Immersion heaters and electric showers are among the highest-draw appliances in a house, and customers rarely have any sense of what they cost to run until someone shows them the number.

This calculator breaks the cost down into daily, weekly, monthly and yearly figures from three inputs: the appliance wattage, how long it runs each day, and the price paid per kWh. It is a useful thing to be able to pull up on a phone when a customer asks whether a system change is worth doing.