The myth: "just get the biggest heater you can afford"
It is one of the most common pieces of advice floating around UK garden sauna forums: buy the strongest heater your budget allows, because "it will heat up faster and you can always turn it down." In practice, an oversized heater does not make a sauna better. It makes the thermostat cycle on and off in short bursts, pushes heat unevenly towards the ceiling, and can push a cabin outside the volume range its manufacturer actually tested it for — which matters for both comfort and your warranty.
Heater sizing for an outdoor sauna is simple arithmetic once you know three numbers: the internal volume of the cabin in cubic metres, the heater's rated output in kW, and a small allowance for glass and uninsulated surfaces. Here is how to do that calculation properly, and why "bigger" quietly stops being "better" long before you reach the top of the range.
kW per m³: the numbers that actually matter
Electric sauna heaters are not rated by room size in m² — they are matched to the internal volume of the cabin, because that is the air mass the heater has to bring up to roughly 80–90°C and keep there. Harvia, the Finnish manufacturer whose heaters we fit as standard, publishes a recommended volume range for each model (figures checked directly on harvia.com, not from memory, as these change between product revisions):
- Harvia KIP 4.5 kW — recommended for 2.8–6.0 m³
- Harvia Spirit SP60 WiFi 6.0 kW — recommended for 5.0–8.0 m³
- Harvia Spirit SP90E 9.0 kW — recommended for 8.0–14.0 m³
Take our own R-ILLY as a worked example: internal dimensions of roughly 186 × 110 × 205 cm give an internal volume of about 4.2 m³. That sits comfortably inside the KIP's 2.8–6.0 m³ band, which is exactly why it is the standard heater on that cabin — not the 6 kW Spirit SP60, and certainly not a 9 kW unit "for extra power".
The glass-front correction
Volume alone is not the whole story. A cabin with a full glass front — like R-ILLY's — loses heat faster through that glazing than it would through 42 mm thermo-pine wall. Most sauna heater calculators (Harvia's own guidance among them) apply a rough uplift of roughly 20–50% to the "effective" volume for every square metre of glass or uninsulated surface, rather than a single fixed multiplier — the exact figure depends on glazing type and how exposed the wall is. We deliberately do not quote one "magic" number here, because we have not been able to confirm a single universal coefficient from Harvia's own documentation; treat any online calculator's glass correction as a starting range, not gospel, and size slightly up rather than down if your cabin has a large glazed area.
Single-phase vs three-phase: the UK wrinkle
This is where UK installs differ from the simple kW/m³ table. Most UK homes only have a single-phase 230V supply. Current draw follows basic Ohm's law arithmetic (I = P/V), so a 4.5 kW heater draws roughly 20 A and a 6 kW heater roughly 26 A on a dedicated single-phase circuit — both comfortably within a standard dedicated circuit breaker. Push past roughly 9 kW and the current on a single-phase supply climbs into the high 30s/low 40s of amps, which is why many manufacturers specify a three-phase supply for heaters from around 9–10.5 kW upward: splitting the load across three phases keeps the current per phase (and the cable size) much more manageable.
In practice, this means almost every garden cube or barrel sauna we sell — heated by a 4.5 kW or 6 kW Harvia unit — runs happily off a single dedicated single-phase circuit, the kind any competent domestic electrician installs in an afternoon. It is only once you move into larger communal cabins needing a 9 kW+ heater that three-phase becomes a real conversation to have with an electrician before you order anything. Either way, UK installations should always be signed off by a qualified, Part P-registered electrician working to BS 7671 wiring regulations — this article is sizing guidance, not an electrical certificate.
Why a bigger heater is not better
- Short cycling — an oversized heater reaches target temperature fast, then switches off and on repeatedly instead of running a steady, even cycle, which feels harsher and wastes energy on start-up draw.
- Uneven heat — too much power in too small a volume pushes hot air straight to the ceiling before it has time to mix, leaving your feet cold while your head overheats.
- Warranty and safety margins — manufacturers test clearances to the thermo-pine walls and benches for a specific heater-to-volume pairing; going above the recommended range can void guarantee cover.
- Electrical cost — a heater that needs three-phase power where a single-phase one would have done adds real installation cost for no comfort benefit.
- Running cost — more kW drawn at start-up (and during cycling) means a higher electricity bill for a result that is, at best, the same temperature reached a few minutes sooner.
R-ILLY heater options and UK pricing
Prices checked directly against our UK (GBP) price list before publishing this guide:
- R-ILLY, no heater: £4,990
- R-ILLY with Harvia KIP 4.5 kW (standard): £5,390
- R-ILLY with Harvia Spirit SP60 WiFi 6.0 kW (smart): £6,090
- R-ILLY with Harvia Legend 240 wood-fired, flue kit included: £6,990
Note that the Spirit SP60 is an upgrade in features (WiFi control, faster response) rather than a requirement for R-ILLY's 4.2 m³ — the standard KIP 4.5 kW already sits well inside the recommended band.
FAQ
What size heater do I need for a small 2–4 person garden sauna?
For a typical cabin of 3–5 m³ — most of our compact cube and barrel models — a 4.5 kW heater such as the Harvia KIP is the standard fit and runs on a single-phase supply.
Can I run a 9 kW heater on a normal UK single-phase supply?
Sometimes, but the current draw is high enough that many manufacturers call for three-phase power at or above roughly 9–10.5 kW. Always check the specific heater's installation manual and get a qualified electrician to confirm before you order.
Does a glass front really change the heater I need?
Yes — glazed and uninsulated surfaces lose heat faster than a solid thermo-pine wall, so cabins with a large glass front are usually sized with a modest upward allowance rather than matched purely to internal volume.
Getting the sizing right before you buy
Every R-ILLY, barrel and cube sauna we sell ships with a heater already matched to its internal volume — so the "do I need bigger" question is mostly one for larger custom or multi-person cabins. If you are unsure which heater option suits your space, our team can check it against the manufacturer's own volume bands before you order. For how installation works in cold weather, see our related guide on installing a garden sauna in winter, and for the broader wood-fired vs electric trade-off see wood-fired vs electric outdoor sauna.
Browse the full range in our UK sauna collection, or email sales@saunado.eu with your cabin size and we'll confirm the right heater before you order.