Parliament is costing a £3bn restoration. We measured its roof in about a second.

In February, Parliament published the costed proposals for fixing the building it sits in. Phase one alone runs to seven years and is capped at £3 billion, and phase one is not the restoration. It is the enabling works: temporary Chambers, a fit-out of the QEII Conference Centre, early work inside the Victoria Tower, underground construction. The full job, on the two options still standing, lands somewhere between £8.4 billion over 19 years and £18.7 billion over 61. In June the National Audit Office called the programme "at a critical stage".

So we did the one thing nobody has to spend seven years on. We measured the roof.

17,500 sq m, 2.4 MW, 765 homes

We ran the same aerial read we run for homeowners, the one that measures your roof from above before an installer ever climbs a ladder, across the Palace of Westminster. The full measurement is here.

• About 17,500 sq m of pitched and turreted roof along the Thames
• An estimated 2.4 MW of solar capacity, near enough 6,000 panels
• Roughly 1.9 to 2.3 million kWh a year
• The yearly electricity of about 765 homes

Those are Solarable's own figures, read from Google's aerial solar data at high imagery quality. They are an estimate of what the roof could physically carry, not a planning application, and any real scheme on a building like this would live or die on a heritage survey rather than on our numbers.

The "you can't do that to a listed building" argument is already over

Here is the part that tends to end the conversation early. Parliament has already put solar on the Palace of Westminster.

Photovoltaic cells were installed on the Palace's flat roof areas as part of the cast iron roofs project, and Millbank House on the estate uses solar thermal panels to pre-heat hot water. Not a pilot, not a proposal. Done, on a Grade I listed building inside a World Heritage Site.

That cast iron roofs project deserves its own moment. It ran for about a decade, concluded in January 2022, cost £80 million, and covered more than 30 individual roofs first laid in the 1850s. Over 90 per cent of Barry's original fireproof cast iron tiles were kept, each one weighing something like 65 kg, lifted off, sent to a workshop in Sheffield, stripped back to bare metal, assessed, repaired and put back where it came from. On time and on budget, while the rest of the programme argued about billions.

If the estate can take 30 Victorian roofs apart tile by tile and put them back, the question was never whether difficult roofs are possible.

Why we bother measuring buildings nobody will let us touch

Because the interesting number is almost never the one being discussed.

Parliament's restoration is measured in decades and billions, and the roof above it is measured in nobody's spreadsheet at all. Solar on the Palace is not going to be what fixes that building. But 17,500 sq m sitting in plain sight, on an estate that has already proved it can do the delicate version of this work, is worth someone saying out loud.

The same is true of the roof you actually own. Most people find out what their roof can do at the end of the process, from whoever is selling them the system. We think that is backwards, so we put the measurement first and let you decide what to do with it.

Parliament will take until at least 2032 to move out. Your roof takes about a minute.