Why don’t the numbers add up to the penny?
Every figure in Wattlas is worked out from unrounded half-hour readings, and rounded once, when it's shown. That's why a list of lines can be a penny, or a hundredth of a kWh, away from its total. The total is the one to trust.
Why not round each half hour?
Because the small differences add up. Here's one sunny day from a real Wattlas household. Rounding each half hour to two decimal places moves it by a few watt-hours at most, sometimes up, sometimes down. By the evening those nudges had added 18 watt-hours, enough to turn 14.56 kWh into 14.58.
Wattlas keeps every half hour exactly as it was read, adds them up, and only then rounds the day's total for the screen. Rounded figures don't always add up to the same total as the unrounded ones.
Why can the cost lines be a penny out?
The same thing happens with money. Each line on the cost card is worked out exactly, and so is the total, from the same unrounded figures. Then each is rounded to the penny on its own. Most days they agree. Some days they don't:
We could make the lines add up by adding up the rounded lines instead, but then the total would be wrong by up to half a penny for every line. The total is the figure you'll compare with your bill, so it's the one we keep exact.
Why fewer decimals for a month or a year?
Your electricity meter, your inverter and your car charger each measure at their own point, so they never quite agree. A car charger wired on its own supply, for example, draws a little standby power, about 0.002 kWh every half hour, that the meter sees but the battery system may not. That's about 3 kWh a month. Over a month or a year these small differences reach whole kWh, so Wattlas shows fewer decimal places as the period gets longer.
Why might my bill be a few pence different?
Your supplier rounds too, and it gets to choose how. HMRC lets businesses round the VAT on a whole invoice down, or work it out line by line, for example by "rounding to the nearer 1p or 0.5p". Each supplier picks its own method, so a bill and an exact figure can be a penny or two apart without either being wrong.
Bigger differences usually have another cause:
- an estimated meter reading on the bill;
- a credit, refund or adjustment;
- a tariff change part of the way through the billing period;
- smart-charge slots your supplier bills a few days later.
Which figure should I trust?
The total. It's worked out from every half hour exactly and rounded once. If you want to check our sums yourself, Wattlas gives you the unrounded figures: in the app, open Performance, then Payback details, then "Get your geek on" to download the daily summary or every half hour as a spreadsheet.
Start monitoring your home energy.
Solar, battery, car charging and energy tariff costs, all in Wattlas.
