The daily heat required to raise water temperature is:
Q (kWh/day) = (V × Cp × ΔT) / 3600
Cp = 4.186 kJ/kg·°C, V ≈ mass in kg (liters of water)Loss-adjusted energy demand:
Q_adjusted = Q / (1 − loss%/100)Solar heat delivered per square meter:
Yield = sunshine × efficiency
Collector area (m²) = Q_adjusted / Yield- Enter your daily water volume and the inlet temperature.
- Set the target temperature for your irrigation or cleaning needs.
- Choose sunshine input and provide local values.
- Enter collector efficiency and expected system losses.
- Set the panel area to estimate the panel count.
- Press Calculate size to see results above the form.
| Scenario | Volume (L/day) | Inlet (°C) | Target (°C) | Sunshine | Efficiency | Loss | Collector (m²) | Panels |
|---|---|---|---|---|---|---|---|---|
| Warm season beds | 150 | 20 | 32 | 5.5 h/day | 55% | 10% | 1.92 | 1 |
| Greenhouse rinse | 300 | 16 | 35 | 5.0 h/day | 50% | 12% | 4.42 | 3 |
| Cooler shoulder season | 250 | 12 | 32 | 3.5 h/day | 50% | 18% | 6.64 | 4 |
Design targets for garden hot-water use
This calculator sizes collectors for warming irrigation or wash-down water. Start with a daily volume that reflects actual tap time and flow rate. A modest 10–20°C lift often protects tender roots and improves cleaning comfort. Higher lifts increase collector area rapidly, so define a realistic target temperature for your season. To estimate volume, time your hose fill into a known container for one minute.
How the heat demand is determined
Daily heat demand is based on water mass and temperature rise. One liter is treated as one kilogram, and water’s specific heat is 4.186 kJ/kg·°C. The tool converts kJ to kWh, then adjusts for distribution and storage losses. Loss settings cover pipe runs, windy locations, and uninsulated tanks. If you use long hoses, set losses higher to stay conservative.
Sunshine inputs and efficiency assumptions
You can enter peak sun hours or daily solar energy per square meter. Peak sun hours are an easy planning proxy, while solar energy values can come from local solar maps or on-site monitoring. Collector efficiency represents delivered heat after optical and thermal effects. Use lower values for flat-plate units in cool air, and higher values for evacuated tubes. Document seasonal values and rerun the calculator before major planting cycles.
Interpreting collector area and panel count
The reported collector area is the minimum area needed on an average day. Panel count is calculated by dividing this area by your single-panel area and rounding up. If you need morning availability, increase storage rather than oversizing collectors. The recommended tank range (about 1.0–1.5× daily volume) stabilizes temperature and reduces cycling. For cloudy weeks, add 10–25% area or lower your temperature target.
Practical placement and safety notes
For consistent output, face collectors toward the equator and avoid shade during mid-day hours. Use insulated piping, short runs, and a tempering valve to prevent scalding at taps. In freezing climates, choose drainback or glycol systems. Maintain clear access for cleaning glazing and checking joints annually.
Which sunshine input should I use?
Use peak sun hours for quick planning. Use daily solar energy when you have map data or monitoring. Both are converted into a per‑square‑meter daily heat estimate using your efficiency setting.
What efficiency value is reasonable?
For many flat‑plate collectors, 40–60% is a practical planning range. Evacuated tubes can be higher in cool air. If you are unsure, start conservative and adjust after reviewing real temperature performance.
How should I set system losses?
Increase losses for long pipe runs, exposed tanks, windy roofs, or poorly insulated hoses. Decrease losses for short runs and well‑insulated storage. Conservative loss settings reduce the risk of undersizing during shoulder seasons.
Why is my collector area very large?
Large areas usually come from high temperature rise, low sunshine, low efficiency, or high losses. Lower the target temperature, reduce daily volume, improve insulation, or reposition collectors to reduce shading and boost received energy.
Should I oversize collectors or storage?
For earlier warm water and smoother output, storage is often more helpful than heavy collector oversizing. Extra storage buffers short clouds and reduces cycling. Oversizing collectors may increase stagnation risk if flow is limited.
Can I use this for greenhouse hydronic loops?
Yes, if you treat your daily loop makeup or draw as the “volume,” and choose realistic inlet and target temperatures. Hydronic systems may have different losses; use a higher loss percentage and validate with measured loop temperatures.