Hot Water Demand Calculator
A 50-room resort at 90 litres per room needs about 4,500 litres of 60°C water a day — roughly 173 kWh of heat. A heat pump delivers that on about a quarter of the electricity a resistance heater burns.
Pick your building type and enter the count — rooms, beds, covers, or people. The calculator sizes daily demand from CIBSE Guide G figures, converts it to kWh (Q = VρcpΔT), and recommends heater output and storage volume.
Your Building
Demand & Sizing Results
| Property | Value | Unit |
|---|
The Formulas
Daily heat energy (sensible heating of water, cp = 4.186 kJ/kg·K):
Heater output needed (storage system — recover the day's demand in the chosen window):
Heat pump electricity (divide heat by COP; resistance heating has COP = 1):
Method & Sources
Every figure in this tool has a source. The demand defaults are editable — if you know your real consumption, use it.
- Unit demands — CIBSE Guide G (Public Health and Plumbing Engineering), typical daily hot water demand at 60°C: hotels 90–135 L/bedroom, restaurants 7–10 L/meal, dwellings 35–45 L/person, offices 4–5 L/person, hostels 70–90 L/bed, hospitals 90–140 L/bed depending on category.
- Water properties — specific heat capacity 4.186 kJ/kg·K, density taken as 1 kg/L across the 27–60°C range (the <2% density variation is inside the demand uncertainty).
- 60°C storage — Legionella control practice per CIBSE Guide G and UK HSE guidance: store at ≥60°C, distribute at ≥50°C.
- Storage at 25–35% of daily demand — sizing convention for peak morning draw, not a regulation. Sharp-peak buildings need more; steady-draw buildings less.
- What we deliberately leave out — simultaneity factors for very large systems, circulation losses, and solar pre-heat. Those belong in a proper design, not a screening tool.
Related Free Tools
Frequently Asked Questions
How much hot water does a hotel room need per day?
CIBSE Guide G gives 90–135 litres per bedroom per day at 60°C — the low end for budget hotels with showers only, the high end for resorts with bathtubs. A 50-room resort at 90 L/room needs about 4,500 litres a day, roughly 173 kWh of heat with Philippine inlet water. Laundry and kitchen are extra.
Why is hot water stored at 60°C?
Legionella bacteria multiply between 20 and 45°C and are killed above 60. Standard practice — CIBSE Guide G and UK HSE guidance — is to store at 60°C or above, distribute at 50 or above, and blend down at the tap. Sizing at 60°C keeps the numbers consistent with that practice.
How do I convert litres of hot water into kWh of heat?
kWh = litres × 4.186 × temperature rise ÷ 3600. Example: 1,000 litres from 27 to 60°C is 1000 × 4.186 × 33 ÷ 3600 = 38.4 kWh. A heat pump at COP 4 delivers that for about 9.6 kWh of electricity; a resistance heater needs the full 38.4 kWh.
What size water heater do I need?
With storage, the heater needs to recover the daily demand within your heating window, not match the peak draw. Heater kW = daily kWh ÷ recovery hours. Example: 173 kWh over 10 hours needs about 17.3 kW of heat output, and the storage tank carries the morning peak.
Does warm Philippine inlet water reduce heating cost?
Yes — substantially. Philippine water arrives at 25–28°C, so heating to 60 is a 33 K rise. In the UK the inlet is 10–15°C, a 45–50 K rise. Litre for litre, the Philippines needs about 30% less energy — and heat pumps run at higher efficiency in warm air too.
How much storage volume should I install?
Common practice for hotels is storage equal to 25–35% of daily demand — enough to ride the morning shower peak while the heater recovers. It is a convention, not a regulation: dormitories and sports clubs peak harder and need more; steady-draw buildings need less.
Want This Checked Against Your Building?
Tell us your room count and what you pay for power. Our engineers will size the system properly and tell you plainly what it saves — and where it would not.
Get a Proper Sizing