A 200 litre cylinder stored at 60 °C delivers about 300 litres of usable water at 40 °C, because every stored litre is blended with roughly 0.67 litres of cold at the tap. That is the V40 ('mixed water at 40 °C') figure defined in the EU ecodesign rules for water heaters (Regulation 814/2013). Expect 1.4–1.6 times the tank volume from a well-stratified cylinder, and as little as 1.1 times from a badly mixed one.
How much hot water is actually usable?
More than the label on the tank, because nobody showers at 60 °C. Blending 60 °C stored water down to a 40 °C shower with 10 °C mains means each litre from the cylinder becomes 1.67 litres at the tap: the hot fraction is (40 − 10) ÷ (60 − 10) = 0.6.
Real cylinders do not reach the theoretical figure, because the last of the stored water arrives cooler as cold mains fills in behind it. A good tank delivers 1.4–1.6 times its nominal volume at 40 °C. A tank whose layers have been stirred together delivers barely more than its own volume.
| Nominal volume | Usable at 40 °C (V40) | 8-minute showers at 9 L/min | Baths at 90 L |
|---|---|---|---|
| 120 L | 170–190 L | 2 | 2 |
| 150 L | 210–240 L | 3 | 2 |
| 200 L | 280–320 L | 4 | 3 |
| 250 L | 350–400 L | 5 | 4 |
| 300 L | 420–480 L | 6 | 5 |
Storing at 50 °C instead of 60 °C cuts the multiplier from about 1.5 to about 1.15, so a 200 litre tank drops from roughly four showers to three. That is the real cost of a low storage temperature, and the reason heat pump cylinders are specified one size up in what size hot water tank you need.
What is V40 and why is it on the datasheet?
V40 is the volume of mixed water at 40 °C that a cylinder can deliver — its hot output blended down to 40 °C — from a fully charged, fully heated start. It is defined in the EU ecodesign regulation for water heaters (Regulation 814/2013), and it is the only number that lets you compare two tanks fairly.
It is worth more attention than nominal volume, because two 200 litre cylinders can differ by 60 litres of V40 depending on how the cold feed is diffused, where the coil sits and how tall the vessel is. When a datasheet quotes only litres, ask for the V40.
Why does stratification decide the answer?
Because hot water floats. A cylinder that has been left alone holds a hot upper layer over a cooler lower one, separated by a thermocline perhaps 100–150 mm thick. Draw water off the top and the hot layer is replaced from below without mixing, so the outlet stays at full temperature until the thermocline reaches it. That is stratification, and it is what turns 200 stored litres into 300 usable ones.
Four things destroy it: a cold feed entering without a diffuser, an oversized secondary return pump circulating continuously, a coil positioned high in the tank, and a squat vessel with a height-to-diameter ratio below about 2:1. Any of them can strip a third off the usable output without changing the standing loss by a single watt.
How can I tell how much hot water is left?
There is no gauge on most cylinders, but there are three reliable proxies:
- Feel the tank down its height, through a gap in the insulation or at the sensor pockets. The point where it turns from hot to warm is the thermocline; everything above it is usable.
- Read the cylinder thermostat. It is normally mounted about a third of the way up. When it drops below setpoint, roughly the bottom third is already cold — which is exactly the point at which reheating should start.
- Fit a second sensor at the top and one at mid-height. Two temperatures give a usable state-of-charge reading for a few euros of parts, and most solar and heat pump controllers already have a spare input.
If the tank feels uniformly lukewarm rather than hot at the top and cold at the bottom, that is the mixing problem, not a heat problem — and it is diagnosed differently from a tank that has simply gone cold, which is the standing loss question.
Frequently asked questions
How many showers can you get from a 300 litre tank?
About six 8-minute showers at 9 litres a minute, because a 300 litre cylinder at 60 °C yields roughly 420–480 litres of blended water at 40 °C. Store the same tank at 50 °C and it falls to around four showers.
What does V40 mean on a cylinder datasheet?
V40 is the volume of mixed water at 40 °C a cylinder can deliver from one full charge — its hot output blended down to 40 °C — as defined in EU Regulation 814/2013. It is the fairest comparison between two tanks, because it captures stratification and cold-feed design as well as raw volume.
Why does my hot water run out sooner than the tank size suggests?
Almost always mixing rather than heat loss. A cold feed without a diffuser, a running secondary return pump or a coil set too high stirs the hot and cold layers together, so the outlet temperature falls long before the stored energy is used.
Does storing at a lower temperature reduce usable hot water?
Substantially. Dropping from 60 °C to 50 °C cuts the blend multiplier from about 1.5 to about 1.15, so a 200 litre cylinder loses roughly one shower's worth of capacity. That is why heat pump cylinders are usually sized one step larger than boiler cylinders.