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Hot Water Calculator (reheat times for heat pumps)

By January 13, 2026February 13th, 20262 Comments

Hot Water Calculator for Heat Pumps

This hot water calculator estimates three things for a cylinder reheat:

  • Heat output (kWh) added to the water

  • Electrical input (kWh) drawn from the meter (using COP)

  • Time taken (hh:mm) based on your heat pump’s thermal power during DHW

If you just want the calculation, you’re in the right place.

If you want the full context on Vaillant DHW settings (schedules, hysteresis, modes, anti-legionella and why reheats can take longer than you expect), the main guide goes into this properly: Vaillant Arotherm Basic Hot Water Settings

This calculator forms part of my Heat Pumps Explained series, which covers how heat pumps work, how they’re sized, and how to interpret performance and running costs, including examples from my own 5 kW Vaillant aroTHERM heat pump installation.

Also check out further Free Heat Pump Tools and calculators.

Hot Water Calculator

Hot Water Calculator

Enter your starting and target temperatures, the litres you’re heating, your heat pump’s DHW power output and the COP of the run.

The calculator uses:

  • Heat (kWh) = Litres × ΔT × 0.001163

  • Electrical (kWh) = Heat ÷ COP

  • Time (hours) = Heat ÷ thermal power output

Defaults are 200 L, 10°C → 50°C, COP 3.0, 5 kW.

Hot Water Calculator

Enter your values. The calculator assumes water is 1 kg/L and specific heat is 4.186 kJ/kg·°C. 1 kWh = 3,600 kJ.

Used only to estimate time. This is thermal output. Electrical kWh still = heat ÷ COP.

Electrical input

–

kWh drawn from the meter

Heat output

–

kWh of heat delivered to the water

Time taken (hours:mins)

–

hh:mm using thermal power output

COP check: Heat ÷ Electrical = – (should match the COP you entered)

Show formulas
ΔT = Target − Start
Heat (kWh) = Litres × ΔT × 0.001163
Electrical (kWh) = Heat ÷ COP
Time (hours) = Heat ÷ Thermal power output

Time in minutes to heat water (from 10C to 50C)

3kW 5kW 7kW 9kW 10kW 12kW 14kW 16kW
50L 47 28 20 16 14 12 10 9
100L 94 56 40 32 28 24 20 18
150L 140 84 60 48 42 36 30 27
200L 187 112 80 64 56 48 40 36
250L 233 140 100 80 70 60 50 45
300L 280 168 120 96 84 72 60 54

What this calculator is useful for

You might use a hot water calculator if you’re:

  • Estimating reheat time after showers or baths

  • Comparing DHW performance between Eco / Normal / Balance / Sweet Spot style modes (if you have these available to you on your heat pump)

  • Working out roughly how many kWh a reheat costs on your tariff

  • Sanity-checking whether a long reheat is “normal” for your cylinder size and target temp

This page focuses purely on the calculation.

Want the full explanation?

This calculator is just one small part of the picture.

The main Vaillant guide covers the stuff that actually affects real-world hot water results, like:

  • How Vaillant hot water reheats work (and why flow temp rises during the run)

  • Why hot water takes priority and pauses space heating

  • Scheduling choices (including time-of-use thinking)

  • Cylinder charging hysteresis and why quick top-ups are expensive

  • Anti-legionella behaviour

  • Recommended baseline settings and common gotchas

Read the full guide here: Vaillant Arotherm Basic Hot Water Settings

If you also want to go deeper on the broader “what makes heat pump hot water cheap and reliable” angle, this one is the companion piece: Best Heat Pump Hot Water Settings (The Golden Rules)

Calculator walkthrough on YouTube

In this video I talk through the calculator and add some commentary and context around it.

Other useful hot water articles

If you want more detail on how heat pumps and hot water work in real homes, these articles could help:

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Mick Wall

Mick Wall

Mick Wall runs Energy Stats UK, where he shares independent data and real-world insights from his own Sheffield home. By tracking solar, battery storage, and heat pump performance, Mick helps cut through the myths and highlight what really works in the UK’s shift to low-carbon heating.

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