How much heat does a hot water cylinder lose per day?

How do I convert standing loss in watts to kWh per day?

Multiply the watts by 24 hours and divide by 1,000: kWh/day = W × 24 ÷ 1,000. The standing loss figure on a cylinder's datasheet or ErP label is a continuous leak rate measured with the tank held hot, so a 54 W tank loses 54 × 24 ÷ 1,000 ≈ 1.3 kWh every day it stays at temperature — heat the boiler, heat pump or solar circuit must replace before a single litre is drawn. Over a year that single small tank leaks around 470 kWh, which is why the watts on the label matter more than they look.

How much does a real cylinder lose per day?

Declared manufacturer data makes the scale concrete. The table converts the standing losses of the Solimpeks Solibuffer range — a representative set of insulated steel tanks from 50 to just over 1,000 litres:

Notice the geometry at work: twenty times the volume loses only four times the heat, because surface area — not volume — drives the loss. Per stored litre, large tanks are far more efficient, which is one argument for a single big store over several small ones. Which class those watts earn is explained in which cylinder energy class to buy.

What determines how much heat a cylinder loses?

Three factors set the loss: insulation, temperature difference, and surface area. Insulation dominates — 50 mm of polyurethane foam roughly halves the loss of a thin 25 mm jacket, and each further increment helps less than the last. The driving force is the gap between stored water and room temperature: a tank held at 65 °C in a 20 °C room loses roughly a third more than the same tank at 55 °C, which is why heat pump systems that store cooler water also leak less. Finally, every uninsulated fitting counts: bare connections, an unlagged first metre of pipework and open thermosiphon loops can add tens of watts on top of the declared figure, since the label value is measured on the bare tank under EN test conditions.

How can I cut the daily loss?

Four measures, in order of payback. First, lag the pipes: the first metre of hot pipework off the tank continuously siphons heat and is the cheapest fix. Second, add a jacket to an older cylinder — topping up to 80 mm of total insulation is the classic sub-one-year payback measure. Third, review the set temperature: storing at 60 °C rather than 70 °C cuts the loss meaningfully, though anti-legionella requirements set the floor. Fourth, fit non-return valves or heat traps on connections to stop thermosiphoning. Whether to let the tank cool between uses is a separate calculation — covered in is it cheaper to leave the hot water on all day — but for a well-insulated tank that stays hot for 24 hours or more, reheat timing matters less than pipe lagging.

Is the lost heat really wasted?

Partly. A cylinder standing inside the heated envelope returns its loss to the house exactly when heating is running, so in winter perhaps half the standing loss offsets space heating. In summer, and in unheated garages or plant rooms, the loss is a pure write-off — and in cooled buildings it becomes a double cost. With water heating accounting for roughly 15% of EU household energy consumption (Eurostat), a daily 2 kWh leak is a permanent line item on the bill regardless of the tariff you pay; the exact money depends on your local price per kWh, but the kilowatt-hours themselves are not negotiable.