Understand your estimate
How to use this result
The explainer takes two electricity bills and splits the change in total into billing-period length, energy used per day, unit price, standing charge and other charges. The parts add up exactly to the change.
Method reviewed · Calculation methods · Evidence and source policy
Find the right inputs
Energy and days on each bill
kWh and days covered
Copy these from the bills. Bills covering different numbers of days make the totals hard to compare, which is why length is shown separately.
Unit price and standing charge
Major currency units per kWh and per day
Enter 28 pence as 0.28. Use the same tax treatment on both bills.
Other charges or credits
Total in major currency units
Enter fees, credits or adjustments as a total. Credits are negative; enter 0 if none.
The calculation
Change = (extra days × earlier daily kWh × earlier price) + (later days × change in daily kWh × earlier price) + (later kWh × change in unit price) + change in standing charge + change in other charges.
A worked arithmetic example
Illustrative arithmetic: an earlier bill of 420 kWh over 30 days at 0.28 per kWh plus 0.50 a day totals 132.60. A later bill of 610 kWh over 33 days at 0.30 plus 0.55 a day totals 201.15, a rise of 68.55. Of that, 11.76 comes from three extra days, 41.44 from higher daily use, 12.20 from the higher unit price and 3.15 from the standing charge. These values are assumptions, not real bills.
What the estimate can tell you
Look at the largest part first. Usage-driven changes point to appliances or weather; price-driven changes point to the tariff. If the bills cover different days, compare cost per day as well as totals.
Questions about this calculation
Why show billing-period length separately?
A bill covering more days uses more energy even when daily use is unchanged. Separating length from daily use stops a longer bill looking like higher consumption.
What if my bill is estimated or the price changed mid-bill?
This explainer takes each bill as a single period at one price. If your bill shows two price periods or an estimated reading, use the bill’s own breakdown and enter the totals; the result will be approximate.