Turn your greenhouse into a calmer heat battery. Choose material, enter size, finish fast now. See how much heat you can store overnight safely.
| Scenario | Material | Volume | Estimated Mass | Usable Energy | Usable Energy (kWh) |
|---|---|---|---|---|---|
| Two water barrels | Water | 0.40 m³ | 400 kg | 18.74 MJ | 5.206 kWh |
| Concrete bench core | Concrete | 0.15 m³ | 360 kg | 3.55 MJ | 0.986 kWh |
| Raised bed soil mass | Soil (moist) | 0.50 m³ | 800 kg | 10.75 MJ | 2.986 kWh |
Thermal mass moderates day–night swings by absorbing heat in daylight and releasing it after sunset. In small garden greenhouses, a 6–12°C swing is common without storage, while added mass can cut that swing when ventilation and shading are controlled.
Water is the top performer for compact storage: its specific heat is about 4,186 J/kg·°C, so each 100 kg stores roughly 0.116 kWh per 1°C change. Concrete and brick are lower (around 800–900 J/kg·°C) but provide structure. Moist soil varies; wetter mixes store more energy but can dry under heat.
Use a realistic temperature range, not the best-case forecast. A 10°C usable swing with 400 kg of water stores about 16.7 MJ (4.6 kWh) before losses. If only 70–85% is delivered to the air, adjust with the usable fraction to match airflow and insulation.
Placement often matters as much as capacity. Put barrels or masonry where sun hits them, but avoid blocking plant light. Position mass along the north wall in northern climates, keep it off cold soil with a small stand, and ensure air can move across the surface for faster exchange.
Compare options by kWh stored per footprint and install effort. Water barrels are inexpensive and modular, but need secure lids and stable bases. Masonry adds permanent weight; confirm floor capacity and keep pathways clear. The calculator documents scenarios so you can iterate before purchasing. For spring starts, target enough storage to cover at least three hours of heat gain.
It estimates how much stored heat actually warms air and plants after losses to glazing, leaks, and ground contact. Use 60–90% depending on insulation, wind exposure, and whether the mass is shaded at night.
Use volume when you know dimensions or container size, and let density estimate mass. Use mass when you have a product label (kg or lb) or when material density is uncertain, like mixed stone or soil.
Water has a high specific heat, so it stores more energy per kilogram per degree than most building materials. It is also easy to package in barrels, tanks, or jugs near beds.
Start with your typical day high and night low inside the structure, not outdoor values. If you vent aggressively, your daytime peak may drop. For planning, use conservative numbers from several similar days.
It reduces heater runtime, but it cannot create heat without an input source. During long cloudy spells, stored energy is limited. Combine mass with insulation, tight sealing, and a backup heater for frost nights.
Place it where it receives sun and where air can circulate across it. Avoid blocking light to crops. Along a back wall, under benches, or between rows with clear walkways are common, effective layouts.
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.