Storing solar heat means something different from storing solar electricity. A battery keeps the electricity from your solar panels for the evening. The biggest surplus from a solar roof, however, falls in summer, and the biggest demand falls in January. Anyone who takes that difference seriously ends up looking at heat storage instead of just electricity storage.
Why not just solve everything electrically?
A heat pump converts electricity into heat, and that can be done with solar electricity too. The problem is timing: in July, an average solar roof produces many times what a home needs that month, while in January the opposite is true. Those summer peaks are already leading to widespread grid congestion and curtailed solar farms, while in winter you have to buy electricity from the grid again to heat your home. Keeping solar heat local — instead of sending the summer surplus to the grid and buying it back in winter — avoids paying twice for the same problem: first a lower feed-in tariff in summer, and later still the full price for winter electricity.
Three ways to store heat
A water buffer stores heat for hours to a few days; fine for day-night differences, too limited for a whole season. Phase change materials sit in between: more compact than water, with a storage period of days to a few weeks. For an entire season — using June’s surplus in December — you need seasonal storage: a buffer specifically designed to bridge months without significant heat loss. Which of the three fits depends on what you’re trying to solve: bridging just a few cold hours calls for a water buffer, cheap and simple enough. Closing the real seasonal gap between summer and winter calls for a seasonal buffer large enough to capture that difference without an impractical amount of volume. For most homes, the choice mainly comes down to how many months of coverage you actually need.
What do you need in practice?
The basic principle is simple: solar panels supply electricity in summer, that electricity heats a thermal buffer via a heat pump or electric element, and that buffer delivers heat months later for space heating and hot water. How much buffer you need depends on your heat demand and the surplus your panels generate — factors we cover in detail in our article on sizing a thermal buffer correctly. In practice this usually means custom sizing, since every roof and every home has a different surplus and demand pattern.
A concrete example
Take a home with an average solar roof: in summer months, output is often three to four times higher than the home’s own electricity use at that moment. Without storage, that surplus goes to the grid, on increasingly unfavourable terms as more neighbours do the same. In winter the picture is exactly reversed: little solar output, while heat demand for heating and hot water is at its highest. Holding on to some of that summer heat until autumn or winter shifts a significant share of annual heat demand to the moment you already generated it yourself.
Who is this interesting for?
This mainly matters for homes moving off natural gas, new builds with plenty of rooftop solar, and situations where the electricity grid can no longer handle demand. The bigger the gap between summer surplus and winter need, the bigger the gain from storing heat instead of just electricity. For a home with few solar panels and a well-insulated shell, a small day buffer is often enough; the more surplus and the higher the winter heat demand, the sooner seasonal storage pays for itself compared to simply feeding more solar electricity back to the grid at an ever-lower rate.