Because we are in the solar energy business, people often want us to compare a solar pool heater to an electric heat pump. There really is no comparison, because solar pool heaters work every day, and you wouldn’t leave an electric heat pump running unless you really wanted to swim every day — it’s too expensive!
“But wait,” you say. “The heat pump sales guy told me a heat pump costs just a dollar a day to run a heat pump in Southwest Florida.”
Not so fast. That’s based on a whole slew of assumptions about the setpoint temperature and the outside air temperature and when you can and can’t use your heater to achieve this miraculously cheap energy. Water heating is about physics (it takes one BTU to heat one pound of water one degree Fahrenheit). Heat pump manufacturer rate their units with a coefficient of performance (COP) that tells you how many units of electricity it takes to heat water under a set of test conditions. It is using this test data that they estimate the amount of electricity it will take to heat your pool. Sadly, the assumptions never seem to quite work out right. If you are trying to heat your 75 degree pool when it’s 65 degrees outside, just forget it. A dollar-a-day won’t buy you one degree.
I recently did a little test that I felt was a good example of how people actually use their pool. The nieces and nephew were coming over for a pool party earlier this month and I wanted to make sure they (and I) had a nice warm pool to swim in. I decided to fire up my electric heat pump to see what happened to my electric bill. On the Wednesday before their Friday arrival I checked the pool temperature, which reached a high of 74ºF. The plan was to run the heat pump on Thursday and Friday (it takes a while for heat pumps to heat a pool). My goal was to get the pool to 85ºF by their Friday afternoon arrival, which I did. The heat pump sales guy would like you to think this would cost about two bucks.
Below is a daily energy use chart that my utility company, FPL, provides to homeowners. It shows the daily energy use for the last 30 days. On some days my net usage was negative (I have a solar electric system). Can you guess which Friday the family showed up?!
It’s not hard to see the two days on which I ran the heat pump. My electricity use was drastically above the average. During the four days leading up to the visit I consumed about 20 kilowatt hours (kWH) per day. On the two days that I ran the heat pump, I averaged 100 kWh per day. The extra 80 kWh per day costs about $9, so that’s $18 to heat my pool for two days (and on the first day it only reached 82 degrees). That’s a far cry from the dollar a day promise made by your friendly heat pump sales guy.
You see, they are taking into account all of the days that you will not heat your pool, like when the pool is already above their assumed setpoint (which is never 85ºF), and when it’s simply too cold for electric heat pumps to work at all. By including these non-heating days in the “average,” they can quote a very low number. Unfortunately, most people we talk to initially request a pool temperature of about 85ºF, which would cost well above $1/day, and the true cost of operating a heat pump is always reported to be far higher. In fact, many people come to us after receiving bills that cost them “hundreds of dollars” to run their heat pump for a month.
What’s interesting is that my pool could have reached 85ºF virtually every day in March with a solar pool heating system, even without a pool cover. This would have cost zero — there is no additional operating cost above your existing filtration pump energy use.
This all might sound like I really hate heat pumps. The truth is that I like heat pumps for what they are. They can heat pools when you are likely to use them, and with proper planning, they can achieve very nice temperatures most of the time. But, there is a cost associated with this luxury, and it’s not a dollar a day. When used in conjunction with a solar pool heater, you get the best of both worlds, tempering your heat pump energy costs. It can be a good choice for heating spas, especially if you don’t attempt to use the spa when it is very cold outside.
Part of the issue people have is understanding how heat pumps work. By using heat pump improperly, they end up with high electric bills. Heat pumps take the latent heat in the ambient air and use a compressor to “multiply” the energy used to generate heat that is then transferred to your pool water. That is the basic idea at least. You will see all kinds of COP efficiency ratings quoted, but keep in mind that these are all quoted based on a set of assumptions — assumptions that are rarely the reality. Here are two things you really need to understand as a heat pump owner or potential buyer:
- Heat pumps work more efficiently when the outside air temperature is high. Don’t run your heat pump on cool evenings, and don’t run your heat pump when it is very cool. The COP will suffer, and you will consume a lot of electricity with little effect on pool heating.
- Heat pump performance increases as the difference between the water temperature and air temperature decreases. If your pool is 75º and it’s 60ºF outside, don’t expect your heat pump to heat your pool efficiently. Once the pool temperature exceeds the air temperature, the efficiency of a heat pump drops off dramatically.
Most heat pumps are rated at 80/80/80, meaning the air temperature is 80 degrees, the humidity is 80%, and the water temperature is 80 degrees. This hardly approximates reality, so you have to look deeper into the real costs of operating a swimming pool heat pump, even in Southwest Florida where conditions are generally good for heat pumps.