Water Heating Energy
Calculator
Results
- Heat required (MJ)
- 31.395
- Heat required (kWh)
- 8.720833
- Input energy (kWh)
- 9.689814
- Energy cost
- 1.647268
Energy results
| Heat required (MJ) | 31.395 |
| Heat required (kWh) | 8.720833 |
| Input energy (kWh) | 9.689814 |
| Energy cost | 1.647268 |
formula-map diagram
- Heat required (MJ)
- 31.395
- Heat required (kWh)
- 8.720833
- Input energy (kWh)
- 9.689814
- Energy cost
- 1.647268
Energy relationship
Formula
Q = m × c × ΔT (c = 4186 J/kg·K, 1 L ≈ 1 kg); input = Q ÷ η= 31.395
Note
This is a simplified model: it applies the standard equation to the numbers you entered and ignores real-world losses, weather variation, tariff structures and equipment tolerances. Wind power assumes air density 1.225 kg/m³ at ISA sea level and the power coefficient you enter (Betz limit 0.593). Verify with measured data or a professional energy audit before making purchasing decisions.
More in Energy and environment
See all →Frequently asked questions
What inputs drive the water heating energy calculation?+
The main inputs are the volume of water, the temperature rise needed (from incoming cold water temperature to your target hot water temperature), and the specific heat of water. More volume or a bigger temperature rise both directly increase the energy required.
What's the underlying formula?+
Energy equals mass of water times specific heat times temperature change, commonly expressed as kWh = (gallons × 8.34 × ΔT°F) ÷ 3412, or in metric terms as kWh = (liters × 4.186 × ΔT°C) ÷ 3600. Specific heat is the energy needed to raise one unit of water by one degree.
Why does incoming water temperature matter so much?+
Groundwater temperature varies by season and region, often ranging from around 40°F in winter to 70°F in summer in temperate climates, so the same target hot water temperature requires more energy in winter simply because there's a bigger gap to close.
Does this account for standby heat loss from the tank?+
A basic calculation only covers the energy to heat the water itself, not ongoing losses from an insulated tank keeping water hot over time. For total energy use, you'd need to add standby loss, which depends on tank insulation and how often it cycles to maintain temperature.
How efficient is my water heater at delivering this energy?+
Electric resistance heaters are close to 100% efficient at converting electricity to heat, while gas heaters lose some energy through the flue, and heat pump water heaters can exceed 100% efficiency by moving heat rather than generating it. Divide the calculated energy need by your heater's efficiency to get actual input energy required.