Plan collector size and storage with confidence. Compare energy savings and payback. Get clear recommendations for your home today.
| Scenario | People | Liters/person | Hot/Cold (C) | Irradiance | Eff / Loss | Solar % | Collector Area | Tank |
|---|---|---|---|---|---|---|---|---|
| Small family | 4 | 40 | 50 / 20 | 5.0 | 55% / 15% | 70% | ~3.2 m2 | ~192 L |
| Larger home | 6 | 45 | 52 / 18 | 4.5 | 50% / 18% | 75% | ~6.1 m2 | ~324 L |
| High usage | 5 | 60 | 55 / 15 | 5.5 | 60% / 12% | 80% | ~5.0 m2 | ~360 L |
Household hot water demand scales with occupants, fixture types, and routines. A four-person home at 40 liters each needs 160 liters daily. With cold water at 20 C and setpoint 50 C, deltaT is 30 C, requiring about 5.58 kWh of heat per day. If usage rises to 55 liters each, the same home needs 220 liters and roughly 7.67 kWh daily.
Collector sizing depends on sunshine, efficiency, and distribution losses. If irradiance is 5.0 kWh/m2/day, efficiency 55%, and losses 15%, useful gain is about 2.34 kWh/m2/day. Targeting 70% solar fraction of a 5.58 kWh load means 3.91 kWh/day from solar, needing roughly 1.67 m2 of collectors. Lower irradiance of 4.0 can push area above 2.08 m2, so local solar data matters.
Storage buffers cloudy hours and peak draws while improving delivery temperature stability. A storage factor of 1.2 suggests a 192 liter tank for a 160 liter daily demand. For higher solar fractions, larger tanks reduce cycling and store midday heat for evening demand. As a rule, increasing storage by 20% can reduce backup starts and improve effective solar contribution.
Financial results depend on energy price, backup efficiency, maintenance, and installed cost. With a 90% backup heater, the avoided input energy for 3.91 kWh/day thermal is about 4.35 kWh/day. At 0.18 per kWh, gross savings are near 285 per year. After 40 annual maintenance, net savings approach 245. A 2500 system then has a simple payback near 10.2 years, while lower energy rates lengthen payback significantly.
Use conservative inputs when uncertain: lower efficiency, higher losses, and winter cold-water temperature. Verify roof tilt, shading, pipe runs, and freeze protection strategy. If recommended area is very large, reduce solar fraction, improve insulation, or consider demand reduction with low-flow fixtures. Always confirm final sizing with product ratings, local codes, and installer guidance. Document assumptions, then revisit settings after monitoring real monthly energy bills carefully.
Use recent meter data if available. Otherwise, 25–35 liters suits conservative use, 40–60 liters fits typical families, and 60+ liters matches high shower or bath frequency. Enter your best average, not a single peak day.
For many homes, 50–75% balances cost and performance. Higher targets can increase collector area and storage needs. If roof space is limited, choose a lower fraction and rely on backup heating during low-sun periods.
Efficiency converts sunlight into usable heat. A small change, such as 55% to 45%, reduces useful gain and increases required area. Use realistic values from product data, then add losses for piping and tank heat loss.
Tank size mainly affects comfort and solar utilization. Too small can waste midday heat and increase backup cycling. Savings change slightly, but reliability improves with adequate storage, especially when aiming for higher solar fractions.
It is a simplified estimate based on energy rate, avoided energy, and maintenance. Installation costs, incentives, fuel type, seasonal performance, and usage changes can shift payback materially. Treat it as a planning range, not a guarantee.
Yes. Enter the backup efficiency that matches your system. Condensing gas may be 90–95%, while heat pumps vary by season. The calculator converts solar thermal output into avoided input energy using that efficiency.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.