Sauna heater size and electrics.
How to size a sauna heater, and the electrics it needs.
A sauna heater is sized to the room’s volume, with extra for glass and stone walls. Get it wrong and the room never gets hot. Here is the maker’s method with worked examples, what supply a 6–9 kW heater needs, who is allowed to wire it in Ireland and the UK, and what heaters cost.
Updated · 4 min read
Size the heater to the room, not the seats
Heater makers size heaters by the volume of the hot room in cubic metres. Measure the inside length, width and height and multiply them. Then add for any wall that is not insulated timber: Harvia’s manual says to add 1.2 m³ for every square metre of glass, brick, tile or concrete, and to multiply the volume of a log-walled sauna by 1.5. Its own example: a 10 m³ room with a glass door needs about the same heater as a 12 m³ room.1
Then pick a heater whose rated range covers your number. Harvia rates its 9 kW Cilindro PC90, for example, for rooms of 8–14 m³.2 A heater that is too small never gets the room properly hot; Harvia says a correctly sized heater brings an insulated room to bathing temperature in about an hour.1
| Room | Volume | Extra for glass or masonry | Size the heater for |
|---|---|---|---|
| Small barrel, about 1.8 m across and 2 m long | About 5 m³ (a barrel is round, so less than length × width × height) | A 1 m² glass door adds 1.2 m³ | About 6 m³ |
| Cabin, 2 m × 2 m × 2.1 m | 8.4 m³ | A 2 m² glass front adds 2.4 m³ | About 11 m³ |
| Log cabin, 2.4 m × 2 m × 2.1 m | 10.1 m³, × 1.5 for log walls | None | About 15 m³ |
These are worked examples of the manufacturer’s method, not a substitute for the heater’s own sizing table. Always check the range printed for the model you buy.
Single-phase or three-phase?
Most homes in Ireland and the UK have a single-phase supply. Many heaters up to about 9–10.5 kW can run on it, but they draw a lot of current. Harvia’s spec for the 9 kW Cilindro PC90 calls for a 40 A fuse and 3 × 10 mm² cable on a 230 V single-phase supply, against 3 × 16 A fuses and 5 × 2.5 mm² cable on 400 V three-phase.2 Bigger heaters are often three-phase only: the HUUM HIVE Mini (6–10.5 kW) is sold for single- or three-phase, the 12–18 kW HIVE for three-phase only.3,4
Ask your electrician two questions before you choose a heater: is there spare capacity at the fuse board and in the supply to the house, and how far is the cable run to the sauna? A long run to the end of the garden needs thicker cable, and that is often the biggest electrical cost.
Who can wire it
| Rule | |
|---|---|
| Ireland | The CRU says electrical work in your home must be done by a Registered Electrical Contractor, and that it is the law.5 A new circuit is restricted work that only a registered contractor may do and certify.6 |
| England and Wales | A room containing a sauna heater is a “special location” under Part P, and a new circuit is notifiable work, including in gardens and outbuildings.7 Use an installer registered with a competent person scheme.8 |
| Scotland and Northern Ireland | Check with your council’s building standards or building control team, and use a registered electrician. |
Wood-burning stoves
Wood stoves are rated by room volume too, and the flue and safety distances to the timber walls matter as much as the stove. Follow the stove’s own installation manual for clearances and heat shields, and fit a guard so nobody leans on it. Our wood-fired vs electric guide compares the two.
What heaters cost
| Heater | Type | Ireland | UK |
|---|---|---|---|
| HUUM DROP 4.5 / 6 / 9 kW | Electric | €607.95 / €689.95 / €804.959 | £429.99 / £488.00 / £569.0010 |
| HUUM HIVE Mini 6 / 9 / 10.5 kW | Electric | €1,031.95 / €1,355.95 / €1,412.953 | See retailer |
| Narvi Kuru 14 / 20 | Wood-burning | €889.95 / €934.9511 | £629 / £66112 |
Prices are from Finnmark Sauna’s Irish and UK stores on 29 October 2026. Compare stockists in the sauna heater supplier directory.
What it costs to run
Harvia says a 6 kW heater uses about 7–9 kWh over a two-hour session.13 That is roughly €2.70–€3.50 at Ireland’s average household price of about €0.39 a kWh14 and £1.85–£2.35 at the UK price cap of 26.32p a kWh for October to December 2026.15 A bigger heater uses proportionally more while it heats up.
