Is it the rate, or is it you?
A bill going up says nothing on its own: the price per unit may have risen, or you may have used more, or both — and people routinely blame the wrong one. Enter two bills and this splits the change cleanly. It uses only the four numbers you type. No national averages, no assumptions, nothing uploaded.
How the split is calculated
With earlier price p₀ and usage q₀, and later price p₁ and usage q₁, the change in the total is split three ways:
| Component | Formula | Reads as |
|---|---|---|
| Rate effect | (p₁ − p₀) × q₀ | What the new price would have cost you at your old usage |
| Usage effect | (q₁ − q₀) × p₀ | What your new usage would have cost at the old price |
| Combined | (p₁ − p₀) × (q₁ − q₀) | The part that exists only because both moved together |
The three add up to the whole change, exactly. Unit price here is your all-in price — total bill divided by kilowatt hours — so it includes standing charges, network fees and taxes. That is the number that matters to a household, and it is also why your all-in price can rise while the advertised energy rate does not.
Reading the result
| If most of it is… | Then |
|---|---|
| Rate effect | The decision that moved your bill was made somewhere — a tariff change, a rate case, a capacity cost pass-through. Find out who decided |
| Usage effect | Your bill moved for reasons inside your own home — weather, occupancy, a new appliance. National headlines about data centres are not the explanation for your number |
| Both | Normal, especially across seasons. Compare the same month a year apart to strip the weather out |
This tool attributes nothing to AI or data centres, and it cannot: your two bills do not contain that information. It exists so the attribution question starts from an honest number. The evidence on the wider question is in Is AI raising your electricity bill?