Appliances & Utilities calculator
What Water Heater Size Do I Need?
Use hot water needed during peak period, incoming water temperature, desired delivery/storage temperature to find water-heater recovery power.
Enter your measurements
What Water Heater Size Do I Need uses hot water needed during peak period, incoming water temperature, desired delivery/storage temperature and the other configured measurements to return water-heater recovery power with a formula written specifically for this tool.
- Hot water needed during peak period — Required
- Incoming water temperature — Required
- Desired delivery/storage temperature — Required
- Desired recovery time — Required
- Heater efficiency — Required
Water-heater recovery power
Enter the measurements above to calculate water-heater recovery power.
How the calculation works
V12 calculation: temperatureRise=max(0,deliveryTemp-incomingTemp); waterMassKg=peakHotWaterVolume×1000; energyKWh=waterMassKg×4.186×temperatureRise/3600; requiredPowerKw=energyKWh/recoveryTime/efficiency. This is the implementation specification for this calculator.
Worked example
Example: Hot water needed during peak period = 80 L; Incoming water temperature = 10 °C; Desired delivery/storage temperature = 50 °C; Desired recovery time = 2 h; Heater efficiency = 90%. Result: Required Power Kw = 2.07 kW; Energy KWh = 3.72 kWh; Temperature Rise = 40 °C.
Before you decide
The result is only as accurate as the measurements entered. Check the exact product documentation for ratings, clearances, installation and safety requirements before relying on the result. For what water heater size do i need, the limiting result on this page is water-heater recovery power.
Manufacturer or safety specification required for the final decision. The CMS formula handles the stated measurement calculation only; installation clearances, ratings, certification and safety instructions must come from the exact product documentation.
Frequently asked questions
What does the Water Heater Size calculator tell me?
It returns water-heater recovery power from the measurements entered on this page. The calculation is specific to water heater size, so the result should be compared with the exact product or specification you are considering.
Which measurements should I take for water heater size?
Measure hot water needed during peak period, incoming water temperature, desired delivery/storage temperature, desired recovery time, heater efficiency. Use clear, usable dimensions rather than nominal room or product labels.
Should I round the water-heater recovery power?
Keep the calculated value for comparison, then choose a real product that meets or stays within the returned water-heater recovery power while allowing for the clearances, tolerances or ratings shown by its manufacturer.