Sizing and calculators
Electric tankless water heater sizing calculator
Enter the fixtures that run together and your cold-water temperature range. The result shows hot-water demand and heat output, without choosing a heater for you.
Your sizing estimate
No estimate yet. Add at least one active fixture and a cold inlet range, then calculate.
Printable blank fixture worksheet
Inlet low: ______ °F; inlet high: ______ °F; outlet setpoint: ______ °F.
Supply voltage: ______ V; documented efficiency: ______; source: __________________.
Write inlet low/high, outlet setpoint, and each simultaneous fixture here. For each row record count, flow low/high, hot-side or mixed flow, and mixed target if used.
| Fixture | Count | Low GPM | High GPM | Flow type / target °F |
|---|---|---|---|---|
| Fixture 1 | _____ | _____ | _____ | ____________ |
| Fixture 2 | _____ | _____ | _____ | ____________ |
| Fixture 3 | _____ | _____ | _____ | ____________ |
| Fixture 4 | _____ | _____ | _____ | ____________ |
Temperature rise = outlet minus inlet. Heat output in Btu/h = 500.4 × hot-side GPM × rise. Divide by 3412.142 for thermal kW. See the method and worked examples.
Heat duty = 500.4 × hot-side GPM × temperature rise (Btu/h); divide by 3412.142 for thermal kW. Mixed hot fraction = (mixed target − inlet) / (outlet − inlet).
Planning only. Confirm the exact maker flow curve and supply limits. Current is not a breaker or wire size. No pipe-loss or safety multiplier is included.
Sources: DOE demand/rise guidance (2012); Stiebel Eltron flow table; CPSC scald guidance. Method v1.0, checked October 15, 2026. Full method: tanklesswaterheatersizingcalculator.com/methodology/.
Planning estimate, not an installation design. Method v1.0, checked October 14, 2026. Equations, assumptions and sources.
Calculate thermal kW from fixture demand, then estimate electric input and aggregate current only when the efficiency and voltage are stated.
Sections7
This electric tankless water heater sizing calculator starts with the heat your water needs. It then separates that heat from electric input power and current. Use it to screen a demand case, not to choose wires or breakers. A model's circuit schedule and the home's available electric capacity still need separate checks.
Estimate thermal kW from simultaneous hot-side flow and temperature rise. Divide by a stated heating efficiency for electric input kW, then divide watts by the chosen single-phase voltage for aggregate current. If efficiency is unknown, only thermal kW is available. None of these results is a breaker size or proof that a heater fits the electric service.
Electric tankless water heater sizing calculator inputs at 240 V
The fixture worksheet follows the same method as the general sizing calculator. Enter the number of draws that run together, low and high flow, and a measured cold inlet range. Select mixed flow if the measurement includes cold water blended at the fixture. The tool adjusts the hot fraction at each inlet temperature.
Select 208 V or 240 V for this case. Check which supply the home has before using the result. This is a single-phase resistance-heating estimate.
It cannot size three-phase units. If the voltage is unknown, find it in the system records. Do not pick a value just because it gives a lower current.
Leave efficiency empty when you lack evidence. This produces thermal kW with electric input and current marked unknown. The optional efficiency-of-1 checkbox loads an ideal mathematical assumption. It does not assign a verified rating to any heater. Label that case as assumed when you share it.
Convert BTU/h to kW and aggregate amps
For a hot-side draw, heat output is 500.4 × GPM × rise in °F. Thermal kW is that result divided by 3412.142. If heating efficiency is e, electric input kW equals thermal kW divided by e. Aggregate current equals input kW × 1000 divided by voltage.
Consider an assumed 3 GPM hot-side load, a 50°F inlet and a 120°F outlet. It requires about 30.8 thermal kW. With an explicitly ideal efficiency of 1, that is about 128.3 A at 240 V or 148.1 A at 208 V. The example compares the same water-heating task at two chosen voltages; it is not the nameplate rating of a product.
A real heater may supply less power at a lower voltage. Its heating elements have a fixed resistance. Do not assume the same model gives the same kW at both voltages. The Tempra manual lists each model with its voltage. Read the row for the unit you plan to use, rather than a larger value from another version.
Why 128 A does not identify a breaker
A whole-house heater may spread its heating load across several circuits. Aggregate current is the sum for this math case. It is not the load on each branch. It cannot tell you the breaker rating or wire size. Nor does it show how much spare power the home has.
Write down the model and supply voltage. Keep the input rating with the maker's circuit list. A service-load check also needs the other loads in the home and the rules that apply there. Empty breaker spaces do not prove spare power. A licensed electrician checks that supply; this page helps describe the water demand.
Do not open a live panel, change a breaker or infer a wire size from this estimate. The consequence of a high current result is to compare the stated setup rules and alternatives. It may be worth considering point-of-use heating for one fixture rather than assigning the full house to a large instantaneous electric load.
Check winter GPM against a flow table
The result shows the heat needed for the entered case. It does not select a product. Read the maker's flow table at the same rise and voltage. Check how little flow can start the heater, too. Then read what the unit does when demand exceeds its output, such as reducing flow to hold the set temperature.
For example, a stable displayed temperature does not prove that every tap receives the requested flow. The heater may preserve temperature while supplying less water. A winter supply can expose this limit even when summer use seems satisfactory. The electric-versus-gas comparison explains the supply trade-offs without treating either fuel as a universal answer.
Frequently asked questions
Should I use UEF as heating efficiency?
No. UEF reports water-heater performance under a specified test pattern. This tool needs the fraction of input power that reaches water for the chosen heating case. Leave it unknown unless you have suitable evidence. Using a test label as a substitute would make the power calculation look more certain than it is.
Does 208 V always mean a larger heater?
For the same required input power, lower voltage means higher aggregate current. But a real model's available power can also change with voltage. Compare the model's voltage-specific specs and flow chart. Do not convert one rating into an assumed fit simply because the calculator can divide watts by a different voltage.
Can I count the main breaker minus other breakers?
No. Adding or subtracting breaker labels is not an electric service-load check. The real assessment considers connected equipment and the relevant load rules. This page does not perform that assessment. Keep the estimated heat demand as an input to equipment planning, while the exact circuit and service rules remain separate.
What if the ideal scenario exceeds the model?
Treat that as a reason to investigate further. Confirm that the simultaneous uses and inlet temperatures describe the intended demand. Check a larger supported capacity or another heating arrangement. Do not round the demand down, hide a fixture, or call a reduced-flow result a match unless that reduced use is an explicit choice.
Electric tankless water heater sizing calculator verdict
Use the electric tankless water heater sizing calculator to keep water demand, power and current distinct. Save the inputs with the result. Revisit the general heater comparison if supply limits make the planned electric load impractical. Check the methodology for the heat-balance assumptions and math examples.
Sources checked October 14, 2026: Stiebel Eltron's manual and flow table. The estimate uses the values you enter. It does not size wires or breakers. It does not design an installation or prove that a model fits your home.