 kWh. All three come from Table 1 of Ofgem's "Review of typical domestic consumption values: decision", published on 27 May 2026. Licence: Open Government Licence for the text, credit Crown copyright and the document title. When you give the number of bedrooms, the calculator takes the DESNZ NEED median for that number instead, listed in the table above.

Export Rates: the Smart Export Guarantee and Supplier Tariffs

Our default export rate is 13.00p per kWh, our calculation from Ofgem figures. The figures are in Ofgem's Smart Export Guarantee (SEG) Annual Report, SEG Year 5, published on 5 December 2025 for 1 April 2024 to 31 March 2025: the total paid to SEG installations and the total they exported. We divide one by the other. Your own rate replaces it when you type one. Ofgem lets the report's text be reused under the Open Government Licence, with a credit to Crown copyright and the document title.

The SEG rules are Ofgem's, and its SEG: Guidance for Generators, dated December 2019, sets out the certification and metering conditions. The SEG began on 1 January 2020. Suppliers with 150,000 or more domestic customers must offer a rate above zero. Solar of up to 50 kW needs MCS certification or an equivalent, and an export meter with an export MPAN. The SEG covers Great Britain only. It is not the Feed-in Tariff, which closed to new applicants on 1 April 2019.

Northern Ireland Electricity and Export Rates

A Northern Ireland postcode uses Power NI's own published rates. The unit rate is 34.15p per kWh, from Power NI's "Current electricity prices" page. The export rate is 11.88p per kWh for 1 October 2026 to 30 September 2027, from its "Tariff rates" page for small generators. We read both pages on 8 November 2026. Licence: No open licence: the Power NI site prints "© All Rights Reserved". We quote its rates as facts, with the date we read them. Licence check: 14 November 2026.

Tax, Grants and Rules: HMRC, gov.uk, Ofgem and the ENA

Dates and rules come from the body that sets them: HMRC for VAT, government guidance for grants and loans, and the network operators' own standard for grid rules. The table lists six rule documents we hold.

Licence: Open Government Licence v3.0 for all six documents, under Crown copyright (for SI 2026/848, Crown and database right). Licence check: 14 November 2026.

VAT on Solar Panels and Batteries

Home solar panels and batteries carry 0% VAT until 31 March 2027 and 5% from 1 April 2027. The source is HMRC's "Energy-saving materials and heating equipment (VAT Notice 708/6)" on gov.uk, which we read on 8 November 2026. The calculator shows your installed cost at both rates.

Grants, Loans and Schemes

This page lists three scheme documents, and none of them feeds the calculator. The Warm Homes: Local Grant covers England and runs to 31 March 2028. It is for households with an income of £36,000 or less, a means-tested benefit or a qualifying postcode, in homes rated EPC D to G. The Warm Homes Loan Scheme is a loan, not a grant: it lends up to £15,000 for solar and £15,000 for a battery, and it needs an MCS installer. ECO4 ends for new measures on 31 December 2026. We explain loans and do not arrange credit.

Grid Connection and Plug-In Solar Rules

The Energy Networks Association's Engineering Recommendation G98 Issue 2 covers a system of up to 16 A per phase, which is 3.68 kW on a single-phase supply. Above that, the G99 process applies. The calculator uses that limit for one note on larger systems. Plug-in solar of up to 800 W has been legal in Great Britain since 27 August 2026, under SI 2026/848. It is not yet permitted in Northern Ireland.

Product Data and the Carbon Factor

Two calculator figures rest on makers' own documents. The battery round-trip efficiency of 89% is our rule: the lower of two makers' UK datasheet figures, for the Tesla Powerwall 3 and the Enphase IQ Battery 5P. The battery verdict uses a life of 10 years: a warranty length, not a measured life, the shortest in the makers' terms we read. The default panel wattage of 475 W is our rule too: the median of the panel wattages four energy suppliers name for their own solar packages. A carbon factor is held, though the calculator does not use it.

DESNZ Greenhouse Gas Conversion Factors

DESNZ published "Greenhouse gas reporting: conversion factors 2026" on 11 June 2026. We hold its UK electricity factor of 0.13096 kg CO2e per kWh. The calculator has no carbon line, so no calculator uses that factor today. Licence: Open Government Licence v3.0, Crown copyright.

Tables We Publish and How You Can Reuse Them

Our cost pages carry tables of our own; this log does not list them yet. The licence for reusing our tables is not decided, so until it is, ask before you reuse one. Our planned page on how to cite our data will hold the wording. Data that belongs to DESNZ, Ofgem or anyone else stays under its owner's licence.

Other Organisations We Cite, and How We Label Them

We hold facts from the Energy Saving Trust for use as an outside benchmark, each with the date shown on its page. When a page quotes one, the page names the Energy Saving Trust page and its date beside the fact. Four of its pages in our records are:

• "Solar panels", last updated 1 October 2026.
• "Rooftop solar panel installation: step-by-step guide", last updated 1 October 2026.
• "Best tariffs for renewable technologies", last updated 19 October 2026.
• "Solar water heating", dated 26 October 2026.

We quote short facts and name the page.

Data We Do Not Use, and Why

Some data is easy to find but not good enough to put a price on your roof. Five kinds stay out:

• A "this month" cost from the MCS Data Dashboard, which we have not exported or checked.
• A cost per kW quoted by another site that we have not checked at its source.
• Other sites' modelled savings and payback.
• Lender rates and finance offers: the calculator uses none.
• Reviews, star ratings and user counts: the calculator uses none.

Leads 2 Trade, the company we earn from by passing it survey leads, supplies none of the data in this log. The calculator has no input for who pays us.

How We Keep This Log Up to Date

The price cap changes every quarter and DESNZ cost data is published once a year. The table gives when each of four kinds of source changes.

Report a Problem With Our Data

If you think a figure or a date is wrong, tell us. Email editorial@solarpanelcostcalculator.uk with the page address, the figure and the source you think is right. We reply within 2 working days.