El Björn TF18 360° electric fan heater with its purple textile air distributor warming a timber-framed temporary site enclosure on a frosty winter day while two workers in hi-vis and hard hats review drawings

How many kW do I need? A practical guide to sizing site heating and choosing the most practical solution

Cold sites slow work down. Cold welfare cabins, workshops and marquees make people miserable, and in buildings they can put you on the wrong side of the law. The usual answer is temporary heating. But ask "how big a heater do we need?" and you'll often get a guess.

This guide shows you how to estimate the heat output a space needs in kilowatts (kW), how to turn that into a number of heaters, and what to check on safety before anything is switched on. If you'd rather skip the maths, our free heating calculator does it for you.

Why heater size matters

Too little heat and the space never gets warm, however long the heaters run. Too much in one place and you get hot spots, wasted power and, with fuel-fired units, more combustion products to deal with.

There's a legal side too. Under regulation 7 of the Workplace (Health, Safety and Welfare) Regulations 1992, the temperature in all workplaces inside buildings must be "reasonable" during working hours. The approved code of practice, HSE L24, says the temperature "should normally be at least 16 degrees Celsius", or at least 13°C where work involves rigorous physical effort. HSE's page on managing workplace temperatures lists portable heaters as one way to make sure work areas are warm enough when they are occupied.

Getting the size roughly right first time is the easiest way to hit those temperatures without overspending on hire, power or fuel.

The quick formula for heat output

Our calculator uses a simple volume-based method:

Heat (W) = length × width × height (m) × building factor × (temperature rise in °C ÷ 20)

Then:

  • kW = W ÷ 1,000
  • BTU/hr ≈ W × 3.412

Step 1: measure the space

Measure the inside length, width and height in metres. For a pitched marquee or a space with a sloping roof, use an average height. Multiplying the three gives you the volume in cubic metres (m³).

Step 2: pick a building factor

The building factor is how many watts each cubic metre needs for a 20°C rise. It reflects how well the space holds heat.

Building type Typical examples Factor (W/m³ at 20°C rise)
Brick or block with cavity wall Offices, workshops, cavity-built site cabins 35
Brick or block without cavity Older or solid-wall buildings with less insulation 50
Timber frame or heavy plastic Marquees, tents, temporary covers, lightly insulated enclosures 85

These are the factors built into the GO2 heating calculator. They're a practical rule of thumb for UK site and temporary heating, not an official standard.

Step 3: set the temperature rise

The temperature rise is how much warmer than the outside air you want the space to be. The calculator defaults to 20°C. On a 0°C day, that takes you to around 20°C inside. If you only need a 15°C lift, the formula scales the answer down to three-quarters (15 ÷ 20 = 0.75). If it's colder outside or you need a bigger lift, the answer goes up.

Worked examples

Here's the formula applied to three common spaces at the default 20°C rise.

Space Size (L × W × H) Volume Factor Heat needed BTU/hr (approx.)
Cavity-wall building 10 × 6 × 3 m 180 m³ 35 W/m³ 6,300 W = 6.3 kW 21,500
Same size, solid wall 10 × 6 × 3 m 180 m³ 50 W/m³ 9,000 W = 9.0 kW 30,700
Marquee or temporary cover 15 × 8 × 4 m 480 m³ 85 W/m³ 40,800 W = 40.8 kW 139,200

Notice how much the building type matters. The solid-wall building is the same size as the cavity one but needs over 40% more heat. The marquee is less than three times the volume of the buildings but needs more than six times the heat of the cavity building, because fabric and plastic hold so little of it.

If the cavity building only needed a 15°C rise, the sum would be 6,300 W × 0.75 = 4,725 W, or about 4.7 kW.

From kW to number of heaters

Once you know the total, work out how many heaters you need:

Number of heaters = total kW needed ÷ rated output of one heater, rounded up

Always round up, and check the answer against these points:

  • Leave some headroom. The calculator gives an estimate. If your total lands exactly on one heater's maximum, step up a size or add a unit.
  • Spread the heat. In a long or open space, two or three smaller heaters spaced out usually warm it more evenly than one big unit in a corner.
  • Allow for heat loss. Open doors, roller shutters, high air change, very cold weather and poor seals all increase the output you need.
  • Check your power supply. Many larger electric fan heaters need a three-phase supply. Check the voltage and rated current on the product page before you order.

Example 1: 6.3 kW cavity-wall building

One 9 kW heater covers it with room to spare. The El Bjorn KH9 Electric Fan Heater has 4.5 kW and 9 kW settings and runs on a 400 V three-phase supply. If you want digital controls with temperature and humidity monitoring, look at the El Bjorn TF9 EBC.

