Buy class B or C: a cylinder's ErP class is set purely by standing loss, and at 200 L class C allows at most 86 W of continuous heat leakage (about 2.1 kWh per day) versus 61 W for class B. Pay for class A or B only where the cylinder is heated by direct electricity or stands outside the heated envelope.
What does the ErP energy class on a hot water cylinder mean?
The class measures exactly one thing: standing loss — the watts of heat the tank continuously leaks to the room while holding hot water at test conditions. Under EU Regulation 812/2013, hot water storage tanks are graded from A+ down to G on this single number; the class says nothing about coil size, recovery speed or build quality. The label is mandatory for tanks up to 500 litres, and larger tanks still publish a declared standing-loss figure and class in their datasheets.
Because the loss happens 24 hours a day, 365 days a year, small differences in watts compound: 25 W of extra standing loss is roughly 220 kWh over a year — see how much heat a cylinder loses per day for the full conversion.
How is the cylinder energy class calculated?
Each class is a formula, not a fixed number: the allowed standing loss S in watts grows with storage volume V in litres, scaled by V^0.4 so that bigger tanks get a larger — but not proportionally larger — allowance.
| Class | Maximum standing loss S (W) |
|---|---|
| A+ | S < 5.5 + 3.16 × V^0.4 |
| A | S < 8.5 + 4.25 × V^0.4 |
| B | S < 12 + 5.93 × V^0.4 |
| C | S < 16.66 + 8.33 × V^0.4 |
| D | S < 21 + 10.33 × V^0.4 |
| E | S < 26 + 13.66 × V^0.4 |
| F | S < 31 + 16.66 × V^0.4 |
| G | S ≥ 31 + 16.66 × V^0.4 |
Worked example at 200 L (V^0.4 ≈ 8.33): class A demands under 44 W, class B under 61 W, class C under 86 W and class D under 107 W. The companion Ecodesign Regulation 814/2013 sets minimum performance requirements for water heaters and hot water storage tanks.
What class do real tanks actually achieve?
Manufacturer data shows the realistic pattern. The Solimpeks Solibuffer range of buffer tanks declares 54 W at 50 L, 62 W at 100 L, 74 W at 170 L and 85 W at 245 L — all class C — rising to 140 W (class D) at 460 L and 195–220 W (class E) at 850–1,030 L. That is typical for well-insulated steel tanks: small and mid-size domestic cylinders land in class B or C, while large commercial stores drift to D and E simply because surface area grows faster than the V^0.4 allowance. Note the efficiency of scale in the same data: twenty times the volume leaks only four times the heat, so one large store beats several small ones.
When is a class C cylinder the rational choice?
Class C is rational whenever the heat filling the tank is cheap or the loss is partly recovered. At 200 L the B-to-C gap is about 25 W, or 0.6 kWh per day. If a heat pump charges the cylinder at a COP of 3, the electrical cost of that gap shrinks to a third; if solar thermal covers summer demand, the marginal heat in the sunny half of the year is essentially free; and a cylinder standing inside the heated envelope returns much of its winter loss to the rooms around it. Against that, A-class insulation — vacuum panels or very thick casings — carries a real price and size premium that may never pay back. A C-class tank that stays hot for a day or more is not a design failure.
When should I pay for class A or B?
Pay for a higher class when every lost watt is billed at full price: cylinders heated by direct electric immersion, tanks standing in unheated garages, lofts or plant rooms, large volumes where the absolute watts are high, and systems that hold temperature around the clock for a recirculation loop — a duty examined in how a secondary return should be controlled. In warm climates the case doubles, because summer standing loss becomes a cooling load. With water heating around 15% of EU household energy use (Eurostat), the class you choose sets a bill you pay every single day the tank is hot.
Frequently asked questions
What is a good energy class for a hot water cylinder?
Class B or C is the normal range for a quality insulated cylinder up to about 300 L; class A is rare and usually requires very thick or vacuum insulation. Large buffer stores above 500 L commonly carry class D or E because surface area grows faster than the allowance formula.
Does the ErP class include the coil or immersion heater efficiency?
No. For a storage tank the class reflects standing loss only — the heat leaked while holding temperature. Coil surface area, recovery rate and heat-source efficiency are separate specifications and are not visible on the label.
Is a bigger tank always a worse energy class?
Not necessarily, because the allowance rises with volume to the power 0.4. In practice small and mid-size tanks reach class B–C while very large stores settle at D–E; per stored litre, however, big tanks lose far less heat than small ones.
How much energy does class B save over class C?
At 200 L the maximum-loss difference is roughly 25 W, which works out to about 0.6 kWh per day or 220 kWh per year. Whether that justifies the purchase premium depends on how the tank is heated and where it stands.