Are Greenhouse Heaters Expensive To Run?

There are lots of different ways you can heat your greenhouse which will influence how expensive greenhouse heaters are to run. Here we will show you different ways to heat your greenhouse, including:

• Ways to heat your greenhouse without using a heater
• The different greenhouse heaters available and their associated costs
• What you can do to reduce the costs of heating your greenhouse.

With all this information at your fingertips we aim to help you make the correct choice when it comes to heating your greenhouse and if you choose to use a heater, ways to keep it as economical as possible. 

Ways To Heat Your Greenhouse Without Using A Heater

It might sound a little strange, creating heat without using a heater, but there are lots of old, tried, and tested methods which you can find gardeners have used to do just that! Our research has found all these ways which have been suggested will help heat your greenhouse:

Using Compost To Heat Your Greenhouse
If you are lucky enough to have a large greenhouse – or this also works for a polytunnel – then harnessing the heat generated as waste decomposes is a good way of adding additional warmth. If you have or had a compost pile in your garden or allotment, you will know that a well-designed compost pile can measure over 100 degrees Fahrenheit. By well-designed we mean a compost pile which is packed tightly, layered up with a mix of garden waste and is turned occasionally to introduce oxygen. The heat generated form the pile will help warm your greenhouse, keeping it warmer than the outside temperature.

Our main concerns with this method are the space it will use up, the odour it might produce, the insects / pests it might harbour and the actual amount of warmth it will add – but it is definitely a ‘free’ heating source.

Using Thermal Mass To Heat Your Greenhouse
‘Thermal mass’ is all about using objects / things that will absorb heat from the sun during the day and release that heat when the sun goes down in the evening. Things like bricks, rocks, clay and water all have good thermal mass so it’s a question of how you can use these items in your greenhouse to take advantage of the warmth they will give off. Here are a few ideas:

• Bricks – you could make raised beds from bricks
• Stones – better than concreate at absorbing and retaining heat, black stones would work best and could be used on the floor of the greenhouse
• Water – water barrels stored inside your greenhouse, sealed containers of water work best as do black containers as they will absorb heat from the sun better than light coloured containers.
  

Again, our concerns with this method are how much space will be taken up and how much warmth will be actually generated. Still a ‘free’ method of heating though.

Using Colour To Heat Your Greenhouse
So, there are differing opinions when it comes to colour and whether you should paint the walls of a greenhouse to either absorb heat or reflect sunlight. This works best with a lean-to greenhouse where you will have a big brick wall which you could paint white to reflect sunlight into the greenhouse or black to absorb heat and release it slowly when the sun goes down.

How much warmth this will add to your greenhouse space might be small, but it’s generated with very little effort or cost on your part so is worth a try.

Using Animals To Heat Your Greenhouse
If you have pets living outside, such as rabbits or guinea pigs, or keep a few hens, have you considered moving them into your greenhouse or polytunnel for the winter months and take advantage of the heat they generate? Normally you would cover your rabbit hutch over to keep it dry and free from snow and add in extra bedding to keep your rabbit snug. If you move the hutch into your greenhouse then you not only provide extra protection to your rabbit, but also any heat they produce will help provide warmth in the greenhouse. Whilst research has shown that a single hen will give off about 10 watts of energy, letting them into your polytunnel or large greenhouse could help generate some warmth.

Again, the sceptic would ask how much heat this would actually generate and potentially what smells or pests it might introduce into a greenhouse environment. But it’s a good method to try if you have rabbits in a hutch as it helps them out too. But, as one of our readers has pointed out, you do need to be extremely careful on sunny days as, even in winter, the temperature can rocket and cause harm to the animals. So she has suggested this method should only be used overnight and not during the day. 

Using Candles To Heat Your Greenhouse
Using candles to generate heat is a very traditional method and a very DIY way to heat your greenhouse. It basically involves creating a little ‘terracotta pot’ heater using a candle – usually a tea light – a terracotta / clay pot with a hole in the base and some bricks or stones to stand the pot on – you might also want a saucer or similar to stand the candle on, depending on where you are placing this homemade ‘heater’ in your greenhouse.

The principle is that the candle – in its saucer or just stood in the greenhouse – is covered by the turned upside down terracotta pot which is lifted slightly off the ground using the bricks or stones to allow air to the candle to enable it to burn efficiently. Try to avoid standing the candle directly below the hole in the base of the pot – your aim is to heat the walls of the pot not to direct the heat through the hole.

