Sizing a solar hot water system means matching collector area to your daily energy demand and the solar resource at your site, then giving the tank enough volume to carry a day of hot water through cloudy spells. This calculator follows the simplified SRCC OG-300 logic installers use during design and rebate applications.
How it works
Daily hot water demand sets the energy the system must deliver, and the local solar resource sets how much each square foot of collector can supply:
demand (gal/day) = people × gallons per person per day
energy (BTU/day) = demand × 8.34 × (hot temp − cold temp)
usable solar per ft² = peak sun hours × 317 × collector efficiency
collector area (ft²) = (energy × solar fraction) / usable solar per ft²
storage (gal) ≈ 1.5 × collector area (clamped to 1–2× daily demand)
backup (kW) = (energy × (1 − solar fraction)) / (recovery hours × 3412)The constant 8.34 is the weight of a gallon of water in pounds, and 317 converts
the daily solar irradiance from kWh/m² to BTU/ft². Multiplying by collector
efficiency gives the heat each square foot actually captures.
Example and tips
A four-person household using 20 gallons per person, heating from 55 to 120 °F, needs about 43,400 BTU per day. At 5 peak sun hours and 55 percent collector efficiency, each square foot delivers roughly 872 BTU, so a 70 percent solar fraction calls for about 35 square feet of collector and around 80 gallons of storage. Keep efficiency realistic: flat-plate collectors run 50 to 60 percent, evacuated tubes higher in cold weather. Always pair the array with a backup heater rather than oversizing for the worst winter week.