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Everyone who uses the hot water regularly.
The guide's rule gives the first two people the most collector each and every further person less, because a household's hot water does not grow in proportion to its size. A household with unusually heavy use — long showers, frequent baths, a large tub — is better served by a yield calculation than by a count of people.
The guide's two US bands; the northern one is given at both ends of its range.
The band only changes the collector added for each person beyond the first two. Outside the United States, and wherever the roof faces away from the sun, is shaded or is steep or flat, a yield calculation for the actual site is what decides the size — the rule here is a starting point, not a design.
Collector area
56 sq ft
Sized to cover most of a summer's hot water for 4 people, by the guide's rule.
- Solar storage for an active system
- 84 gallons
- Collector per person beyond the first two
- 8 sq ft
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Solar Water Heater Collector Size Calculator: 56 sq ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- US Department of Energy, Heat Your Water with the Sun — A Consumer's Guide (DOE/GO-102003-1824, December 2003): size to meet 90% to 100% of summer hot water needs; about 20 sq ft (2 m²) of collector for each of the first two family members, then 8 sq ft (0.7 m²) per additional person in the Sun Belt or 12 to 14 sq ft (1.1 to 1.3 m²) in the northern United States
- The same guide: for active systems the solar storage tank grows with the collector, typically 1.5 gallons per square foot of collector; a 66-gallon system usually serves one to three people, an 80-gallon system three or four, a 120-gallon system four to six
Inputs used
- People in the household
- 4
- Climate band
- Sun Belt (southern United States)
Intermediate steps
- Solar storage for an active system
- 84 gallons
- Collector per person beyond the first two
- 8 sq ft
Confidence note: Sized to cover most of a summer's hot water for 4 people, by the guide's rule.
What this calculation does not cover
- This is the contractor's rule of thumb the Department of Energy publishes, for climates in the United States. It sizes the collector to cover most of the summer's hot water, not the year's, and it does not know the roof, the collector type or the site. A solar water heating design elsewhere, or for a household that uses far more or less than usual, needs a yield calculation from the site's solar data and the collector's rating.
- The roof decides what the collector achieves. Orientation, tilt and shading change the output far more than a person or two on this page, and none of them is an input here.
- Flat-plate and evacuated-tube collectors are not told apart. The rule counts collector area, and two collectors of the same area do not deliver the same heat in the same climate.
- The storage figure is the guide's rule for active systems, and it grows in step with the collector because a large collector over a small store overheats when demand is low. Integral collector-storage and thermosyphon systems carry their storage differently.
- Nothing here covers freeze protection, overheat protection, the backup heater, controls or the regulations that apply to hot water systems where you build.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-25 · v1.0.0
Regulatory standards & verification citations2
- US Department of Energy, Heat Your Water with the Sun — A Consumer's Guide (DOE/GO-102003-1824, December 2003): size to meet 90% to 100% of summer hot water needs; about 20 sq ft (2 m²) of collector for each of the first two family members, then 8 sq ft (0.7 m²) per additional person in the Sun Belt or 12 to 14 sq ft (1.1 to 1.3 m²) in the northern United States
- The same guide: for active systems the solar storage tank grows with the collector, typically 1.5 gallons per square foot of collector; a 66-gallon system usually serves one to three people, an 80-gallon system three or four, a 120-gallon system four to six
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