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Water Heating Energy Calculator
Computes the energy needed to heat a volume of water by a given temperature rise (specific heat of water 4.186 kJ/(kg·K)) at a heater efficiency, the daily and annual energy and cost for repeated heating, and the heating time for a given heater power.
When to use
You want the energy and cost of hot water for showers, a tank or a kettle, the annual cost of a water heater, or how long a heating element takes to heat a tank.
Do not use when: You need standby losses of a storage tank or heat-pump water heater performance under varying conditions (enter its COP × 100 as the efficiency for a rough estimate), or non-water fluids (use specific-heat).
Formula
heat_delivered_kwh = 4.186 × volume_liters × temperature_rise_c / 3600; energy_kwh = heat_delivered_kwh / (efficiency_percent / 100); energy_mj = energy_kwh × 3.6; daily_energy_kwh = energy_kwh × heatings_per_day; annual_energy_kwh = daily_energy_kwh × 365; cost = energy × energy_price_per_kwh; heating_time_hours = energy_kwh / heater_power_kw
Q = m·c·ΔT with c = 4.186 kJ/(kg·K) and 1 kg per litre (density variation with temperature ignored, < 2 %). The efficiency converts delivered heat into purchased energy; storage standby losses are not included.
Inputs
| Parameter | Type | Unit | Required | Description |
|---|---|---|---|---|
volume_liters | number | L | yes | Volume of water heated per heating in litres (1 L ≈ 1 kg). Range: > 0, ≤ 1000000 |
temperature_rise_c | number | °C | yes | Temperature increase in °C (or K), e.g. 40 to heat 15 °C mains water to 55 °C. Range: > 0, ≤ 100 |
efficiency_percent | number | % | default 90 | Share of the purchased energy that ends up in the water: 90–100 electric resistance, 60–95 gas, 200–400 for a heat-pump water heater (COP × 100). Range: ≥ 10, ≤ 500 |
energy_price_per_kwh | number | per kWh | no | Optional price per kWh of electricity or gas in your currency, for the daily and annual cost. Range: ≥ 0, ≤ 100 |
heatings_per_day | number | default 1 | How many times per day this volume is heated (e.g. number of showers). Range: ≥ 0, ≤ 100 | |
heater_power_kw | number | kW | no | Optional heater input power in kW, to report the heating time of one volume. Range: > 0, ≤ 1000 |
Outputs
| Output | Type | Unit | Description |
|---|---|---|---|
energy_kwh | number | kWh | 4.186 × volume_liters × temperature_rise_c / 3600 / (efficiency_percent / 100). |
energy_mj | number | MJ | energy_kwh × 3.6. |
heat_delivered_kwh | number | kWh | Useful heat before efficiency losses: 4.186 × volume × ΔT / 3600. |
daily_energy_kwh | number | kWh | energy_kwh × heatings_per_day. |
annual_energy_kwh | number | kWh | daily_energy_kwh × 365. |
daily_cost | number | daily_energy_kwh × energy_price_per_kwh; only when a price is given. | |
annual_cost | number | annual_energy_kwh × energy_price_per_kwh; only when a price is given. | |
heating_time_hours | number | h | energy_kwh / heater_power_kw; only when heater_power_kw is given. |
Example
50 L by 40 °C at 100 % efficiency: {"volume_liters":50,"temperature_rise_c":40,"efficiency_percent":100} → {"energy_kwh":2.326,"energy_mj":8.372,"heat_delivered_kwh":2.326,"annual_energy_kwh":848.8}
50 L by 40 °C at 90 %, twice a day, 0.30 per kWh, 2 kW element: {"volume_liters":50,"temperature_rise_c":40,"efficiency_percent":90,"energy_price_per_kwh":0.3,"heatings_per_day":2,"heater_power_kw":2} → {"energy_kwh":2.584,"daily_energy_kwh":5.168,"annual_energy_kwh":1886.3,"daily_cost":1.55,"annual_cost":565.89,"heating_time_hours":1.29}
GET https://tttkmbb.com/api/v1/calculate/water-heater-energy?volume_liters=50&temperature_rise_c=40&efficiency_percent=100
Machine access
- API:
GET https://tttkmbb.com/api/v1/calculate/water-heater-energy(query parameters) orPOSTwith a JSON body{"inputs": {...}} - Schema: https://tttkmbb.com/api/v1/calculators/water-heater-energy · Markdown: https://tttkmbb.com/energy/water-heater-energy.md · JSON definition: https://tttkmbb.com/energy/water-heater-energy.json
- MCP: server
https://tttkmbb.com/mcp, toolrun_calculator with calculator_id="water-heater-energy" - OpenAPI operationId:
calculate_water_heating_energy - Freshness:
static. Authentication: none. Rate limit: fair use (see rate limits).
Sources
- Wikipedia – Specific heat capacity (reference)
- U.S. DOE Energy Saver – Sizing a New Water Heater (government)
FAQ
How much energy does a shower use?
A 10-minute shower at 9 L/min uses 90 L; heating it from 15 °C to 40 °C (ΔT 25 K) needs 2.6 kWh of heat, about 2.9 kWh of electricity at 90 % efficiency.
How do I model a heat-pump water heater?
Enter its coefficient of performance times 100 as efficiency_percent (COP 3 → 300 %); the calculator then divides the heat by 3.
Why does a gas heater cost less despite lower efficiency?
Efficiency converts heat to purchased energy; cost also depends on the price per kWh, and gas is usually cheaper per kWh than electricity. Enter the gas price per kWh (price per m³ ÷ ≈ 10.5 kWh/m³).
Related calculators
- Specific Heat Calculator — Heat energy for other substances and units.
- Electricity Cost Calculator — Cost of the kWh at several tariffs.
- Solar Payback Calculator — Solar electricity that could supply the water heater.