Thermal Storage Sizing Calculator
A 1,000-litre tank cycling across 40 K stores 46.5 kWh of heat — enough to carry a 10 kW load for about 4.7 hours with the heat pump off. Charge it during solar hours and that heat is nearly free.
Convert litres to kWh or kWh to litres (Q = VρcpΔT), see how long the store carries your load, and what shifting the charge to cheap or solar hours saves per month. With an honest word on when PCM beats water.
Your Store
Storage Results
| Property | Value | Basis |
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The Formulas
Stored energy (water: cp = 4.186 kJ/kg·K, 1 kg/L):
Carry time and charging cost:
Method & Sources
- Water sensible storage — 4.186 kJ/kg·K; 1,000 L across 40 K = 46.5 kWh. The same basis as the iSTOR buffer sizing in our Feedwater Pre-Heat tool.
- The usable ΔT is the honest variable — storage is only as big as the temperature swing your system can actually use. Heating buffers manage 30–50 K; chilled-water loops often only 5–10 K, which is why cold-side storage in water gets bulky.
- PCM (phase-change) storage — stores most of its energy in melting at near-constant temperature, so it beats water by several times per litre where the usable ΔT is narrow. That is the design logic of the Karnot FLX warehouse system's hot and cold PCM batteries.
- Standing losses — a well-insulated tank loses roughly 1–2 kWh/day per 1,000 L; a few percent per daily cycle. Bare tanks lose far more — see the Insulation Calculator.
- Charging economics — default ₱4/kWh represents self-generated solar power; replace with your own off-peak or PPA rate. The saving scales with the spread, not the absolute prices.
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Frequently Asked Questions
How many kWh does a hot water tank store?
kWh = litres × 4.186 × usable ΔT ÷ 3600. A 1,000 L tank across 40 K stores 46.5 kWh — a 10 kW load for ~4.7 hours. A tank that only swings 10 K stores a quarter of that: the usable ΔT is everything.
What is thermal storage actually for?
Three jobs: run the heat pump when power is cheapest (solar or off-peak) and use the heat later; cover short peaks so a smaller machine runs steadily; and ride through defrost or brief outages. The same logic works for chilled water and ice on the cold side.
Why use PCM instead of water?
Water wins on cost when space allows and the temperature swing is wide. PCM stores most of its energy in melting at near-constant temperature — so where the usable ΔT is narrow (cooling loops, dehumidification reheat) it stores several times more per litre. That is why our FLX warehouse system uses PCM batteries, not giant tanks.
How much does tank heat loss matter?
A well-insulated tank loses roughly 1–2 kWh/day per 1,000 L — a few percent of a daily cycle. Bare or thinly-lagged tanks are a different story: check the Insulation Calculator.
Thinking About Load Shifting?
Storage plus solar plus a heat pump is how our Measure → Shave → Prove projects flatten demand peaks. Tell us your peak window and we will tell you what a store would do to it.
Ask About Peak Shaving