Why your heating bill depends on the building, not the tariff

People shop hard for an energy tariff and barely look at the thing that determines most of the bill.

The tariff sets the price per unit. The building decides how many units you need. Between two London flats of the same size, the second number varies far more than the first — and unlike the tariff, you cannot switch it after you move in.

What the EPC actually says

Every property marketed for sale or rent in England must have an Energy Performance Certificate, rated A to G. It is a modelled estimate rather than a measurement: an assessor records the construction, insulation, glazing, heating system and controls, and software produces a rating and an estimated annual cost.

Two things follow from "modelled".

It does not know how you live. The estimate assumes standard occupancy and standard heating patterns. A household that works from home all winter and one that is out ten hours a day get the same certificate and very different bills.

It can be out of date. Certificates are valid for ten years. A flat that has had new windows since — or has had its boiler quietly fail into inefficiency — carries the old rating either way.

The things that actually drive the bill

In rough order of impact for London housing stock:

Whether the walls are insulated. London has a great deal of solid-wall Victorian and Edwardian housing, which loses heat far faster than cavity-wall construction and is expensive and disruptive to fix. This is the single biggest determinant and the hardest to change.

Glazing. Single glazing, or old double glazing with failed seals, in a period property is a large ongoing cost. In a conservation area, replacement may also be restricted.

Which floor you are on, and what surrounds you. A mid-floor flat with heated neighbours above, below and either side is cheap to heat almost regardless of its rating. A top-floor flat with an uninsulated roof, or a ground-floor flat over an unheated void, is the opposite. Two identical flats in one block can differ substantially on this alone.

The heating system and, critically, its controls. A modern boiler with a working thermostat and thermostatic radiator valves beats an efficient boiler with no controls. Electric-only heating, common in studio conversions, is usually the most expensive option per unit of heat.

Whether you control it at all. Some blocks have communal heating with a fixed service charge, meaning your usage and your bill are only loosely related.

What to ask at a viewing

• What is the EPC rating, and when was the certificate issued?
• What is the heating — gas boiler, electric, communal? If electric-only, expect a higher bill.
• Is it solid wall or cavity? Anything pre-1930s in London is likely solid wall.
• Which floors are above and below, and are they occupied and heated?
• Can I see a recent bill? The most reliable answer available. Current occupiers will often share one, and it beats every model.

Then, and only then, the tariff

Once you know the building, the tariff is worth ten minutes.

The specific thing to avoid is doing nothing: when you move in, you inherit whichever supplier serves the property, on their standard variable tariff, which is rarely the best available. That default is where a lot of people quietly sit for years.

Take meter readings on the day, photographed with a timestamp, at both properties. Then compare what is actually available rather than accepting the inherited default. It is a ten-minute job with a return that repeats every month.

Where the housing data helps, and where it stops

We hold housing tenure for every London borough — the split between owner-occupied, private rented and social rented households.

That is genuinely informative about what kind of place somewhere is. Bromley, Havering and Bexley are around two-thirds owner-occupied; Tower Hamlets, Hackney and Islington are closer to a quarter, with much larger private and social rented sectors.

But it does not tell you about the building fabric, and we would rather say so than imply otherwise. We hold no data on construction date, wall type, glazing or EPC ratings — those are properties of individual buildings, not of areas, and no borough average could capture them.

That is the honest limit here. Tenure mix tells you something real about a neighbourhood. The heating bill is a question about one specific flat, and the only way to answer it is the certificate, the recommendations page and a look at a recent bill.

The pattern, again

The recurring point across everything we write: area data narrows a search, and building-level facts decide it.

Crime, transport, schools and green space for every borough are on the borough pages. They will tell you whether an area is worth viewing in. They will not tell you whether the specific flat is cold.