How long does a hot water cylinder take to heat up? Times by size and element

Heat-up times worked out for 25 to 300 litre cylinders with 2, 2.4, 3 and 3.6 kW elements, why yours may take longer, and what a heat pump does differently.

Last reviewed 2026-10-28

A cylinder's heat-up time comes down to two numbers: how much water, and how big the element is. Heating water from 15°C to 65°C takes 0.058 kWh per litre, so a 180 litre cylinder needs about 10.5 kWh. Divide by the element size and you have the time.

Heat-up time from cold, by size and element

Cylinder	Energy (15 to 65°C)	2 kW	2.4 kW	3 kW	3.6 kW
25 L	1.5 kWh	45 min	35 min	30 min	25 min
50 L	2.9 kWh	1.5 h	1.2 h	1.0 h	50 min
80 L	4.7 kWh	2.3 h	1.9 h	1.6 h	1.3 h
135 L	7.8 kWh	3.9 h	3.3 h	2.6 h	2.2 h
180 L	10.5 kWh	5.2 h	4.4 h	3.5 h	2.9 h
250 L	14.5 kWh	7.3 h	6.1 h	4.8 h	4.0 h
300 L	17.4 kWh	8.7 h	7.3 h	5.8 h	4.8 h

Worked out from the energy needed, with no losses. A real cylinder takes a little longer.

These are the physics with no losses. Real cylinders take a little longer, and the element size is on the cylinder's data plate.

A close photograph of the data plate on the side of a hot water cylinder, the label stating capacity, pressure rating and date of manufacture. — The data plate on the side of the cylinder gives its capacity and element rating.

Why yours takes longer than the table

Controlled or night rate power. Many New Zealand cylinders are on a tariff where the lines company switches them off at peak times. The cylinder only heats in the hours it has power.
Colder incoming water. In winter the cold supply can be well under 15°C, which adds energy to every litre.
A failing element or thermostat. If a cylinder that used to recover quickly now takes all day, or never gets hot, the element or thermostat is the usual cause. That is electrical work.
A cylinder too small for the house. If the hot runs out mid-shower every day, it may simply be undersized: see what size cylinder do I need.

Heat pump cylinders

A monoblock heat pump hot water cylinder standing on pavers beside a New Zealand house, with the fan and compressor built into the top of the tank and clear space around it for airflow. — A heat pump cylinder heats with a fan and compressor on top instead of a big element.

A heat pump heats more slowly than a big element but uses far less power for each litre. Haier rate the HP250M1U1 at 42 litres per hour of recovery. Some heat pumps also have a backup element for very cold mornings. Compare them in heat pump cylinders.

Small cylinders heat fast

A 25 litre underbench unit with a 2.4 kW element, like the Thermann 25L plug-in ($740), is hot again in about half an hour. See small cylinders.

Ready to pick one?

All our cylinders are listed with the price on them.

See the range