As the candle warms the pot, the warmth it holds will be released into the surrounding air and in this way you have created a little, DIY candle heater.

With this method of ‘heating’ you are only producing a small amount of warmth – for a cold frame or mini greenhouse this could help to keep it frost free but for a full size greenhouse this is unlikely to produce sufficient warmth. It might prove useful if you only have a few plants to keep frost free as you could group them around the terracotta heater, but it would be a case of trying it to see how successful it is.

Caution should always be taken when using an open flame and hot wax and this is our main work with using this method. There are so many potential flammable materials in your greenhouse, including the greenhouse itself if it is plastic. So be really careful if you decide to try this method and read up everything you can before setting up a candle terracotta pot DIY heater.

If you have tried any or all of the above methods and you find they don’t produce sufficient heat for your greenhouse, then you might want a more conventional way to provide heat – that being to add a greenhouse heater. With any greenhouse heater you will incur the original purchase price of the heater plus the fuel required to power it – be that electric, natural gas, propane gas or paraffin.

In this section we will take a look at the different types and their associated running costs.

Are Greenhouse Heaters Expensive To Run? Electric Greenhouse Heaters

Electric greenhouse heaters are the easiest way to add heat as they simply rely on you having an electric supply to your greenhouse for you to plug the heater into. There is no fuel to purchase or monitor, no pipes or regulators . . . simply select a suitable greenhouse heater for the size of your greenhouse, plug it in and turn it on – it’s that simple.

However, the cost of running an electric heater is likely to be higher than using a gas or paraffin heater, depending on how powerful (what the wattage of the heater is) the heater is and how long it is running for every day.

Let’s take a look at some of the electric greenhouse heaters that are available to purchase and show you how to calculate how much they cost to run.

Electric heaters for your greenhouse can be divided into two main categories – electric tubular heaters or electric fan heaters.

When you look at the details of electric tubular heaters you are likely to discover that they are usually low wattage heaters, designed to produce a low level, background heat. They tend to be perfect for use in smaller greenhouses, but you may need to use multiple tubular heaters in larger greenhouses to produce sufficient warmth to keep the space frost free or above.

Looking at the electric tubular heaters which we stock at Two Wests, we have put together a table below which shows the wattage and size of heaters available.

What you then need to consider is how many of these heaters you would need to heat your greenhouse – so you need to look at the size of your greenhouse and how many watts of power are recommended to keep that space frost free. To help you with this we have put together this second table.

So now you can calculate how many heaters it would take to keep your greenhouse frost free by comparing the wattage of the different size heaters and seeing which sizes will fit around the wall of your greenhouse. If we assume that your greenhouse is insulated, then the table below shows you which heaters you would need.

What you might decide, by using the table above, is that it would be more cost effective and more efficient to use a higher wattage fan heater rather than so many low wattage tubular heaters.

One of the simplest and most efficient ways to heat your greenhouse is to use an electric fan heater. Fan heaters not only produce heat – there are lots of different heaters on the market with different power ratings so the power should be available to produce the heat you require – but they also incorporate fans which will circulate the heat quickly and efficiently around the greenhouse – so avoiding hot and cold spots. 

Let’s take a look at which electric fan heater would be suitable for heating your insulated greenhouse:

The table above clearly shows the difference in costs as the power / wattage of the fan heater increases. What will help with the expense of running an electric fan heater is that they normally incorporate a thermostat of some type – enabling you to ‘set’ a temperature you wish to maintain, and the heater will then automatically turn off and on to maintain the temperature. By using a thermostatically controlled heater this should mean the heater isn’t running when it doesn’t need to so helping to keep running costs efficient.

If you don’t have access to electricity in your greenhouse, or don’t want to use an electric heater, then you could look to use either a gas or paraffin heater.  

Are Greenhouse Heaters Expensive To Run? Natural Gas Greenhouse Heaters

Natural gas – also referred to as ‘mains gas’ as it is the gas we receive in our home if we have gas central heating – is extracted as a gas from the ground or seabed and is a mix of gases but is mainly comprised of methane. 

Natural gas is one of the most economical fuels in the UK but is one of the least popular types of fuel used in a greenhouse because it needs to be piped into the greenhouse, so has to be installed by a suitably qualified person, who would also need to fit your heater. However, if you are lucky enough to have mains gas in your greenhouse then it will prove an efficient and cost-effective way to heat your growing space.

