What size heat battery do I need? Start with demand

Size a heat battery for the heat it must deliver between charges, not the number of bedrooms alone. Household count is a starting clue; showers, baths, flow rate, space-heating demand and the charging window decide the result.

The short answer

For hot water, estimate daily useful heat and peak flow demand separately. For space heating, use a room-by-room heat-loss calculation and a clear operating schedule. Then compare those needs with independently verified or maker-published output data.

The helper above gives a rough discussion figure. It is intentionally not a model recommendation: our product dataset remains empty until capacities and test conditions are verified from primary documents.

Capacity and power are not the same

Capacity, measured in kWh, says how much energy can be stored. Power, measured in kW, says how quickly it can charge or release heat. A store can have enough total energy yet fail to deliver the flow rate a household expects.

Questions that change the size

• Are showers supplied at the same time?
• Is there a large bath or high-flow rain shower?
• How cold is the incoming mains water in winter?
• Can the unit recharge between morning and evening demand?
• Must it supply hot water only, space heating only, or both?
• What happens when stored heat runs out?

Avoid false precision

Cylinder-equivalent litre claims depend on temperatures and test methods. Ask for the assumptions behind the equivalence and compare useful energy at your required outlet temperature.

Leave space for real life

An occasional guest does not necessarily justify a permanently oversized store. A predictable second charge window may be cheaper. Discuss normal demand and the busiest credible day with the designer.

Your next step

Keep a seven-day hot-water diary: shower duration, baths, simultaneous use and times of day. Give it to the installer alongside your tariff schedule and measured space.