Renewables
Solar Water Heating Sizing Calculator
Size solar thermal collector area and cylinder
Updated July 12, 2026 · Live
What this tool does
Sizes the solar thermal collector area and hot-water cylinder from household size, hot-water use, temperature rise, collector efficiency and sun hours.
Enter household size, hot-water use per person, the temperature rise, collector efficiency and sun hours; the calculator returns the collector area and a cylinder size.
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Formula Used
How the solar water heating sizing works
The daily hot-water volume is the household size times the per-person use. The energy to raise that volume by the temperature rise comes from the specific heat of water — litres times 4.18 kJ times the rise, divided by 3600 to reach kWh. Dividing that daily heat demand by the usable sun and the collector efficiency gives the collector area, and the cylinder is sized to about a day’s hot water.
What you enter
Enter household size, hot-water use per person, the temperature rise, collector efficiency and sun hours; the calculator returns the collector area and a cylinder size. Every field is editable, and the result recalculates the moment you change a value.
Reading the result
The collector area and cylinder size are a starting specification for a solar thermal system. Solar water heating typically covers most summer hot water and a share of winter, with the existing boiler or immersion topping up.
Safe by design
This calculator does the arithmetic of sizing — quantities, geometry and cost. It deliberately stops at the line a certified installer must sign off. It will never tell you a cable or fuse size, an earthing arrangement, a battery charge voltage, or a roof structural load: those are regulated calculations where a wrong number causes fire, electrocution or collapse. All electrical work must be carried out by a qualified, certified installer, and this page is not an installation guide.
Assumptions and limits
Nothing time-sensitive is baked in. Electricity prices, export tariffs and local peak sun hours all change and differ by location, so the calculator asks you to enter your own figures rather than embedding a database that would silently go stale. It sizes for an average day; a run of dull days leans on the back-up heater. Collector efficiency varies with temperature and type, and the cylinder rule of thumb is a starting point a specifier will refine.
Using it with the rest of BuildMetricLab
This calculator is one step in planning a system. Pair it with the other solar sizing, geometry and cost tools to go from an energy figure to a panel count, a layout and a costed bill of materials. Every BuildMetricLab tool runs entirely in your browser — no sign-up, nothing sent to a server, and the formula is shown on the page so the maths can be audited.
Sources & methodology
Sizes the solar thermal collector area and hot-water cylinder from household size, hot-water use, temperature rise, collector efficiency and sun hours. Every result is calculated from the values you enter, all inputs are editable, and no time-sensitive prices or tariffs are embedded.
Frequently asked questions
How much hot water does a person use?
Around 40–50 litres a day of hot water per person is typical for baths, showers and taps, though it varies widely by household.
What temperature rise should I enter?
It is the hot-water target minus the cold mains feed — often around 30–40°C. Enter it as a difference, not an absolute temperature.
Will solar cover all my hot water?
It typically covers most summer demand and a share in winter, with the boiler or immersion topping up when the sun is weak.
Flat plate or evacuated tube?
Evacuated tubes are usually more efficient in cold, dull conditions; flat plates are cheaper and robust. Enter the efficiency of the type you are pricing.
Spotted something off?
Calculations or display — let us know.