This article is not about what size of pump to choose, it is about comparing costs between pumps of the same size, in terms of their up-front cost and running costs. To read about what size pump to choose, see the article on Filtration and Pumps.
The cost of running a pump is dependent on how may watts it uses as electricity is measured in terms of kilowatt hours, or the quantity of watts, in thousands, used in an hour. The larger the pump the greater the watts it uses, however pumps of the same pumping power can differ considerably in their watt usage. An energy efficient pump that delivers 4200 gallons per hour may run at 550 watts while another that delivers the same volume of water may run at 850 watts.
Does that mean that over time the 550 watt pump will save money? Not necessarily. Pumps are only warrented for one or two or three years, generally. If you live in an area where electricity is cheap and the pumps you are considering are warranted for a year or two, then the added cost of the energy efficient pump, and its replacement, may be greater than the energy costs it will save over the same time period.
If, on the other hand, you live where the cost of electricity is high and you are looking at a pump with a three year warrenty that uses significantly less electricity than the other, then the energy efficient pump may more than pay for its extra up-front cost over the long term.
How to know? A Life Cycle Cost Analysis. Find the cost of electricity in your area, figure how long the pump should last, (generally at least two times the warranty), the number of watts it runs on and do the math. Here’s an example.
Say your pump uses 550 watts/hour and you plan to have it running continuously. It has a warranty of two years, so figure it will last four. 550 watts per hour in one day amounts to 13,200 watts. Divide that by a thousand to get kilowatts; 13.2. Multiply that by 365 days in a year = 4,818. Multiply that by 4 years = 19,272 kilowatts over its lifetime. Multiply that by the cost of electricity in your area, say $.20 = $3,854.
Do the same for the 850 watts/hour pump and the result is $5,957. There is a difference in energy costs of usage over their lifetime between the two pumps of $2,102. The difference in purchase price of the pumps will be in the vicinity of $200.00.
Does this mean the energy efficient pump is the most economical over time? In this scenerio it does, which assumed a twenty cent energy cost, a lifetime of four years and continuous running for all of that four years, yes. If the pump is run half that time, say 12 hours a day or six months of the year the energy differential would drop to $2,901, still a considerable savings. Reduce the life time to two years or run the pump for less often and the difference will be still less. In some areas the cost of electricity is much less so the cost of running the pump will be much less.
Let’s look at the same scenerio as the first example above, but instead of an energy cost of twenty cents per kilowatt, we’ll assume eight cents per kilowatt. We’re looking at a pump that uses 550 watts/hour, running continuously. It has a warranty of two years, so figure it will last four. 550 watts per hour in one day amounts to 13,200 watts. Divided by a thousand to get kilowatts gives us 13.2 per day. We multiply that by 365 days in a year = 4,818, and that by 4 years = 19,272 kilowatts over its lifetime. Now we multiply that by $.08 (eight cents) per kilowatt and we have a total cost over four years of $1,541.76 for the 550 watt pump and plugging in the 850 watt number in place of the 550 we get $2382.00 over four years. A difference of about $840.00. Reduce the life of the pump to two years and you have a difference in running costs over that two years of $427.00.
As you see, the less the pump is run, the cheaper electrical energy is in your area and the shorter the life of the pump, the less is the differential in running costs between energy-efficient and non eneregy-efficient pumps. In some cases, when the pump will not be run continuously, when it is fairly small and doesn’t use a lot of wattage and the up-front cost of the pumps is significant, it may make more sense to go with the less expensive pump, especially if you are trying to reduce up-front costs of building a pond.
To know exactly which is the most economical way for you to go, do the Life Cycle Cost Analysis. It’s just arithmetic, so once you’ve gathered the necessary data and got your head around the variables, plug them into the formula above and you’ll know which way to go.