Enter a current in amps and the voltage (UK mains is 230 volts) and this converts amps to watts. Power in watts is simply current times voltage.
UK mains is a nominal 230 volts, so a device drawing 10 amps uses about 2,300 watts. This is the same relationship used for fuses and cabling: a standard 13A plug at 230V can carry up to roughly 3,000 watts. To go the other way, use the watts to amps calculator, and to turn watts into energy used, see watts to kWh.
Here is where this is actually useful for your bill. The appliances that pull the most amps are the ones that make heat, and those are the ones costing you real money: an electric shower, a fan heater, a kettle, a tumble dryer. If a plug or an extension lead feels warm in use, that is current turning into wasted heat in the wiring, and it is both a fire risk and a sign you are overloading it. Anything drawing near 13 amps deserves its own wall socket, not a shared four-way. Sort the wiring safely first, then look at what those high-amp appliances are costing you with the running cost calculator.
What I would actually do: stop guessing an appliance's draw from its amp rating and read the wattage off the label instead, because that is the number that turns straight into pennies. Amps matter for safety and fuses; watts matter for the bill.
Common questions
How do I convert amps to watts?
Multiply the current in amps by the voltage. At UK mains voltage of 230V, 10 amps is 10 times 230, which is 2,300 watts, or 2.3kW.
How many watts is 13 amps?
At 230 volts, 13 amps is about 2,990 watts, just under 3kW. That is why a standard 13A UK plug is the limit for high-power appliances like heaters and kettles.