Instantaneous Water Heater Calculator
Calculate the required power of an instantaneous (tankless) water heater, the achievable hot water flow rate, or the outlet temperature. Enter the heater power, cold water inlet temperature, and target hot water temperature to get instant results.
Instantaneous Water Heating Performance Calculator
Instantaneous Water Heater Formula
Where:
- \(P\) — power of the heater (kW)
- \(T_h\) — target hot water temperature (°C)
- \(T_c\) — cold water inlet temperature (°C)
- \(Q\) — hot water flow rate (L/min)
- \(0.073\) — constant combining the specific heat capacity of water (\(\approx 4.19\ \text{kJ/(kg·K)}\)), the conversion from L/min to kg/s, and a small safety margin
These are three forms of the same energy-balance equation: the electrical power fed to the heater must equal the heat needed to raise the incoming water from \(T_c\) to \(T_h\) at flow rate \(Q\). Rearranging the equation lets you solve for whichever quantity is unknown.
Instantaneous Water Heater – Calculation Example
Example: a small point-of-use electric tankless heater rated at 3 kW heats incoming water from 10 °C to 55 °C. What flow rate can it deliver?
\[ \Delta T = T_h - T_c = 55 - 10 = 45\ \text{°C} \\ Q = \frac{P}{\Delta T \times 0.073} = \frac{3}{45 \times 0.073} = \frac{3}{3.285} \approx 0.91\ \text{L/min} \]
Result: the heater can supply about 0.91 L/min of water at 55 °C — enough for a single hand-washing basin, but not for a shower.
Reverse check: to get 12 L/min at the same 45 °C temperature rise you would need \(P = 12 \times 45 \times 0.073 \approx 39.4\ \text{kW}\) — a whole-house unit requiring a three-phase supply.
How to use the calculator
- Choose the Calculation mode: Performance to find the flow rate, Power heater (P) to find the required power, or Hot water temperature (Th) to find the outlet temperature. The input field you are solving for is hidden automatically.
- Enter the Power heater in kilowatts (kW) — the rating plate of the appliance shows this value.
- Enter the Cold water temperature in °C. Use the coldest expected mains temperature (typically 8–12 °C in winter in temperate climates).
- Enter the Hot water temperature in °C. Most applications use 40–45 °C at the tap; 55 °C is common for storage or appliance feed.
- If you selected the power or temperature mode, enter the Water consumption in L/min.
- Read the result below the inputs.
What an instantaneous water heater is and why sizing matters
An instantaneous (tankless) water heater heats water on demand as it flows through the unit, instead of storing hot water in a tank. Because there is no reservoir, the heater must deliver all the required heating power at once: the colder the incoming water and the higher the desired outlet temperature, the more power is needed for the same flow rate. This is why tankless units range from compact 3 kW point-of-use devices to 30 kW+ whole-house units.
The physics behind the calculator is a simple energy balance. Raising 1 liter of water by 1 °C takes about 4.19 kJ, so the heat flow required is proportional to the flow rate and the temperature rise \(\Delta T = T_h - T_c\). With an electric heater, the electrical power in kilowatts must match this heat demand; the constant 0.073 in the formula converts units and adds a modest allowance for heat losses.
Interpreting the result: if the calculated flow rate is lower than the demand of your tap or shower, the water will simply come out cooler than requested at that flow rate (or the unit will throttle the flow, if it has electronic control). Typical demands are 4–6 L/min for a shower, 2–4 L/min for a kitchen sink, and 1–2 L/min for a washbasin. When selecting a unit, always size it for the winter cold-water temperature, not the annual average.
To plan the water side of the installation, you may also find the Pipeline Flow Rate calculator useful; for the electrical supply side, check the appropriate cable with the Wire Gauge calculator, since high-power heaters need dedicated wiring.
Assumptions, limitations and practical tips
- 100% efficiency is assumed. Real electric tankless heaters are 98–99% efficient, so the error is negligible, but gas-fired units lose 15–25% of the input energy through the flue — for gas, divide the result by the unit's efficiency.
- Constant properties. The calculation uses the specific heat of water at moderate temperatures and assumes it is constant with temperature; the effect over the 10–60 °C range is well under 1%.
