One of the questions we get asked many times is how much will it cost to heat my swimming pool. We wish we had a simple answer that we could give out but we do not, there are so many variables that it is impossible to give a definitive answer so we will try explain why it is so difficult to work out the cost of heating a swimming pool and make a comparison of the different methods of pool heating.
In principal it is easy to know who much heat you need to put in to your swimming pool to heat it up and therefore easy to work out the cost. We have units of measurement for how many kilowatts (kw) of energy it takes to raise the temperature of fixed volume of swimming pool water. For example a small 8,000 litre pool like a 12ft splasher pool will need about 150 kw of energy to raise the temperature from 10 deg C (tap water) to 25 deg C (swimming pool water). With electricity at 8p per kw/hr then it will cost £12.00. Simple. Well yes it is but this example assumes that the pool water does not lose any heat and never will. If only this were true!
In reality the swimming pool water will lose heat not only while you are heating it but as soon as the heater gets the water to the desired temperature it starts to lose heat and you have to constantly replace that lost heat. There are many ways a pool can lose heat and the rate at which it looses heat depends on many factors.
Whilst swimming pools can lose heat through the sides by far the biggest amount of heat is lost from the surface of the pool and the biggest factor here is evaporation. Keeping a cover on the pool will cut down huge amounts of heat loss compared to not having one. In fact on a calm, warm, sunny day a solar cover will not only cut your heat losses it can warm the water. Other factors affecting swimming pool heat loss are how much colder the air is compared to the pool water. This is pretty obvious but the warmer you heat your pool the faster it loses heat. A pool kept at 32 deg C will lose heat at twice the rate of a pool kept at 26 deg C against an ambient temperature of 20 deg C. So if you are trying to save on your heating bills ask yourself if you can tolerate water a few degrees cooler? A small change in temperature can mean a big saving in heating costs.
What do you do if your coffee is to hot? Do you blow on it? Well image your pool as the cup of coffee and the wind as your breath. The wind speed over your pool can have a huge affect on heat loss so making the pool more sheltered or covering with a dome or enclosure can make big savings in heating costs.
So heating a swimming pool is really all about replacing the heat that the pool has lost. No matter how much you do to insulate and shelter the pool there is always going to be big losses in heat. So lets have a stab at estimating the heat required to maintain your pool temperature. Most heaters have a thermostat and these typically detect when your pool drops by 1 deg C and switch them on and they stay on until the temperature is back to where you set it. Lets say your pool drops 1 deg C each night so in the morning your heater has to get it back up and it stays there until evening and then overnight it drops 1 deg C again. Not very realistic but its only an example. Lets assume a season from May to mid September of 135 days.
The table below shows how much heat input is required each day and for the season for various pool sizes.
| Pool Volume (litres) |
Typical size |
Heat req’d per day (kw) |
Heat req’d per season (kw) |
| 8,000 |
12ft Round |
9.3 |
1256 |
| 16,000 |
15ft Round |
18.6 |
2512 |
| 25,000 |
18ft Round |
29.1 |
3925 |
| 40,000 |
12 x 24ft |
46.5 |
6280 |
| 64,000 |
15 x 30ft |
74.4 |
10048 |
| 100,000 |
20 x 40ft |
116 |
15700 |
|
Hopefully from this table you can get a rough idea of the total heat you will have to put in to your swimming pool. Remember though that the figures above are based on a 1 deg C loss per day. On colder nights it may be more on warmer days it may be less.
The table below lists how much it would cost to put in various amounts of heat using different types of heater. Cost assumptions are shown underneath.
| KW req’d |
Electric |
Gas |
Oil |
Heat Pump |
| 1,200 |
£119 |
£39 |
£67 |
£28 |
| 2,500 |
£249 |
£81 |
£140 |
£59 |
| 4,000 |
£398 |
£129 |
£224 |
£94 |
| 6,000 |
£597 |
£194 |
£336 |
£141 |
| 10,000 |
£995 |
£324 |
£560 |
£235 |
| 15,000 |
£1,493 |
£485 |
£840 |
£353 |
|
Cost assumptions
Electricity – 9p per kw/hr heater efficiency 92% cost per kw output 9.78p
Gas – 2.75p kw/hr heater efficiency 85% cost per kw output 3.24p
Oil – 45p per litre = 4.2 p per kw/hr heater efficiency 75% cost per kw output 5.6p
Heat pump - 9p per kw/hr heater efficiency 400% cost per kw output 2.25p
Conclusions.
For small pools up to about 15ft round the lower purchase cost of an electric heater makes it a good buy. But if your pool is any bigger then the electric option is less attractive. Taking the longer view, say 5 years of ownership, the extra purchase cost of a heat pump is more than paid back in lower fuel costs.
