Water Heating Calculator
About Water Heating Calculation
This water heating calculator determines the energy to heat water between two temperatures, the energy consumption of the heat source with efficiency taken into account, the heating time, and the cost of one heating cycle. For natural gas, it also calculates gas consumption and the hot water flow rate in litres per minute. The calculation is suitable for electric water heaters, gas boilers, and domestic hot water systems.
Guidelines and recommendations
Thermal energy required to heat water
Temperature difference. The required temperature rise is first calculated as the difference between the final and initial water temperatures. If the final temperature is not higher than the initial temperature, the temperature rise used for the energy calculation is taken as 0 °C.
ΔT = max(T2 - T1, 0)
Water density and specific heat capacity. The calculation uses the approximation 1 litre of water = 1 kg and a specific heat capacity of water of 4.186 kJ/(kg·°C). After converting joules to kilowatt-hours, this corresponds to a factor of 0.0011627778 kWh/(l·°C).
Q = V · ΔT · 0.0011627778
Here, Q is the thermal energy that must be transferred to the water, kWh, V is the water volume, l, and ΔT is the temperature rise, °C. For example, doubling either the water volume or the temperature difference also doubles the required amount of heat.
Efficiency and source energy consumption
Efficiency. Efficiency indicates what share of the source energy is converted into useful heat delivered to the water. When efficiency is taken into account, source energy consumption is calculated by dividing the required thermal energy by the efficiency expressed as a decimal value from 0 to 1.
Qsource = Q / η
For example, at 90 % efficiency, η = 0.90, so delivering 5 kWh of heat to the water requires about 5.56 kWh of source energy. If efficiency is not taken into account, the calculation uses η = 1, meaning no additional losses are included.
Typical efficiency values. For electric resistance heating, efficiency for direct water heating is often taken as close to 100 %. For gas boilers, a common indicative range is about 85-95 %, although the value from the technical documentation of the specific appliance should be used whenever possible. When natural gas is selected, the calculator uses a default efficiency value of 90 %.
Natural gas consumption
Gas calorific value. Natural gas volume is calculated from the amount of fuel energy required by the heater. The default calorific value is 10.5 kWh/m3. The actual energy content of natural gas depends on its composition and supplier, so for a more accurate result the value provided by the gas supplier should be used.
Vgas = Qsource / Hgas
Here, Vgas is natural gas consumption, m3, Qsource is the required fuel energy including efficiency losses, kWh, and Hgas is the gas calorific value, kWh/m3. The higher the appliance efficiency and gas calorific value, the lower the calculated fuel volume.
Heater power and heating time
Useful heating power. In this calculation, power means the rate at which the heater can transfer heat to the water. Heating time is therefore determined from the thermal energy required by the water and the useful heating power, while efficiency separately affects the amount of source energy consumed.
t = Q / P
Here, t is the heating time, h, Q is the thermal energy required by the water, kWh, and P is the useful heating power, kW. The calculated time is also converted to minutes by multiplying by 60. When electricity is selected, the calculator uses a default power of 2 kW, while for natural gas it uses 20 kW. These are starting values and can be replaced with the power of the actual appliance.
Gas boiler power. For a combi or instantaneous gas boiler, use the power available for domestic hot water production. If the manufacturer specifies a separate domestic hot water output, that value is preferable for the calculation.
Hot water flow rate
Flow rate in litres per minute. For a gas heater, the calculator also determines the volume of water that it can theoretically heat continuously at the specified power and temperature rise. This result does not depend on the total water volume entered for the energy calculation.
q = P / (60 · ΔT · 0.0011627778)
Here, q is the hot water flow rate, l/min. At the same heater power, reducing the required temperature rise increases the available water flow. For example, the same boiler can provide a higher flow rate when heating water by 30 °C than when heating it by 45 °C.
Cost calculation
Electricity. The cost is calculated by multiplying the source energy consumption in kWh by the tariff per 1 kWh.
Cel = Qsource · tariffel
Natural gas. For gas, the cost is calculated from the estimated fuel volume in m3 and the tariff per 1 m3.
Cgas = Vgas · tariffgas
The calculator does not assign a currency to the tariff. The final cost is displayed in the same monetary unit in which the tariff is entered.
Temperatures and European standards
Typical temperatures. For cold mains water, a common indicative range is 5-15 °C depending on season and region. For stored hot water, values of about 50-60 °C are commonly used. For a specific system, actual or design temperatures should be entered whenever available.
European standards. The following related European standards are used when designing domestic hot water systems and assessing their energy performance:
- EN 806 - “Specifications for installations inside buildings conveying water for human consumption”.
- EN 12831-3:2017 - “Energy performance of buildings. Method for calculation of the design heat load. Part 3: Domestic hot water systems heat load and characterisation of needs”.
- EN 15316-3:2017 - “Energy performance of buildings. Method for calculation of system energy requirements and system efficiencies. Part 3: Space distribution systems for domestic hot water, heating and cooling”.
These documents are used when selecting parameters and designing water supply and heating systems. The basic energy calculation in this calculator is based on the thermal balance of the water, the specified useful heating power, and, where applicable, efficiency and the calorific value of natural gas.
FAQs
Why does efficiency affect gas or electricity consumption but not heating time?
Heating time is calculated from useful heating power, meaning the power actually transferred to the water. Efficiency represents the additional source energy required to provide that useful output. Therefore, lower efficiency increases gas or electricity consumption, but with the same useful heating power it does not change the calculated heating time.
Why does water volume not affect the gas boiler flow rate in l/min?
The flow rate of an instantaneous heater is determined by the boiler power and the required temperature rise. Total water volume affects the total energy demand, gas consumption, and operating time, but not the number of litres the boiler can heat each minute under constant conditions.
What natural gas calorific value should I use?
For an initial estimate, the default value of 10.5 kWh/m3 can be used. For a more accurate natural gas consumption calculation, use the calorific value provided by the gas supplier or specified for the local gas network.
What power should I enter for a combi gas boiler?
Use the useful output available in domestic hot water mode. On some boilers this differs from the space-heating output, so if a separate domestic hot water rating is provided in the technical documentation, that value should be used.
Why can the actual hot water flow rate and heating time differ from the calculated values?
In practice, heating is affected by heat losses from the appliance and pipework, changes in inlet water temperature, power modulation, and control behaviour. For instantaneous heating, actual flow conditions and heat exchanger performance also matter, so the calculated result should be treated as an engineering estimate for the specified input conditions.