- Single draw point. The flow figure applies to one outlet at a time. If two fixtures run simultaneously, add their flow rates before calculating the required power.
- Hard water and scaling reduce heat transfer over time, especially at high temperature settings; periodic descaling keeps the actual performance close to the calculated value.
- Electrical supply. Units above ~12 kW normally require three-phase power; verify that your supply and breaker rating match the heater before buying.
- Scald safety. Delivering water above 49 °C at a tap creates a scalding risk; where higher temperatures are needed (e.g. dishwashers), prefer a point-of-use boost or a thermostatic mixing valve.
Quick reference: required power for common flow rates
Required power (kW) for a cold water temperature of 10 °C, calculated with the formula above:
| Flow rate (L/min) | Th = 40 °C (ΔT = 30 °C) | Th = 45 °C (ΔT = 35 °C) | Th = 55 °C (ΔT = 45 °C) |
|---|---|---|---|
| 2 | 4.4 | 5.1 | 6.6 |
| 4 | 8.8 | 10.2 | 13.1 |
| 6 | 13.1 | 15.3 | 19.7 |
| 10 | 21.9 | 25.6 | 32.9 |
| 12 | 26.3 | 30.7 | 39.4 |
Use the table for a rough orientation, and the calculator for your exact inlet temperature — even a few degrees of difference in \(T_c\) changes the result noticeably.
Frequently Asked Questions
How do I calculate the size of a tankless water heater by hand?
Multiply the flow rate in L/min by the temperature rise in °C and by 0.073: \(P = Q \times (T_h - T_c) \times 0.073\). For example, a shower at 6 L/min with a 35 °C rise needs \(6 \times 35 \times 0.073 \approx 15.3\) kW. Always use the coldest winter inlet temperature, not the annual average, or the heater will underperform in winter.
Can I use this calculator for gas tankless water heaters?
Yes, but adjust for efficiency. Electric units convert nearly 100% of input power to heat, while gas units lose 15–25% through the flue. If a gas heater is 80% efficient and the calculator says 30 kW is needed, divide by 0.80 to get the required gas input of about 37.5 kW. Also note that gas models are usually rated in BTU/h: 1 kW ≈ 3412 BTU/h.
What flow rate do I need for a shower, kitchen sink, or washbasin?
Typical demands are 4–6 L/min for a shower, 2–4 L/min for a kitchen sink, and 1–2 L/min for a washbasin. If two fixtures may run at the same time, add their flow rates before calculating power — a 6 L/min shower plus a 3 L/min tap means sizing for 9 L/min. Otherwise the water will come out cooler than expected when both are open.
How many kW do I need for a whole-house tankless water heater?
For a typical shower (6 L/min) with winter inlet water at 10 °C and 40 °C output, you need about 13 kW. Covering a shower plus a sink simultaneously at 45 °C takes roughly 26 kW. Whole-house electric units range from 24 to 36 kW and almost always require a three-phase electrical supply and dedicated wiring — verify your home's supply capacity before buying.
How do I convert the result to gallons per minute (GPM)?
The calculator works in L/min; to convert, note that 1 US gallon ≈ 3.785 liters, so divide L/min by 3.785 to get US GPM. For example, 6 L/min ≈ 1.6 GPM, and a typical US shower head at 2.5 GPM equals about 9.5 L/min. For imperial gallons (UK), 1 gal ≈ 4.546 L.
Why does my tankless heater deliver cooler water in winter?
Because the inlet water is colder, so the same power produces a smaller temperature rise. A 24 kW unit delivering 12 L/min at a 30 °C rise in summer might see a 45 °C rise needed in winter, cutting the achievable flow to about 7.3 L/min. This is physics, not a fault — size the heater for winter conditions or accept a lower winter flow rate.
What hot water temperature should I set on a tankless heater?
Most people shower comfortably at 38–42 °C, so setting 40–45 °C at the tap is efficient and safe. Temperatures above 49 °C at a tap create a scalding risk, especially for children and elderly people — use a thermostatic mixing valve if the appliance or a remote bathroom needs hotter water. Higher settings also increase scaling in hard-water areas.
Maintaining your water heater is definitely important. I am going to check mine as soon as I get home. I will evaluate any problems that I can find. Perhaps I will find something amiss. Do you think duct tape would seal cracks in the side?