Sauna heater sizing at Kigali's 1,450 m altitude

Start with volume

Our base figure is 0.85 kW per cubic metre for a well-insulated cabin lined in timber. A 2.0 × 2.4 m cabin with a 2.1 m ceiling holds about 10.1 m³, so at sea level the starting point would be about 8.6 kW. That is a starting point, not an answer.

It assumes a properly insulated cabin with a continuous vapour barrier, a sensible vent path and a door that seals. When any of those is missing, no heater fixes it. We would rather add insulation than kilowatts, every time.

Then correct for altitude

Thinner air carries less heat per cubic metre, and a sauna heater's whole job is to warm air and stones which then warm the room. At Kigali's elevation we multiply the volume figure by 1.12, which in practice often means the next heater up from the one a sea-level chart would pick.

It costs a little on day one, and it is the difference between a cabin people use and one they apologise for. The classic symptom of a heater sized for another country is a room that climbs to 70 °C and stalls there.

Higher up, the correction grows. In Musanze, at roughly 1,850–2,500 m, we recalculate on the lodge's actual elevation rather than using Kigali's factor, because the gap between the bottom and the top of that range is another step on the heater.

Then correct for glass

Glass loses heat far faster than an insulated wall. We add about 0.42 kW for every square metre of glazing, so a full glass wall of 5 m² adds 2.1 kW and a glazed corner of 9 m² adds nearly 3.8 kW.

This is not an argument against glass. On a Kigali ridge the glass is the reason to build the cabin where you are building it. It is an argument for putting the glass into the drawing before the heater is chosen, and for accepting the heater that results rather than negotiating it down.

A sizing table for Kigali

The table applies our formula — volume × 0.85 × 1.12, plus 0.42 kW per square metre of glass — and rounds up to the next heater we actually fit: 4.5, 6, 8, 9, 10.5, 12, 15, 18, 21 or 24 kW.

Anything above about 8 kW wants a three-phase supply. Run your own dimensions through the price configurator, which uses the same arithmetic, or see how the sizing fits into a complete custom sauna build.

What it actually costs to run

A heater does not draw its rating continuously. It pulls full load during heat-up — twenty to forty minutes depending on insulation — then cycles to hold temperature at a third to a half of its rating. An evening in a four-person cabin with a 12 kW heater comes to roughly 12 kWh rather than 12 kW for every hour the cabin is warm.

Insulation moves that figure more than anything else you can buy. The same heater in a well-insulated cabin spends far more of the session idle, which is why the best-value cabin in our range is a small one built properly rather than a large one built to a price. REG tariffs and cost per session are set out in sauna electricity in Rwanda.

Single-phase or three-phase

Up to about 8 kW you can usually stay single phase, if the supply and the cable allow. Above that, three phase. REG supplies 400 V three-phase and charges the same RWF 56,000 meter fee for either (REG New Connection Policy), so the decision is about the board and the cable run rather than the meter.

That cable run is the line that varies most. Twelve metres across a garage is trivial; forty metres down a terraced garden is a real cost, and it is dramatically cheaper before the garden is laid than after.

Why cold water is cheap to hold at 1,450 m

A chiller does not cool water in the abstract. It fights the gap between your target temperature and the air around the tub, and in Kigali that gap is unusually small.

The city runs at roughly 26–28 °C by day and 16–17 °C at night, averaging 20.7 °C over the year (Climates to Travel). A hot lowland city sits several degrees warmer by day and, far more importantly, barely cools at night. For a tub held at 8 °C it is the overnight difference that decides the electricity bill: in Kigali the compressor spends the night doing very little.

That buys three things: lower electricity, a smaller unit for the same tub, and a longer working life, because compressor wear tracks running hours far more closely than age.

The product nobody else here is selling

Because the nights are cool, a shaded, insulated, lidded plunge held at 15–18 °C frequently needs no chiller at all in Kigali. Mains water and night air do the work, and 15–18 °C is the temperature most people actually enjoy and use again and again.

We say so at quotation stage, even though it takes a substantial item off our own invoice. A client talked into a compressor they did not need tells their friends exactly that.

For a true 4–6 °C ice bath you do need a machine, and we size it properly. Between those two positions sits a genuinely useful middle that almost nobody in this city is offered.

Insulation and the lid still decide everything

None of the above helps an uninsulated tub standing in the afternoon sun. Heat gain through the shell and evaporation from an open surface will beat any chiller you attach to it.

We build tubs as an insulated shell with closed-cell insulation in the cavity, insulate the base as well as the walls, and supply a lid that fits. The lid is the cheapest component in the system and the one that saves the most money — and because most people stop using a heavy lid within a month, ours are light enough to lift with one hand.

Sizing the chiller, honestly

1 HP for tubs up to about 260 litres, 2 HP to about 570, 4 HP to about 1,200 — then a step up if the tub sits in sun, serves a team back to back, or is held at 4 °C rather than 8–10 °C.

The most common chiller fault we meet is not a fault at all: a unit sold with a tub larger than the one it was sized for. It was never going to reach setpoint, and no service visit will change that.

Send us a model number before you buy something yourself. It is a two-minute answer, and it can save you a very expensive ornament.