Standing loss is the rate at which a hot water storage tank loses heat while held at operating temperature, expressed in watts and measured with the store at 65 °C in a 20 °C room. Under EU Regulation 812/2013 it sets the tank's ErP energy label class: a 300-litre class B tank may lose no more than about 70 W — roughly 1.7 kWh per day.
How is standing loss measured?
Standing loss quantifies the heat a tank leaks through its insulation while simply standing full and hot, with no water drawn. In the standardised test the store is held at 65 °C in a 20 °C ambient — a 45 K difference — and the average power needed to maintain that temperature over 24 hours becomes the standing loss figure S in watts. EN 12897 defines the test procedure for indirectly heated storage water heaters, and EU Regulation 812/2013 puts the resulting number on the energy label of every tank sold in Europe.
A watt figure is easy to translate: 70 W of standing loss equals about 1.7 kWh of heat per day, or roughly 600 kWh per year — energy the heating system must replace before a single litre reaches a tap.
How does standing loss set the ErP label class?
Since 26 September 2017, hot water storage tanks carry energy label classes from A+ down to F, and the class boundaries are pure functions of storage volume V in litres:
| Class | Maximum standing loss (W) | 300 L example |
|---|---|---|
| A+ | 5.5 + 3.16 · V^0.4 | ≈ 36 W (0.9 kWh/day) |
| A | 8.5 + 4.25 · V^0.4 | ≈ 50 W (1.2 kWh/day) |
| B | 12 + 5.93 · V^0.4 | ≈ 70 W (1.7 kWh/day) |
| C | 16.66 + 8.33 · V^0.4 | ≈ 98 W (2.4 kWh/day) |
The classes reward insulation, not volume — which is why a well-insulated 500 L buffer tank can hold a better class than a poorly insulated 200 L cylinder.
How much does standing loss cost per year?
The spread between classes is larger than most buyers expect. For a 300 L tank, class C allows 98 W while class A+ allows 36 W — a 62 W difference that compounds to roughly 540 kWh per year. Reheated electrically at €0.30/kWh that is about €160 every year; reheated by a heat pump with a seasonal COP of 3 it still costs €50–55. Over a 20-year tank life, insulation class quietly outweighs most purchase-price differences, particularly for tanks standing in unheated rooms where every lost kilowatt-hour is truly gone.
How do you reduce standing loss in practice?
Four levers matter most:
- Insulation thickness and material — 50–100 mm of PU foam is the standard; vacuum insulation panels push into class A+ territory.
- Connection discipline — uninsulated ports and single-pipe thermosiphoning through connected pipework can rival the tank's own losses; heat traps and siphon loops stop them.
- Lower set temperature — where hygiene allows, every 5 K less cuts losses by roughly 10%; a hygienic tank that stores only heating water enables exactly this without legionella risk.
- One large tank instead of two small — surface area per litre falls as volume grows, which the V^0.4 formulas reflect.
Solimpeks publishes the ErP energy class and measured heat loss of every Solibuffer model (50–1,000 L) in its catalogue and on its product pages.
Will the tank energy label change soon?
A revision is in motion. The European Commission ran four parallel consultations from November 2025 to January 2026 on replacements for the 2013 ecodesign and labelling regulations, with a rescaled A–G label, EPREL QR codes and an expected ~60 TWh per year of final energy savings by 2030 across the heating product groups. Adoption was foreseen for 2026, so check the current official status before relying on class names in specifications — the measured watt figure, unlike the letter, will remain the durable comparison number. The wider label system is covered in the EU energy label guide.
Frequently asked questions
What is a good standing loss for a 200-litre cylinder?
For 200 L the class B limit works out near 61 W (about 1.5 kWh/day) and class A near 44 W.
Do buffer tanks have energy labels too?
Yes. Regulation 812/2013 labels hot water storage tanks up to 500 litres, and the 814/2013 ecodesign maximum applies to vessels up to 2,000 litres — buffers included. Larger commercial vessels fall outside the label but are still specified by their measured standing loss.
Is a bigger tank always worse for heat loss?
In absolute watts yes, but per litre no: surface area grows more slowly than volume, so one 500 L tank loses less than two 250 L tanks of the same insulation standard. The V^0.4 term in the class formulas encodes exactly this geometry.
Does standing loss include pipe and circulation losses?
No — the test isolates the tank itself. Real installations add connection losses and, above all, DHW circulation losses, which in continuously pumped systems often exceed the tank's own standing loss several times over.