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Energy Accumulated in Heated Water - kWh

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Water is often used to store thermal energy. Energy stored - or available - in hot water can be calculated

E = cp dt m                           (1)


E = energy (kJ, Btu)

cp = specific heat of water (kJ/kgoC, Btu/lb oF) (4.2 kJ/kgoC, 1 Btu/lbmoF for water)

dt = temperature difference between the hot water and the surroundings (oC, oF))

m = mass of water (kg, lbm)

Example - Energy stored in a 1000 liter water tank

Water is heated to 90oC. The surrounding temperature (where the energy can be transferred to) is 20oC.

The energy stored in the water tank can be calculated as

E = (4.2 kJ/kgoC) ((90 oC) - (20 oC)) (1000 liter) (1 kg/liter)

    = 294000 kJ

    = (294000 kWs) (1/3600 h/s)

    = 81.7 kWh    

  • 1 J (Joule) = 0,1020 kpm = 2.778x10-7 kWh = 2.389x10-4 kcal = 0.7376 ft lbf = 1 (kg m2)/s2 = 1 watt second = 1 Nm = 1 ft lb = 9.478x10-4 Btu
  • 1 Btu (British thermal unit) = 1055.06 J = 107.6 kpm = 2.92875x10-4 kWh = 251.996 calorie (IT - International Table calorie) = 0.252 kcal = 777.649

Example - Solar Energy stored in a 200 US gallons Water Tank

A solar energy water buffer tank with 200 US gallons is heated 200oF.

The solar energy stored can be calculated as

E = (1 Btu/lbmoF) (200 oF) (200 US gallons) (8.3 lbm/US gallon)

    = 332000 Btu  


Hot Water Energy Calculator

Energy Stored in Water - Liters/kWh


Energy Stored in Water - US gallons/Btu's

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