Energy bills are written to be paid, not understood, but the structure is simple once you know the parts. Read it properly and you can see where the money goes and whether you are even being charged for what you used.
The kilowatt-hour
Everything is billed in kilowatt-hours, kWh, which is the unit of energy. One kWh is a thousand watts running for an hour: a one-bar electric fire for an hour, or a 100-watt item for ten hours. Your meter counts kWh, and that is what you pay for. Once you think in kWh, the running cost of anything is just its power times the hours it runs times the price per kWh, which is exactly what the calculator does.
Unit rate and standing charge
A bill has two charges. The unit rate is the price per kWh, what you pay for energy actually used. The standing charge is a fixed daily fee you pay regardless of usage, covering the cost of being connected. This matters: if you use very little, the standing charge can be most of your bill, so cutting usage has limits, and a tariff with a lower standing charge may suit a light user better.
Estimated versus actual
If the bill says estimated, the supplier has guessed your usage rather than read the meter, and the guess can be well out. Submit your own meter readings, or fit a smart meter, so you are billed for what you actually used rather than a pessimistic estimate that leaves you in credit or a cheery one that builds a debt.
Finding the cost
With the unit rate in hand, you can finally answer where the money goes. Note your meter before and after running something for a known time, and you have measured it directly. More simply, the heat-making appliances and the always-on ones are the usual culprits, as the electricity and heating pages explain.