What size heat pump does your home need?
Most sizing guides just count bedrooms. This one works off what your household actually draws — shower flow, length and how many people go through the bathroom before work — then tells you the tank size, the recovery rate and how long a full reheat takes.
Your household
Temperatures
The heat pump
Tank size you need
Reclaim CO₂ 250 L · Sanden 250 L. Your household draws about 166 L of stored hot water in the morning rush, and a tank only gives up about 80% of its nameplate before the outlet runs cold.
Mixing 60 °C stored water with 15 °C mains to reach 41 °C means only 58% of the flow leaves the tank. That’s why a 250 L tank goes much further than it first looks.
Estimate only. Real-world sizing also depends on pipe runs, whether you have a bath, simultaneous draw-off (two showers at once halves your effective capacity), and how cold the mains actually runs at your address in July. We check all of it on the site visit — this gets you in the right ballpark before anyone quotes you.
How the maths works
Only part of your shower comes from the tank
Hot water has to be stored at 60 °C to control Legionella, but nobody showers at 60 °C. A tempering valve blends stored hot with cold mains down to about 41 °C at the head.
So the hot fraction of the flow is (shower − mains) ÷ (tank − mains). At 41 °C from a 60 °C tank and 15 °C mains that’s (41−15) ÷ (60−15) = 0.58 — so a 9 L/min head pulls about 5.2 L of hot and 3.8 L of cold every minute.
You can’t use the whole tank
Water stratifies. The hot sits on top, cold mains fills from the bottom, and once you’re into the last fifth the outlet temperature has fallen away. We size on 80% usable, which is the accepted figure.
Reheat time
Energy to heat a tank is litres × 4.186 × ΔT ÷ 3600 kWh. Divide by the heat pump’s heat output (input kW × COP, not input alone) and you get hours. A 1 kW unit at COP 4.5 is putting out 4.5 kW of heat.
Where this stops and we start
This gets you in the right ballpark before anyone quotes you, which is the point — it’s hard to tell whether a salesperson has oversized you if you have no reference. What it can’t account for is pipe run length and dead-leg volume, whether you have a bath to fill, simultaneous draw-off from two bathrooms, and how cold your mains genuinely runs in July at your address. We check all of that on the site visit.
What size heat pump hot water system do I need?
As a rough guide: 1-2 people 170-200 L, 3-4 people 250-280 L, 4-5 people 300-315 L, 6+ people 400 L. But household size alone is a poor guide — shower length and flow rate matter more. Two long showers at 15 L/min will out-draw four short ones at 9 L/min. The calculator above sizes off actual draw-off rather than a bedroom count.
Why does a 9 L/min shower only use about 5 L of hot water?
Because a shower is a mix. You store water at 60 °C (legally required to control Legionella) but shower at about 41 °C, so the tempering valve blends in cold mains at roughly 15 °C. At those temperatures around 58% of the flow comes from the tank and 42% is cold — so a 9 L/min head draws about 5.2 L/min of stored hot water and 3.8 L/min of cold. That's why a 270 L tank serves far more showering than 270 ÷ 9 would suggest.
How long does a heat pump take to reheat a full tank?
Between 3 and 6 hours for a typical residential unit heating a full tank from cold mains to 60 °C. A 1 kW compressor at COP 4.5 delivers 4.5 kW of heat, which lifts about 315 L through a 45 °C rise in roughly 4.4 hours. Heat pumps are deliberately slow and steady — that's how they hit a COP of 4+ rather than the 1.0 an electric element manages.
Can you only use 80% of the tank?
Roughly, yes. Hot water stratifies — it sits in a hot layer on top with cooler water below. As you draw off the top, cold mains enters the bottom and the boundary rises. By the time you're into the last 20% the outlet temperature has dropped enough that it no longer counts as usable hot water. All sizing should be done on usable capacity, not the number on the badge.
What if two people shower at the same time?
It halves your effective capacity and doubles the draw rate, which is the situation that most often catches out an undersized tank. If simultaneous showers are normal in your house, tell us at the quote and we'll size up a step — the extra cost of the bigger tank is far less than the cost of swapping it out later.
Want us to check the sizing properly?
Free site visit, fixed quote back in 2 business hours, VEU rebate already applied.