Peak draw is the largest simultaneous hot water demand a system must serve — the busiest half hour of showers, baths and taps running together. It, not daily consumption, dictates cylinder size and coil power. As a working rule, allow roughly 50 litres of stored hot water per simultaneous shower; a home whose three bathrooms are used at once must store for that overlap, not for the daily average.
Why does peak draw matter more than daily volume?
Two households can use an identical 200 litres of hot water per day and need completely different cylinders. If those litres are spread across the day, a modest store recovers between draws and never runs out. If they concentrate into one morning rush — three showers, a bath and a kitchen tap inside forty minutes — the store must hold nearly the whole demand at once, because no realistic heat source reheats fast enough mid-rush. Sizing to the average is the classic mistake behind "we run out every morning": the cylinder was never wrong about the day, only about the half hour that matters. The full sizing method for larger properties is in What size cylinder does a multi-bathroom property need?
How do you estimate peak draw?
Add up the worst realistic overlap, fixture by fixture. Approximate storage demand per event, expressed as litres of stored water at cylinder temperature (later blended down at the tap):
| Draw event | Approx. stored hot water |
|---|---|
| Shower (8–10 min, standard head) | 40–50 L |
| Bath | 60–80 L |
| Basin handwash | 3–5 L |
| Kitchen sink / washing-up | 10–20 L |
A two-bathroom family peak of two showers plus kitchen use therefore needs roughly 110–120 litres of usable hot water in the peak window, before any recovery contribution. Note the word usable: because of mixing at the cold feed and the dead volume below the coil, a cylinder delivers only part of its nameplate litres at full temperature — a good store with strong stratification keeps that fraction high, as explained in How much usable hot water does a cylinder actually give?
How do recovery rate and peak draw interact?
Storage covers the rush; recovery rate decides how soon the next rush can start. A high-power coil fed by a boiler can partially regenerate the store during the peak itself, letting a smaller cylinder survive a long demand sequence, while a heat pump reheats more slowly and at lower temperature — pushing the design toward more stored litres and larger coil surface instead of raw power. This trade is the practical reason heat pump cylinders are bigger than boiler cylinders for the same household. The wrong balance shows up as either morning shortfalls or an oversized tank paying standing losses all year.
How is peak draw handled in hotels and commercial buildings?
By statistics rather than addition. A 60-room hotel never sees all 60 showers running, so designers apply diversity: demand data or national sizing methods (such as the summation methods in EN 12831-3) estimate the probable peak, typically concentrated into a sharp morning and evening spike. Storage-led designs hold a large stratified volume charged overnight; generation-led designs pair smaller stores with high recovery power. Solar thermal fits the storage-led pattern naturally — collectors charge a large store through the day for the evening peak — which is why hotels are a classic solar application; Solimpeks supplies exactly this combination of collectors with large stratified stores for such projects.
What happens if peak draw is underestimated?
The store empties mid-peak, delivery temperature collapses, and — with a thermostatic mixing valve fitted — flow at the taps effectively turns cold until the source recovers. Chronic underestimation also drives harmful workarounds: turning the store thermostat far up to stretch the litres raises scald risk and losses. Signs your cylinder is undersized (or oversized) are catalogued in How do I know if my hot water cylinder is the wrong size?
Frequently asked questions
How many litres of hot water does one shower use?
A typical 8–10 minute shower draws roughly 40–50 litres of stored hot water once cold blending at the mixer is accounted for. Longer showers and high-flow rain heads push the figure up substantially.
Is peak draw the same as daily hot water consumption?
No. Daily consumption is the total over 24 hours; peak draw is the largest simultaneous or back-to-back demand, usually a morning or evening window. Cylinders are sized primarily to the peak, with recovery rate covering repetition.
What size cylinder do I need for three bathrooms?
Count the realistic worst overlap, at roughly 50 stored litres per simultaneous shower plus bath and kitchen events, then check usable volume rather than nameplate volume. Multi-bathroom homes commonly land in the 250–400 L class; the property-specific method matters more than any single figure.
Does a bigger cylinder solve every peak draw problem?
No — oversizing buys standing losses and slow solar or heat pump reheat of a large volume. The better lever is balance: enough usable storage for the true peak, adequate recovery power for repetition, and good stratification so the stored litres are actually deliverable.