How we wrote this page
Prices, sizes and lead times come from each seller’s own website. Every one links to the page we read, and we last checked them on 22–29 October 2026 for the Olladay sauna industry directory. Prices are the lowest listed for that product, before delivery, base and electrics unless the seller says otherwise, and can change at any time. Sizing follows the heater maker’s published method; the worked examples are our arithmetic. Heater prices are as listed on the retailer pages linked below. Spotted something wrong, or sell saunas and want your prices checked? Email us.
Olladay makes booking software for saunas. Nobody pays to be listed or ranked on these pages, and the tables are in price order, not in order of preference.
Next steps
- How much a sauna costs
- Planning permission in Ireland
- Planning permission in the UK
- Back to the sauna buying guide
References
- Harvia. Spirit electric sauna heater: instructions for installation and use (manufacturer manual). resources.saunaplace.com/pdfs/Harvia-Spirit-IOM.pdf
- Harvia. Cilindro PC90 9 kW electric sauna heater, product page (manufacturer). www.harvia.com/en/products/HPC900400/cilindro-pc90-90-kw-steel
- Finnmark Sauna (Ireland). HUUM HIVE Mini electric sauna heater (retailer). ie.finnmarksauna.com/products/huum-hive-mini-electric-sauna-heater
- Finnmark Sauna (Ireland). HUUM HIVE electric sauna heater (retailer). ie.finnmarksauna.com/products/huum-hive-electric-sauna-heater
- Commission for Regulation of Utilities. Electrical works in your home. www.cru.ie/consumer-information/home-safety/electrical-works/
- Safe Electric. Restricted and controlled electrical works. safeelectric.ie/help-advice/controlled-restricted-electrical-works/
- HM Government. Approved Document P: electrical safety, dwellings (2013 edition). assets.publishing.service.gov.uk/media/5a802da7ed915d74e622ceed/BR_PDF_AD_P_2013.pdf
- Planning Portal. Electrics: building regulations (England and Wales). www.planningportal.co.uk/permission/common-projects/electrics/building-regulations
- Finnmark Sauna (Ireland). HUUM DROP electric sauna heater (retailer). ie.finnmarksauna.com/products/huum-drop-electric-sauna-heater
- Finnmark Sauna (UK). HUUM DROP electric sauna heater (retailer). finnmarksauna.com/products/huum-drop-electric-sauna-heater
- Finnmark Sauna (Ireland). Narvi Kuru wood-burning sauna heater (retailer). ie.finnmarksauna.com/products/narvi-kuru-wood-burning-sauna-heater
- Finnmark Sauna (UK). Narvi Kuru wood-burning sauna heater (retailer). finnmarksauna.com/products/narvi-kuru-wood-burning-sauna-heater
- Harvia. The cost of heating a sauna is less than you think (manufacturer). www.harvia.com/en/ideas-and-trends/sustainability-and-safety/the-cost-of-heating-a-sauna-is-less-than-you-think
- Eurostat. Electricity prices for household consumers, bi-annual data (nrg_pc_204), Ireland, 2,500–4,999 kWh a year, all taxes included. ec.europa.eu/eurostat/databrowser/view/nrg_pc_204/default/table
- Ofgem. Changes to energy price cap between 1 October and 31 December 2026. www.ofgem.gov.uk/news/changes-energy-price-cap-between-1-october-and-31-december-2026
Questions
What size sauna heater do I need?
Work out the room’s volume in cubic metres, add 1.2 m³ for every square metre of glass, brick or tile, and multiply by 1.5 for log walls. Then choose a heater whose rated range covers that number. A 9 kW heater, for example, may be rated for 8–14 m³.
Can a 9 kW sauna heater run on a single-phase supply?
Many can. Harvia’s 9 kW Cilindro PC90 needs a 40 A fuse and 10 mm² cable on single-phase. Ask an electrician whether your supply and fuse board have the capacity.
Does a sauna need three-phase power?
Not usually for a home sauna. Larger heaters, often 12 kW and up, are commonly three-phase only.
Can I plug a sauna heater into a socket?
Not a full-size sauna heater. It needs a dedicated, hard-wired circuit installed by a registered electrician. Some infrared cabins and very small heaters plug in.
How much does a sauna heater cost?
Listed prices for the HUUM DROP start at €607.95 or £429.99 for 4.5 kW. Wood-burning stoves such as the Narvi Kuru are listed from €889.95 or £629.