Water heaters
Choose a storage heater by first-hour rating, and work out the heat a tankless unit must add.
Guides and calculators
Water heaters
Water Heater Size Calculator: First-Hour Rating Worksheet
Add up your busiest hour of hot water, then compare it with a storage heater's first-hour rating. Includes the formula, usage values and what they leave out.
Water heatersTankless Water Heater Calculator: Flow, Rise and BTU
Work out the heat a tankless water heater must add from your hot water flow and temperature rise, why winter matters, and how to read a maker's flow chart.