Water Heating Energy Calculator

Raising a mass m of liquid by ΔT takes E = m × c × ΔT, where c is the specific heat capacity. Mass comes from volume × density. For water near room temperature c is about 4.18 kJ per kg per kelvin and the density about 1 kg per litre — the labelled sample values on the page, from the NIST fluid property tables; both vary a little with temperature and a lot for other liquids (oils, brines, glycol mixes), so they are inputs. In imperial units the same formula runs in lb, BTU/(lb·°F) and °F; for water c ≈ 1 BTU/(lb·°F), which is the definition of the BTU.

A heater of power P delivers P × time of energy; the time is the energy divided by the power. Not all the energy consumed reaches the water: the efficiency (from the heater data — resistive immersion elements are near 100 %, gas and oil burners are not) and a loss allowance for the tank and pipes both increase the input energy, the time and the cost. Both are your figures; leaving them at the ideal is flagged. The “kW for one hour” figure is simply the input energy read as an average power, useful for checking a supply or a circuit.

Cost is the input energy in kWh times the price per kWh you enter, in whatever currency you use; fuel prices per litre or per therm have to be converted to a price per kWh of delivered heat first. No tariff is assumed and no standing charges are included.

Only sensible heat is counted: boiling, steam raising and freezing involve latent heat many times larger and are outside the page (an end temperature above the boiling point is flagged). Heat-up while the water is also being drawn off, heat exchangers and heat-pump coefficients of performance are not modelled. Nothing leaves the browser; the same four anonymous usage counts as the rest of the site apply.