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Free tool
Heat Pump Sizing Calculator
Tell us who uses the hot water and how they bathe, and get the storage tank and heat pump that cover the morning rush, reheat in time for the evening, and what it costs to run against an electric geyser or LPG.
1. Who uses the hot water?
With a shower or tap on hot water.
2. How do they bathe?
Bathtubs and kitchen
All tubs together.
3. Where is it?
Winter: water 18 °C, air 14 °C. We size for winter and cost a typical month.
Temperatures and timing
55 °C keeps Legionella out.
Before the evening rush.
4. Your energy prices
A 14.2 kg cylinder at ₹920 is about ₹65/kg.
Recommended
150 L tank + 3 kW heat pump
For 295 L of hot water a day at 40 °C
From the tank each day
175 L
at 55 °C, mixed down to 40 °C
Reheats a full tank in
2.4 h
63 L/h at winter output
Runs on a winter day
3.0 h
8.3 kWh of heat
Heat pump output in winter
2.72 kW
of 3 kW rated
Why this tank
- Morning rush (70% in 2 h)
- 123 L at 55 °C
- Every bathroom showering at once
- 96 L at 55 °C
- Heat pump makes during the rush
- 127 L
- Usable share of a tank
- 80%
Running cost, a typical month
Saves about ₹1,113 a month over an electric geyser — ₹13,400 a year.
Good to know
- If the farthest bathroom is more than about 15 m of pipe away, add a hot water return line with a small circulation pump, so the hot water doesn't take a minute to arrive.
- Hard water scales a heat pump's heat exchanger and cuts its output. Above about 200 ppm, soften the water first.
- Run the heat pump in the warmest hours of the day where you can — it makes more heat per unit of electricity.
Indicative sizing for planning and budgeting. Final selection depends on the site and on the actual pump, resin or heat pump chosen — our engineers check every quote against it.
How this calculator sizes a heat pump system
A heat pump water heater is two parts: a storage tank that holds hot water for the moments everyone wants it, and a heat pump that refills the tank with heat between those moments. Size one without the other and you either run out of hot water in the morning or pay for a heat pump that sits idle.
Step 1: how much hot water
Daily hot water at shower temperature (40 °C) is built from how people actually bathe:
| Lifestyle | Shower flow | Length | Per shower |
|---|---|---|---|
| Economy (bucket bath, low-flow) | 6 L/min | 5 min | 30 L |
| Standard shower | 9 L/min | 6 min | 54 L |
| Luxury rain shower | 14 L/min | 9 min | 126 L |
We add bathtub fills (about 180 L each) and kitchen use (about 5 L a person).
The tank stores water hotter than you use it — at 55 °C — and a mixer blends it down with cold. So a tank delivers more shower water than its volume:
Litres from the tank = litres at 40 °C × (40 − cold) ÷ (55 − cold)
With 18 °C winter water, each litre at 40 °C takes only 0.59 L from the tank.
Step 2: the tank
The tank must cover the harder of two moments:
- The morning rush. By default 70% of the day’s hot water in 2 hours, minus what the heat pump makes during those 2 hours.
- The busiest minute. Showers running together for one shower length: every bathroom in a home, and in a hotel or hostel the first four plus 30% of the rest — thirty rooms never all shower in the same minute. A heat pump can’t keep up with that minute by minute, so the tank has to hold it.
Only about 80% of a tank comes out at usable temperature before incoming cold water mixes in, so the tank is the larger of the two divided by 0.8, rounded up to a standard size.
Step 3: the heat pump
Heat to reheat a tank (kWh) = litres × 4.186 × (55 − cold) ÷ 3,600
The heat pump must reheat the full tank within the recovery window — 4 hours by default, before the evening rush — and supply the day’s heat in no more than 16 hours of running. Heat pumps are rated at around 20 °C air and give less in winter: about 80% at 7 °C. We size for your region’s winter.
A bigger heat pump makes more hot water during the rush, which means a smaller tank. The calculator shows the combinations that work and recommends the one with the smallest heat pump.
