Variable Speed Pump Electrical Savings
You have probably heard the claims about fantastic variable speed pump electricity savings up to 80% or 90% less than what you pay to run your current filtration pump. While these claims might feel too good to be true, and for some pools they are too good to be true, there is science behind the claims that help support these numbers. Specifically the affinity laws for centrifugal pumps states that when the motor RPM is reduced by half, the electrical draw to the pump is reduced by eight times. This non-linear drop in power consumption is how variable speed pump manufacturers are able to claim such lofty electrical savings numbers.
In the real world, and on your pool, this is actually a much more dynamic equation. If you were to simply run a variable speed pump around the clock on 1/4 RPM, or about 860 RPM, then yes you would be using substantially less power than a single speed pump running on the same schedule. About 80% to 90% less. Unfortunately this is probably not going to work on most (many) pools since pools typically require at least some periods of higher flow daily. Generating chlorine with a salt system, closing the pressure switch on a heater, or running any number of peripheral devices or features such as vacuum cleaners (suction and pressure side), waterfalls and in-floor cleaning systems all need more flow than 860 RPM is going to give you. Depending on your unique pool and plumbing installation 860 RPM might not even be able to overcome the head resistance of your system and you might need a higher RPM to move any water at all.
To learn more about how a variable speed pump works under real world running conditions you can read this article about variable speed pump schedules. What this article shows is how the best variable speed pump run schedule is one that incorporates periods of higher flow along with long periods of minimum (minimal) RPM operation which vastly increases the total volume of water filtered with very little electrical usage. The ability to dial in the exact flow rates and minimum RPM values for your pump is made possible by knowing how much water is moving through your plumbing system. For more information about this critically important instrumentation component that most pools do not have you can read this article about pool flow meters.
Pump RPM = 3450
Amperage draw = 10A
Pump RPM = 1730
Amperage draw = 1.85A
As you can see from this example taken from a Pentair Superflo VS pump (video below), running on a 2" plumbing system, installed with 240 volt electrical supply, the full amperage draw under maximum RPM is just over 10 amps. When motor RPM is reduced by half, the actual amperage draw was only 18% (up from theoretical 12.5%) of the full RPM electrical draw. While most pools will not be able to operate at or under 860 RPM, most pools will be able to run less than 1725 RPM while still overcoming the resistance to flow and actually moving water through the filtration system. At less than half RPM the hourly electrical savings that you experience versus full speed operation of the pump are substantial.
How Much Electricity Do Variable Speed Pumps Use?
When you pay for electricity you do so based on kWh (kilowatt hours). This is an easy calculation to make. First determine how many watts a powered device is using. You can read the wattage display on your pump if it has one, or you can use a wattage meter. In this case we can calculate the Watts for this pump because we know both the current in amps, and the voltage in volts.
Power (in watts) is equal to the voltage multiplied by the current. In this case that is 10 Amps multiplied by 240 Volts. This is 2400 Watts, which you can convert to Kilowatts by moving the decimal point three places to the left. In this case that would be 2.4 kW. To determine kWh simply multiple the watts by every hour the pump is running. In this case running at full RPM this pump uses 2.4 kW every hour. One hour of running would be 2.4 kWh, two hours of running would be 4.8 kWh, and four hours of running would be 9.6 kWh.
3450 RPM for four hours = 9.6 kWh ($0.11 / kWh = $1.06)
1725 RPM for four hours = 1.78 kWh ($0.11 / kWh = $0.195)
Eleven cents per kWh is the current electrical rates in Arizona so these were used just as an example in the above calculation so you can see actual dollar values assigned to each pump RPM. Even at this absolute minimum operating schedule example of four hours of running time you would still be over $30 every month in electrical usage. It is also worth noting that almost no pool professional would agree or recommend to run your pool for only four hours daily, however I have chosen this number specifically as this is most often the lowest filtration running time that pool owners reference as using. Since it costs less than 20 cents to run a pump at half speed, versus $1.06 at full speed, you could save a substantial amount of money every month by running only one hour per day on high speed, and then make up the rest of your filtration with lower RPM operation. With one hour of full RPM and four hours of half RPM, a very simplified running schedule, you would still only be at $0.46 daily for electrical costs. This means with almost no effort to dial in flow rates and RPM for maximum savings we already took the monthly bill of $31.80 and cut it down to $13.80. Converted to electrical cost savings you would experience in total, that is $18 per month, $216 per year, or $1080 after five years of operation.
You should not be running your pump for only four hours per day but even if you are you will still save more than the purchase price of a new variable speed pump in electrical savings. For pool owners who run their single speed pump for more than four hours per day, which I really hope that you are, the savings add up even faster. When you crunch the numbers for electrical costs and consider that filtering the water slower over longer periods of time is beneficial, there really is no reason left to hold out against this new pump technology. You should be excited that a new technology exists that allows you to dramatically reduce the operating costs for one of the single largest electrical draw appliances in your home. If you are ready to make the switch check out some of the best variable speed pumps currently available.
Variable speed pool pump schedules
Cost breakdown for 24 hour VS pump electrical use
Most popular variable speed pumps
Variable speed pool pump reviews
The Swimming Pool Steve blog
Proof of variable speed pump savings
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