Divide BTU/h by 3.412 to get watts. Multiply watts by 3.412 to get BTU/h. The conversion changes the units. For a fair comparison, both ratings also need to use the same temperature conditions.
Worked examples
1,000 BTU/h is about 293 W. A 500 W rating is about 1,706 BTU/h. A room requiring 650 W needs about 2,218 BTU/h at the design conditions. Keep extra digits until the final step when comparing close results.
Check the rating next to the number
Product pages sometimes shorten BTU/h to BTU. In a heat-output specification, look for the rate and the test condition. A figure at ΔT50 and one at ΔT30 cannot be compared directly just by converting units. See our guide to ΔT50 and ΔT30.
Keep electrical input and water-based output clear
A heating element’s wattage describes its rated electrical input. A water-based rail’s wattage describes its heat output at stated test conditions. Check the specification for the operating mode you intend to use. The bathroom BTU calculator can give you a starting room requirement to compare against.
Frequently asked questions
What if the catalogue figures do not convert exactly?
Small differences can be due to rounding. Ask for the manufacturer’s figures and rating conditions if the numbers differ significantly.
See the bathroom heating and BTU guide for the full process from room heat loss to choosing a rail.