Gas smart meter data analyser: methodology
Where your data goes
Nowhere. The page reads your file with your browser’s own file reader and does every calculation in the tab. There is no upload and nothing is stored when you close the page. The privacy policy says the same.
Reading the file
The file is read by the same parser as the electricity analyser: any common CSV layout, the timestamp and usage columns found by their contents, clock changes placed correctly, register readings turned into usage, and every guess reported so you can correct it. Only complete days, with at least 95% of their readings, count towards the results. Gas readings often arrive a day or two late, so the last day of a download is often incomplete and left out.
Cubic metres to kWh
Most smart gas meters measure volume. Bills turn it into kWh with a formula set in law, and this page uses the same one (GOV.UK):
kWh = m³ × 1.02264 × calorific value ÷ 3.6The 1.02264 corrects for temperature and pressure. The calorific value is the energy in each cubic metre, in MJ/m³; the network must keep it between 38 and 41, and your bill gives the figure it used. We start with 39.5, which makes a cubic metre 11.22 kWh, and you can enter your own. Older imperial meters measure cubic feet, converted at 0.0283 m³ each. A file already in kWh is used as it is; a file that doesn’t say is read as kWh, and the page warns you.
Finding the heating
A meter can’t tell heating from hot water, but their shapes differ. Heating burns steadily for hours; a shower is a burst of 20 to 30 kW for a few minutes. So an interval counts as heating when the boiler burned at least 0.8 kW on average above the always-on floor (below), for 60 minutes or more in a row. A combi on low fire runs at 4 to 6 kW and cycles on and off, so 0.8 kW averaged over a half-hour is a boiler genuinely running.
The hours a day are the average of those intervals across your complete days. The times shown on the chart are the intervals where the heating ran on more than half of your days. Two things blur the line: a long bath can look like an hour of heating, and heating on a very low setting may not reach the threshold. Neither changes the picture much over a week or more.
Overnight heating
We add up the heating gas from 23:00 to 06:00 on each day, less the always-on floor, and average it over all your complete days. To put a yearly figure on it, the same seasonal weights as the yearly estimate (below) scale it up, so a cold January night counts for more than a mild October one. That figure answers “what would nights like mine cost over a year”, so it is only as representative as the nights in your file.
The always-on floor
The floor is what your home burns with the heating off and everyone asleep. For each night we average the readings from 23:00 to 06:00 that aren’t heating, and take the median across nights so that one odd night doesn’t decide it. If the heating ran every night, we use the median of every reading that isn’t heating instead. We used to take each day’s single lowest reading; that understates anything that fires in bursts, which uses gas in some half-hours and not others.
A modern combi with the heating off burns nothing between uses, so the floor should be close to zero. Above 0.1 kW, about 876 kWh a year, the page flags it, and says which kind it looks like. A pilot light burns at a steady rate, so its half-hours differ only by the meter’s rounding. A boiler keeping its water hot fires every so often, so some half-hours have twice the gas of others. When the quiet readings vary by more than 25% of their average (their standard deviation over their mean), we call it bursts and point to the keep-hot setting; otherwise we point to a pilot light. It is a pointer, not a diagnosis. If the heating runs nearly all day, the floor may be the heating itself, and the page says so. If anyone can smell gas, the advice is always to leave and call 0800 111 999 (0800 002 001 in Northern Ireland).
A year from part of a year
With 12 months of data the yearly figure is measured. With less, each day is weighted by the share of a year’s gas a typical gas-heated home uses on a day in that month, so a winter download isn’t multiplied up as if every day were winter. These shares are our approximation, not a published series; the nearest published check is National Gas’s distribution-network demand, which swings about four to one between January and August. Every home differs: a house that uses a lot of hot water swings less, a draughty one more. With under a week of data the page shows no yearly figure at all.
One part of your gas doesn’t follow the weather at all: the always-on floor (below). That part is counted at the same rate every day of the year, and only the rest is weighted by month. Otherwise an autumn download from a home with a large constant draw would be multiplied up as if the draw were heating, and overstate the year.
| Month | Share of a year’s gas |
|---|---|
| January | 16.2% |
| February | 14.1% |
| March | 12.1% |
| April | 8.1% |
| May | 5.1% |
| June | 3.0% |
| July | 2.5% |
| August | 2.5% |
| September | 3.5% |
| October | 7.1% |
| November | 11.1% |
| December | 14.6% |
Against a typical home
The comparison is with Ofgem’s typical consumption values, revised in July 2026: a typical home uses 9,500 kWh of gas a year and a high-use home 14,000 kWh (Ofgem). With less than a week of data, your days are compared with a typical home’s on the same days of the year, using the same seasonal weights; with more, the yearly estimate is compared with Ofgem’s yearly figure. Ofgem’s typical home is an average, not a target: a family of five in a large house will use more without anything being wrong.
Heating against hot water
With at least two weeks of June to August in your file, we take summer days as hot water and cooking with the heating off, and the rest of the year’s gas as heating. Mains water is colder in winter, so this slightly understates hot water’s share.
Costs
Costs use the price cap unit rate for gas, the rate your file states if it has a cost column, or a rate you enter. They price the gas only: the standing charge is the same whatever you use, so it plays no part in what a change would save.
What it doesn’t do
- Tell you why. It shows when and how much; whether the heating was left on or needed is yours to judge.
- Correct for the weather. A cold week uses more gas than a mild one.
- Homes with a hot water cylinder heated by the boiler, which blur heating and hot water.