How A Variable Speed Pump Will Save You Money
Calculating Savings With A Variable Speed Pool Pump
One of the biggest false arguments against variable speed pumps versus single speed pumps is that you can just use a single speed pump and turn it off part of the day. This is how people used to save money with their pool pumps and the argument exists today that this is a viable option. In my expert opinion this is a fallacy and one that is argued mostly by people who do not understand the math behind exactly how (and how much) energy you save by using a variable speed pump. Let’s look at this equation of a variable speed pump versus a single speed pump that only runs part of the day.
First, a single speed pump of even moderate size will usually have the ability to meet the turnover rate for filtration of the average pool many times over per day. This part of the logic is sound. Single speed pumps move a lot of water and most pools can easily turn off the pump for part of the day and still meet one, two or three turnovers of the pool water per day. For the purposes of illustration we can use the calculated flow rates for a generic 1.5 horsepower pump.
Example 1.5HP Pump – This example pump is rated to move 65 GPM assuming 40 ft pounds of head. This calculates to 3900 gallons per hour and a potential for 93,600 gallons per 24 hours. Even if you apply the semi-commercial standard of three turnovers of the pool water per day through the filtration system this means this 1.5 HP Pump could effectively filter a 31,000 gallon pool. That would be a 20’x40′ rectangle pool with an 8′ deep end. Again, this is based on three turnovers per day which is great, but more than what the average residential pool owner aims for. The average residential pool gets by with a single turnover per day which is enough to keep all but the heaviest bather load pools clean and clear. This is not ideal but it is very common.
Looking at some common numbers a 16’x32′ pool with a six foot deep end might be around 17,000 gallons. Using a minimal approach to filtration this means you need to pump 17,000 gallons per day through your filter. A single speed 1.5 HP Pump might be able to do this in 4.36 hours of run time. Since one horsepower is equal to 746 Watts, this means the 1.5 HP pump might use 1119 Watts which is approximately 5 Amps at 230 volts. In the real world this pump will actually draw more than this amount with just over 7 amps at 230 volts being a more realistic number (more on this later in the article), but for this example we will use the extremely conservative (unrealistic) calculation of 5 amps. When you pay for electricity you are paying based on kWh (kilo-watt hours). To determine the kWh rating for this pump simply multiply the Watts times the hours that the pump will run. At 4.63 hours and 1119 Watts, this gives you a total of 5181 Watt-hours or 5.181 kWh. To meet the bare minimum turnover you would need to pay for 5.181 kWh of power consumption. Now let us try this equation again but this time using a comparably sized variable speed pump.
Example 1.5HP Variable Speed Pump – For simplicity an easy comparison can be made using an established energy law for centrifugal pump motor energy consumption versus RPM speed. The pump affinity law states that for a reduction in RPM’s by half there is a reduction in energy consumption of eight times. This means if you reduce your pump motor RPM by half then you reduce the electricity it uses by eight times. It is the nature of this non-linear reduction in energy consumption that is the backbone of the efficient technology that variable speed pumps are based on. Turning a single speed pump off for part of the day can simply not compete with the exponential decrease in electricity you use as you dial down the motor RPM of your pump.
A pool pump that uses 1119 Watts per hour at 3450 RPM (single speed pump RPM’s) will use only 140 Watts at 1725 RPM. At 862.5 RPM’s the motor is using only a scant 17.5 Watts. Most variable speed pool pumps go as low as 500 to 600 RPM’s so you can really see how this can reduce on the electricity that you use. It is important to note that these are all theoretical numbers which most likely will not be reproduced on your system due to pump and plumbing system inefficiencies. This also helps to illustrate how horsepower ratings mean nothing since a 1.5 HP pump should run at 1119 Watts – but double this number is actually much more accurate once you take into account energy losses and service factor for the motor.
For this example a single speed pump uses 5.181 kWh of energy to pump 17,000 gallons of water (3450 RPM for 4.36 hours). You could run a similar variable speed pump at 862.5 RPM for over 63 hours for the same amount of energy it takes to run a single speed pump for 4.63 hours. Do you see how drastically these numbers do not line up? Every time you reduce your motor RPM by half the energy used goes down by eight times. Most variable speed pumps go as low as 600 RPM or so, and the electrical cost savings from this are unmistakable. This is purely a math exercise as well and does not even begin to factor in that filtering your pool 24 hours per day is vastly superior to having your pool sit stagnant for almost 20 hours every day.
To put this calculation another way, for every 24 hours that you run a single speed pool pump you can run a similar size variable speed pool pump at 1/4 the speed for 1536 hours. 24 hours of inefficient operation versus over 1500 hours of low RPM, more energy efficient operation. If you have a pool where you can get away with a lot of low RPM running then you are a very strong candidate to get a good return on your investment in a variable speed pump.