No three-phase supply? The El Bjorn TF3 heating and drying fan is rated at 2.8 kW and comes in 110 V or 230 V. 6.3 ÷ 2.8 = 2.25, so you'd round up to three units, which also helps spread the heat.

Example 2: 9.0 kW solid-wall building

9.0 ÷ 9 = 1.0, so a single 9 kW heater would be working flat out with nothing in reserve. Step up to the El Bjorn KH12, which is rated at 12 kW with a 6 to 12 kW power setting. Or use two 9 kW units at either end of the building.

Example 3: 40.8 kW marquee

40.8 ÷ 18 = 2.27, so round up to three 18 kW heaters (54 kW in total) spaced around the structure. The El Bjorn KH18 is rated at 18 kW with a 9 to 18 kW power setting. For 360° air distribution, the El Bjorn TF18 360° gives 9 or 18 kW. For very large spaces, the El Bjorn TF36 EBC offers 18 or 36 kW and the TF72 EBC offers 36 or 72 kW.

You can see the full range in our El Björn collection.

Do you need extension cables?

Heaters rarely sit right next to the distribution board. Match the cable to the heater's rated current and plug type. For example, the KH9 page lists a rated current of 13 A, the KH12 18 A and the KH18 25 A. We stock 400 V 16 A, 400 V 32 A and 415 V 63 A three-phase extension cables. Temporary site supplies should be planned and checked by a competent electrician. HSE's guide HSG141, Electrical safety on construction sites, covers this.

Safety checks before you switch on

Fumes, gas and carbon monoxide

Regulation 7(2) says heating must not release fumes, gas or vapour into a workplace at levels likely to be injurious or offensive. HSE's construction guidance on carbon monoxide flags LPG heaters in enclosed spaces, including welfare facilities, as a source of CO. HSE advises:

  • adequate ventilation at both high and low level, and checking that vents haven't been blocked to "stop draughts" in cold weather
  • CO detectors where appropriate
  • a competent person for all LPG and gas installation work
  • proper maintenance, because blocked burners can raise CO levels

Electric heaters avoid combustion fumes altogether, which is one reason they're often chosen for enclosed spaces. For LPG specifically, see HSE's LPG guidance.

Fire risk

HSE's HSG168, Fire safety in construction, lists heaters as a potential ignition source on site. It recommends:

  • replacing naked flame and radiant heaters with fixed convector heaters or other types with no red element, wherever possible
  • making sure heaters aren't covered or bunched up against combustible material
  • fitting flame-failure devices on gas heaters in temporary site accommodation
  • keeping separate gas cylinders outside the accommodation
  • turning off fuel supplies after use and doing an end-of-day check

Always keep heaters clear of clothing, sheeting and stored materials, and include them in your fire risk assessment.

Permanent heating is a different job

Everything above is for sizing portable or temporary heaters. It doesn't replace a full heating design. For a permanent installation, use a qualified heating engineer.

Frequently asked questions

How many kW heater do I need for a room?

Multiply the room's length × width × height in metres by a building factor (35 W/m³ for cavity walls, 50 for solid walls, 85 for timber or plastic), then by your temperature rise ÷ 20. Divide by 1,000 for kW. A 10 × 6 × 3 m cavity-wall room needs about 6.3 kW for a 20°C rise.

How do I size heating for a marquee or tent?

Use the 85 W/m³ factor for timber frame or heavy plastic. A 15 × 8 × 4 m marquee needs about 40.8 kW for a 20°C rise, which is three 18 kW heaters once you round up. Our marquee heating calculator does the sum for you.

How do I convert kW to BTU?

Multiply watts by 3.412. For example, 6,300 W × 3.412 ≈ 21,500 BTU/hr.

What temperature should a workplace be in winter?

HSE's approved code of practice says workplaces inside buildings should normally be at least 16°C, or at least 13°C where work involves rigorous physical effort. Those are minimums, not comfort targets.

Is one big heater better than several small ones?

Not always. Several smaller heaters spread around a space usually give more even heat, and losing one unit doesn't leave you with nothing. One large unit can make sense in a compact space with a suitable power supply.

Are electric or gas heaters better for enclosed spaces?

Electric heaters don't produce combustion fumes. If you use LPG or other fuel-fired heaters, HSE says you need adequate high and low-level ventilation, CO detectors where appropriate, and competent installation and maintenance.

Get the right heaters for your site

Start with our free heating calculator, then browse the El Björn heater range. If you're not sure which units suit your space or power supply, request a quote or email sales@go2supplies.co.uk with your measurements and building type. Need something we don't list? Try GO2 Source.

Can't source it? GO2 supplies it.

Sources

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