When looking at the cost of natural gas it is measured in units of kWh (like electricity) - kilowatt per hour – and as of October 2023 the average price is 7p per kWh – although this price can vary depending on supplier, so you are best to check on your gas bill as it will state there the kWh rate you are paying.

Knowing what you pay per kilowatt per hour will enable you to calculate the costs you could incur when heating your greenhouse – simply compare the wattage of your heater and multiply this by the kWh. However, as your gas heater is likely to have a thermostatic control which will turn the heater on and off throughout the day to maintain the temperature you have set, realistically you are calculating the maximum charge – it could cost less.

When we compare this 7p per kWh for natural gas, to 34p per kWh for electricity (as checked in October 2023) then we can see this is a cost effective method. However, if you have not got mains gas piped to your greenhouse, it could prove costly to have it installed. Plus, the heater needs to be installed by a qualified fitter. Once these costs have been overcome you will benefit from lower running costs and a fuel source which you don’t need to keep topping up (like propane or paraffin).  

Are Greenhouse Heaters Expensive To Run? Propane Gas Greenhouse Heaters

Basically, propane gas is a form of LPG – liquified Petroleum Gas – which can be liquified and stored in propane gas bottles. Propane gas is not affected by the cold weather in the UK as it has a low boiling point of minus 42 degrees Celsius, making it ideal for outdoor use, including your greenhouse.

It is also available in a range of different sized bottles, from 13kg up to 47kg, making it easy to store – we recommend a 19kg gas bottle or above when being used with a greenhouse heater.

Propane gas is an ideal fuel for a heater thanks to its high-octane rating, which makes it highly efficient, producing more energy per cubic metre than other forms of fuel.

When we study the costs of using a propane gas heater to keep our greenhouse warm, your shopping list of costs includes:

• The price of the heater – different brands and KW of heaters will affect the price
• The costs of the parts needed to connect the heater to the gas bottle – this includes the gas pipe, gas clips and regulator – although you will find with some greenhouse heaters these are included free
• The on-going costs of the fuel – so the costs of the cylinder of propane gas, which again will vary dependent on supplier and size of bottle.

Looking at the price of gas bottles, if you were to purchase our recommended size of 19kg in October 2023 you would find them available from as little as £36.95 up to £124.99, with most companies charging around £50 a bottle. We can then calculate how much it would cost to run a propane gas heater for one hour using a formula provided by one of our heater suppliers:

For 1KW of heat you would use 0.074kg of gas per hour.
Therefore, if a 19kg bottle cost £50, 0.074kg of gas would cost 19p per hour. 

Comparing this to the costs we have seen previously, currently electric is proving the most expensive at 34p per hour, followed by propane gas at 19p per hour and natural gas at 7p per hour.  

What You Can Do To Reduce The Costs Of Heating Your Greenhouse

If you do need to use a heater in your greenhouse in order to use it over the colder months – even if it is only to overwinter more tender plants – then there are things which you can do to ensure your heater is used efficiently and that it doesn’t become expensive to heat your greenhouse because you are keeping it too warm.

The first thing to do is to decide what temperature you need to keep your greenhouse and this all depends on what you are using it for and what plants are going to be in residence during the colder months.

If you are a specialist grower you probably know exactly what temperature your greenhouse needs to be to ensure your plants thrive. However, for most amateur gardeners, their greenhouse will simply be a place to overwinter plants and possibly start sowing seeds earlier in the year to get a longer growing season. From the aspect of overwintering, look at the plants you have and what temperature they will need in order to remain healthy.

Many plants just need to be kept frost free (so don’t want to be in temperatures less than 4 degrees Celsius) including begonia tubers, cannas, dahlias, geraniums, gladiolus corms etc. If you only need to keep your greenhouse like this then use a max min thermometer to monitor the temperature in your greenhouse prior to adding a heater. If you find that the temperature inside your greenhouse is almost this anyway, then try the following:

• Insulate your greenhouse with horticultural bubble wrap
• Cover the plants in your greenhouse with fleece
• Group small plants under a cloche.  

Our blog posts are created from our personal knowledge, information gathered by speaking to other gardeners or manufacturers in the gardening industry, by reading gardening magazines and devouring information from books and the internet. We aim to be as accurate as we can, so if you find a mistake, please remember, we’re only human. if you have any queries you can contact us today!