How to read the calendar
Twelve rows for the months, twenty four columns for the hours of the day. Each square is the average temperature in the room for that hour across the month, worked out from 2024 and 2025. Pale squares are comfortable. Blue squares are heater weather, which a heater can fix. Amber, berry and plum squares are the hours when the room is warmer than 25°C, which nothing cheap can fix.
The model does not heat the room, so in winter both versions sit in the blue. That is honest: the panel roof stops the room cooking far more than it stops it cooling. Where it pays in winter is the heater, explained in how much heat goes out through the roof.
What the numbers say across England
Under polycarbonate, the average town gives 1,320 too-warm waking hours a year. Under panels: 317, a cut of 76%. The hottest hour of the year drops from 47°C to 34°C. Hounslow tops the league table; Buxton has the fewest hot hours.
What it leaves out
Blinds, trees, a neighbour's extension, a fan, a dog that insists on the sunny spot: none of these is in the model, and all of them change your room. The full list of assumptions is on how we model it. If your room is shaded or faces north, choose north: it is the nearest match.