What size hot water cylinder do I need?

1. Cylinder size vs. recovery rate

A hot water cylinder stores a fixed volume of hot water. When that store is exhausted, you wait for the boiler, heat pump or immersion heater to reheat it — the recovery time depends entirely on the heat source, not the cylinder size. A 150-litre cylinder on a 20 kW boiler coil reheats in roughly 30–45 minutes; the same cylinder on a 2 kW immersion might take 3–4 hours. Two things to size together, not separately.

The sizing tables below give the cylinder volume. The heat source spec (boiler kW output, heat pump COP and rated output, or immersion element wattage) determines whether that volume recovers in time for the next morning peak. If in doubt, go one size up and let the heat source set the practical ceiling.

2. Sizing by number of people

The table below shows typical cylinder sizes by household, split by heat source. These are industry norms — not code minimums — and assume normal usage (one shower per person per day, plus typical kitchen use).

3. Heat source makes a real difference

A gas boiler coil can transfer heat to the cylinder at 15–25 kW or more, which means a 150-litre store can recover to full temperature in under an hour. A 1.8–3 kW electric immersion heater takes 3–5 hours for the same volume. Heat pumps land in between — typically 2–3 kW of useful heat output for hot water, so recovery can take 2–4 hours. If your home relies on an immersion or heat pump, size the cylinder generously (one step up from the table) or run a timed off-peak charge to use cheap-rate electricity and keep the cylinder full for the morning peak.

4. How to size yours in three steps

1. Count the people and note the fuel type. Use the table in Section 2 to find the recommended range for your household. Gas units can use a smaller tank; electric units need more storage to cover the same morning rush.
2. Check the First Hour Rating on the label (or ask your installer for the cylinder's standing-heat-loss and heat-up time). The FHR tells you how much hot water the cylinder delivers in its busiest first hour — not just what it stores.
3. Add a usage cushion if your habits are heavy. Back-to-back long showers, a large bath, a household that runs the dishwasher during the morning peak — any of these push you toward the high end of the range. When in doubt, choose the larger size; the cost difference between a 50-gallon and a 60-gallon tank is modest, and running out of hot water is genuinely unpleasant.

Or just use the calculator: Hot water cylinder size calculator → Enter your household size and fuel type and it returns the recommended cylinder capacity and FHR target directly.

5. Small hot water cylinders: point-of-use and under-sink

Under-sink water heaters (also called local water heaters, or 'over-sink' units in Australia) store 3–15 litres (1–4 gallons) and exist to cut the wait for hot water at a single tap, not to supply a shower or bath. A typical shower draws 40–60 litres (10–16 gallons) of hot water, which is several times the capacity of an under-sink unit. These small heaters are useful extras but cannot replace the main whole-house unit.

For tankless (on-demand) water heaters — also called combination boilers or 'combi boilers' in the UK, sizing is by flow rate in gallons per minute (GPM) or litres per minute, not by tank volume. Combi boilers and inline instantaneous heaters are sized by simultaneous outlet demand, not storage.

6. Common sizing mistakes

• Replacing like-for-like without checking usage. If the old unit never kept up, replacing it with the same size guarantees the same shortfall. Take the opportunity to size correctly.
• Ignoring fuel type. Gas and electric units of the same nominal capacity deliver very different amounts of hot water per hour. A 40-gallon gas unit often beats a 50-gallon electric unit over the morning rush.
• Going too big. An oversized cylinder keeps a large volume of hot water warm around the clock, wasting energy on standby losses. Match the size to your actual demand — not to your maximum imaginable demand.

Common questions

What size hot water cylinder do I need for a family of four?

A 150–180 litre indirect cylinder is the typical choice for a family of four in the UK, Australia and New Zealand. If your home has high morning demand — back-to-back showers — err toward the upper end. The cylinder's actual heat-up rate (recovery) is set by the boiler or heat pump output, so a larger cylinder with adequate heat input will keep up better than a small one on a powerful boiler.

What size hot water cylinder do most homes use?

A 150-litre indirect cylinder is the most common choice in UK, Australian and New Zealand homes with a central heating boiler. Homes with a heat pump, solar thermal, or off-peak electric immersion heater often go larger — 180–250 litres — because those heat sources take longer to recover.

Is a small 7-litre water heater enough for a shower?

No. A single shower typically uses 40–60 litres (15–20 gallons), so a 7-litre under-sink unit runs out in the first minute. Small under-sink heaters exist only to cut the wait at a remote tap — they are not shower heaters.

Should I buy a 150 litre or 210 litre cylinder?

150 litres suits most households of 3–4 people with a gas boiler. Choose 210 litres if you have 5 or more people, rely on slower heat sources (heat pump, solar, off-peak electric), or regularly run multiple showers and baths simultaneously.