Climate
| Region | Winter water | Winter air |
|---|---|---|
| North India plains — Delhi, Punjab, Lucknow | 12 °C | 8 °C |
| Hills — Shimla, Dehradun, Darjeeling | 6 °C | 3 °C |
| West India — Gujarat, Rajasthan, Pune | 18 °C | 14 °C |
| East India — Kolkata, Patna, Guwahati | 16 °C | 13 °C |
| South plateau — Bengaluru, Hyderabad | 19 °C | 16 °C |
| Coastal — Mumbai, Chennai, Kochi | 24 °C | 22 °C |
These are typical winter mornings. If you know your own water temperature, enter it under “Temperatures and timing”.
Worked example
A family of five in Gujarat, standard showers, three bathrooms:
| Step | Value |
|---|---|
| Hot water at 40 °C | 5 × 54 L showers + 25 L kitchen = 295 L a day |
| From a 55 °C tank (18 °C winter water) | 295 × 22 ÷ 37 = 175 L |
| Three showers at once | 3 × 54 × 0.59 = 96 L, less 6 L made meanwhile = 90 L |
| Tank | 90 ÷ 0.8 = 113 L → 150 L |
| 3 kW heat pump in winter | about 2.7 kW: reheats 150 L in 2.4 hours |
| Recommendation | 150 L tank + 3 kW heat pump |
Running cost
Heat a day (kWh) = litres at 40 °C × 4.186 × (40 − cold) ÷ 3,600 × 1.1
The extra 10% covers heat lost from the tank and pipes. An electric geyser turns each kWh of electricity into about 0.95 kWh of heat. A heat pump moves heat in from the air, delivering 3–4 kWh of heat per kWh of electricity: its COP. Running costs use average water and air temperatures across the year.
For the family above at ₹8/kWh, a typical month costs about ₹320 with a heat pump against ₹1,430 with an electric geyser — a saving of about ₹1,100 a month, or ₹13,000 a year.
Before you buy
- Store at 55 °C or above, or run a weekly 60 °C cycle, so Legionella can’t grow in the tank.
- Hard water scales the heat exchanger and cuts output year after year. If your water is above about 200 ppm, soften it first.
- Long pipe runs waste water and patience. Beyond about 15 m to the farthest bathroom, a hot water return line with a small circulation pump brings hot water to the tap in seconds.
Questions people ask
What size heat pump do I need for a family of five? ＋
For five people taking regular showers in West or South India, a 3 kW heat pump with a 150–200 L tank is typical. The tank covers everyone showering in the morning rush; the heat pump reheats it in two to three hours. Colder regions, rain showers or bathtubs push both up — the calculator works it out for your case.
How big should the hot water tank be? ＋
Big enough for the morning rush, after allowing for what the heat pump makes during it — and at least big enough for every bathroom showering at once. Only about 80% of a tank comes out hot before mixing cools it, so the tank is sized at 1.25× the hot water it must deliver.
How much does a heat pump save over an electric geyser? ＋
A geyser turns one unit of electricity into one unit of heat. A heat pump moves heat from the air, delivering three to four units of heat per unit of electricity in Indian conditions. Running costs fall by roughly 70–80%. For a family of five in Gujarat at ₹8/kWh, that is about ₹1,100 a month.
Does a heat pump work in a North Indian winter? ＋
Yes, with less output. At around 7 °C air, a heat pump gives roughly 80% of its rated heat at a COP near 3. The calculator sizes for winter water and air temperatures, so the system still keeps up in January. In hill stations with near-freezing nights, add a backup heating element.
What temperature should hot water be stored at? ＋
Store at 55 °C or above. Below that, Legionella bacteria can grow in a tank. Most showers are taken at about 40 °C, so stored water is mixed down with cold — which is why a 55 °C tank goes further than its volume suggests.
What is COP? ＋
Coefficient of performance: units of heat delivered per unit of electricity used. A COP of 4 means 1 kWh of electricity puts 4 kWh of heat into the water. COP rises in warm air and falls in cold.
Built by Devyami
The people behind this calculator install these systems
We've been designing water systems since 1996, with 25,000+ projects delivered — from India's first manufactured heat pump to pumps, softeners, automation and our own IoT platform. These tools use the same rules our engineers use on site.
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