The first question about thermal storage is always the same: how big does this thing get? Fair enough. Nobody wants to sit through a technical explanation only to still not know whether the system fits in a garden, a basement, a utility room or an outbuilding.
The honest answer is that it depends on the situation. That’s also the least useful answer you can give, so let’s walk through the variables that actually set the size.
The building’s heat demand
The most important factor is how much heat is needed per year. A well-insulated new-build is in a completely different range from a 1970s house with average insulation. An apartment block has a different profile from a detached home. A school or a workshop different again.
It’s not primarily about square metres but about kilowatt-hours of heat per year, split between space heating and hot water. Hot water tends to be underestimated. A passive house needs little space heating, but the residents still shower. In well-insulated homes, hot water can end up at a third or more of the total heat demand.
Sizing a buffer therefore doesn’t start with intuition or with standard kits. It starts with actual usage.
Resident behaviour
A building doesn’t have a heat demand. The people in it do. That’s by far the biggest variable, and it rarely shows up neatly in a calculation.
A single occupant in a well-insulated home easily uses half of what the design manual predicts. A family with four teenagers who each shower twice a day, want their bedrooms warm, and are doing homework at four different desks in November sits comfortably at double. Same house, same roof, same insulation — completely different buffer.
The same applies to generation. People who are up at six and work from home use their solar power differently from a dual-income family that’s out of the house from eight in the morning. The buffer’s charging profile looks completely different, regardless of identical PV.
For sizing this means one thing: a Caldum buffer isn’t dimensioned for a house, it’s dimensioned for a household. In new construction we estimate based on family size and life stage. In existing buildings it’s far more useful to look at actual consumption data — the behaviour is already baked in there. In collective systems much of it averages out, which is one of the advantages of scale: the eight-shower family cancels out against the neighbour who takes one cold bath a week.
Ignore this and look only at square metres and U-values, and you end up with a system that’s either too small for the actual users, or needlessly expensive because it’s sized for assumptions nobody in that house actually lives up to.
The degree of self-sufficiency you want
Not every system needs to be fully autonomous. Sometimes the goal is maximum self-sufficiency — which means the buffer has to bridge longer periods and gets larger. In other cases a smaller system is enough, mainly to shave peaks or support a hybrid installation.
Fully off-grid sounds attractive, but it’s rarely the cheapest solution. Caldum can run autonomously in off-grid energy systems and island-mode applications, and does where that’s required. But in most cases near off-grid is more sensible technically and economically: the building stays connected and uses the grid as backup, not as the main source. Buffer size follows from that choice, not the other way around.
How much can be charged?
A buffer can only deliver heat if it gets charged. With Caldum that’s done electrically — from solar panels or from the grid at favourable moments. Available charging energy varies by location. A house with a large south-facing roof and plenty of PV has a different charging profile from a terraced house with limited roof area. A commercial building can generate a lot but may have a completely different usage rhythm.
Grid capacity matters too. In places with grid congestion, charging power can’t simply be turned up. There you spread the charging in time rather than relying on high peaks — which incidentally fits seasonal storage perfectly, because there’s no rush.
Capacity versus power
In energy storage, two things get confused: capacity and power.
Capacity is how much energy fits in total. Power is how fast it can come out. A large buffer with insufficient delivery power doesn’t work well — a shower doesn’t get its hot water fast enough. A buffer with plenty of power but too little capacity is empty by mid-February.
Both need to match. In homes the peak demand is often short and sharp (morning showers, a cooking burst, a cold morning when everyone turns the heating on at once). In larger buildings demand is more even. In collective systems simultaneity actually helps: not all residents shower at the same time, so a shared buffer is more efficient than the sum of individual ones.
Home, block or district
A single home asks for something different than a block or a district. For one house, the buffer is tied to the home’s own energy system and one household’s behaviour. For a block or a small collective, scale advantages appear: not everyone has peak demand simultaneously, so a shared buffer can be smaller than the sum of individual ones.
At district level it gets more complex. Pipe lengths, distribution losses, temperature regimes, ownership, governance and liability all enter the picture. A buffer can still be useful, but the overall system design starts to matter more than the buffer itself. Caldum is built modularly precisely to handle those scale jumps without becoming a different product each time.
Placement
A thermal buffer has to sit somewhere. That sounds trivial, but in practice it’s one of the more important design questions. In new construction you can plan for foundations, plant rooms or integration into an outbuilding from the start. In existing buildings you work with what’s there: underground in the garden, in or beside a storage shed, in a plant room, or collectively placed for several homes.
Which option is right depends on space, installation access and maintenance. A buffer has to fit not just technically but practically — otherwise it exists on paper but never gets built.
Why water tends to get too large
Water is fine as a storage medium for short-term heat. That’s why boilers and buffer tanks use water. But its energy density is limited. Try to hold enough heat for weeks or months and you end up with a tank the size of a lorry — for an average household, around a hundred cubic metres to bridge a winter. That doesn’t fit in most gardens, front yards or plant rooms, and building a tank of that size is no small intervention either.
Caldum uses a storage medium with substantially higher energy density than water. The same amount of heat fits in a much smaller volume, and seasonal storage becomes practical for homes, buildings and small collective systems — instead of something that only works if you happen to have a field to spare.
From rule of thumb to practice
There is no universal standard size that works for every home. Good sizing starts with a handful of sober questions: how much heat is needed per year and when, how much can be generated locally, how much self-sufficiency is wanted, and where can the buffer physically sit? Capacity follows from those answers. That’s less simple than “one size fits all”, but it avoids disappointment. A buffer that’s too small doesn’t deliver what you expect. A buffer that’s too large makes the system needlessly expensive.
We’re using the first Caldum installations to test those calculations under real conditions. How does heat demand actually run across a year? What are the real losses? How does the control system behave alongside PV, grid power or an existing installation? How much comfort margin is needed? Models matter, but only practice shows where the assumptions creak.
So the question “how big should the buffer be” stays important. It just doesn’t start with litres or kilowatt-hours. It starts with the heat demand, and with how the building actually uses heat.