# How big should your hot-water cylinder be? > Size the storage from your busiest hour and the recovery rate of the water-heating system that will serve it. [View the web page](https://heatuneed.com/hot-water-system-calculator/) ## Volume alone does not tell you whether the hot water will last A cylinder starts with stored heat and keeps adding heat while taps are open. The useful answer is therefore its first-hour supply: the mixed water already in the tank plus what its heat pump, immersion heater or boiler can recover in that hour. This tool sizes those two parts together and recommends the smallest listed cylinder that clears the peak. ## How the calculation works The model puts storage and recovery on one consistent mixed-water demand, so a fast heater can reduce the required storage without hiding a genuine busiest-hour shortfall. 1. **Build the mixed-water demand** — Occupants times litres per person gives the daily volume at the tap temperature. The busiest-hour percentage finds the back-to-back peak, then a 15 percent design allowance covers normal uncertainty. 2. **Convert stored heat into usable litres** — Water stored above the delivery temperature is mixed with cold water. The temperature-rise ratio converts nominal cylinder litres into mixed litres, after allowing 10 percent for unusable volume and imperfect stratification. 3. **Add one hour of recovery** — Heating output divided by the energy needed for one litre at the tap gives the litres recovered per hour. Added to storage, this is the first-hour supply used to choose the smallest listed cylinder that clears the peak. 4. **Choose the first size that passes** — Each listed cylinder is checked against the reserved busiest-hour demand. The first one whose stored and recovered litres cover that peak becomes the recommendation; the final product still needs its manufacturer-rated first-hour performance checked. ## What the calculator asks for ### Daily and first-hour demand Daily mixed hot water is the number of occupants times litres per person. The busiest-hour percentage is the share of that daily total used within the busiest 60 minutes: 120 L during the morning peak out of 200 L for the whole day is 60 percent. The sizing model then adds a 15 percent design reserve. ### Cylinder size and recovery Usable stored water is 90 percent of nominal volume, multiplied by the ratio between the stored-water temperature rise and the mixed delivery-temperature rise. One hour of heating output is converted into the same mixed-water litres and added to storage. The recommendation is the next standard volume whose first-hour supply meets the reserved peak. - 100 L - 150 L - 200 L - 250 L - 300 L - 400 L ### Water-heating system and recovery The selected system supplies a starting useful output. Heat pumps commonly recover more slowly than immersion heaters and indirect boiler cylinders, so the same household may need more stored volume. Product selection should use the manufacturer's tested first-hour rating. - **Heat-pump water heater** — 1.5 kW - **Electric immersion heater** — 4.5 kW - **Gas boiler with an indirect cylinder** — 18 kW - **Oil boiler with an indirect cylinder** — 18 kW - **LPG / propane boiler with a cylinder** — 18 kW - **Other / use manufacturer recovery output** — 6 kW ## How the result is decided The result recommends the smallest listed volume whose usable storage plus one hour of recovery meets the busiest-hour demand with the design reserve. The selected cylinder is then marked as fitting or falling short. This is an early cylinder-sizing screen, not a plumbing design. It assumes 90% of nominal tank volume is usable, a 15% peak reserve, ideal mixing and constant output. Real first-hour rating, stratification, coil performance, ambient temperature, backup elements, hygiene cycles and controls are product-specific. Verify the manufacturer's first-hour rating and local plumbing and legionella requirements with a qualified installer. ## Common questions ### Why can a smaller tank sometimes meet the same demand? Because tank size is only the stored half of first-hour supply. A heater with more thermal output replaces hot water while it is being used, and a hotter tank produces more mixed water through a thermostatic valve. Product selection should still use the manufacturer's tested first-hour rating, because real stratification and controls are not ideal. ### Should a heat-pump water heater be larger than my current tank? Often yes. Compressor-only recovery is much slower than gas or electric resistance recovery, so extra storage helps it ride through a morning peak without bringing on an inefficient backup element. The table shows that trade directly: lower the entered heat-pump output and the recommended volume rises. ### What heating output should I enter? Use the useful thermal output actually reaching the cylinder. For a heat-pump water heater, use heat-pump-only output unless you intentionally want backup resistance counted. For an immersion heater, use its element rating. For an indirect cylinder, use the lower of the available boiler output and the coil's rated transfer output. ### Does storing water hotter create free capacity? It creates more mixed litres from the same tank, not free capacity. The heater must put in more heat, standing loss rises and scald risk becomes more serious. Follow the appliance instructions and local hygiene rules, and use a correctly specified thermostatic mixing valve where required. ### How should I set the busiest-hour share? Use 100% when most of the household's showers or baths can happen back to back. Use a lower share only when hot-water use is genuinely spread across the day. If unsure, keep 100%: underestimating the peak directly undersizes the cylinder. ### What must be checked before choosing the product? Verify the manufacturer's tested first-hour rating, recovery conditions, coil rating where applicable, physical dimensions, plumbing code and legionella controls. For a heat-pump water heater, also check its air-volume, condensate, noise and electrical requirements. ## Planning the rest of the building's heat system too? Heatuneed scans rooms with LiDAR and calculates heat loss through walls, windows, doors, floors and roofs — the evidence needed to size space heating alongside the hot-water system. - [Check app availability](https://heatuneed.com/app-store/)