Great news! Making money by cutting your carbon footprint to zero is a new reality. I own a large house built in the 1970s. I’ve already cut its energy bills and carbon footprint to zero, making a 15% return on investment in the process (see my previous Zero Energy Project blog post for details). Now I’ve done the same on my swimming pool. It was quicker, cheaper, and made a higher return on investment to “go zero” on our pool than it did on our house.

By “going zero” on our pool I saved about $3,000 a year and cut about 9 tons of carbon dioxide emissions a year. We did this by installing energy saving devices that I call the “pool fab four”: a new variable-speed pool-water circulating pump, an electric-powered pool vacuum/cleaner, a heat pump pool heater and about five solar panels to power it all. Going zero on our house saved us about $11,000 a year and cut our carbon footprint about 43 tons per year, so the savings on the pool made a significant addition to my total gain from going zero in the first place.

I am a physicist by training so I have a solid understanding of technologies like solar panels, heat pumps, and pool-water circulating pumps. I also have an MBA from Harvard Business School and spent 20 years as President or CEO of private and public companies. So I also understand how to use discounted cash flow analysis (the standard way companies analyze investments) to make financial calculations like internal rate of return (IRR, a measure of return on investment) and payback period (how long it takes to get your investment back). I keep detailed records of my pool’s propane and electricity use in order to measure the effect of each improvement we made to our pool. My results are based on my own experience rather than generalized claims made by manufacturers or installers.

In going zero, I only took measures that made financial sense, i.e., upgrades that paid for themselves with the savings on heating and electricity bills. This excluded doing many things typically done in deep-energy retrofits for swimming pools:

  • I replaced my old fixed-speed pool-water circulating pump with a variable-speed pool-water circulating pump and set its speed to the lowest setting needed to circulate all the pool water once each day, the rate of exchange required to keep the water clean. This reduced the pump speed from 3,450 rpm for about 12 hours a day to about 2,000 rpm for 24 hours per day. The laws of physics state that the electricity used by a pump goes up as the cube of the rpm, so slowing the pump down more than compensates for the doubling of the time that the pump is on. This effect is so powerful that it cut the electricity used to circulate the water by 87%, saving us over $2,000 a year. The new pump paid for itself in the first year with a return on investment of 100% a year. I wish I could invest my 401k in pool pumps!
  • I installed an electric-powered Dolphin™ pool vacuum/cleaner to replace the old pump-driven pool vacuum cleaner. This is safe because it is connected to a GFCI (ground-fault circuit interrupter) electrical socket and because we take it out of the pool and disconnect it whenever anyone is in the water. This alone saved us about 2,000 kilowatt-hours of electricity per year or over $400 worth. It pays for itself in about 3 years with a return on investment of 55% per year. This keeps the pool cleaner too, which eliminates the dreaded summer rite of scrubbing algae off the walls of the pool.
  • We use the pool only in summer, so an air-source heat pump pool heater is appropriate. My pool heat pump is about four to six times as efficient as my old propane-fired heater. In many areas, electricity is also less expensive than propane as a fuel in dollars per btu delivered, if you use the electricity to run a heat pump. We installed a single heat pump, one designed especially for pools, at a total installed cost of $6,000. The savings on the propane bills, after accounting for the extra electricity the heat pump uses, will pay for the heat pump in about 16 years. That’s a return on investment of about 4% per year after tax.
  • I did not add a pool bubble blanket to insulate the pool. This is because with the heat pump and solar panels, the cost of heating our pool is very low, just over $100 a year. This is using the highest measure of the cost of electricity from my solar panels (7 cents per kilowatt-hour) and was calculated as the total installed cost of the panels divided by the guaranteed electricity output of the panels. The running cost of my solar panels is zero. Including the storage rack, a pool bubble blanket for our pool would cost about $1,000 and might last ten years (the warranty is eight years) for an annual cost of about $100 a year. Spending $100 a year on a pool blanket to save a small fraction of $100 a year in heating costs makes no sense. We didn’t like the idea of having a large piece of rolled up bubble-wrap in the garden either.
  • We did not use a pool chemical blanket either. Pool chemical blankets are liquids, usually alcohols, you pour into the pool that create a thin layer on the surface that reduces evaporation (and heat loss) from the pool. Over four weeks in the summer of 2017 I did experiments with and without a pool chemical blanket. I found that they do reduce the loss in temperature of the pool overnight by about 1°F (the pool naturally loses about 2-3°F overnight) but they cost about $50 a week. I can replace that lost heat with my heat pump and solar panels at about a third of that cost and with none of that inconvenience.
  • I opted not to install the standard unglazed, mat-style solar hot-water panels because solar electric (photovoltaic or P.V.) panels powering a heat-pump water heater are more cost effective. Plus, the net-metering agreement with our utility allows us to accumulate a credit for excess electricity generated on sunny days so we can use that electricity on cloudy days. With solar hot-water panels you are out of luck getting warm water on cloudy days and there’s less opportunity to store the excess from sunny days. You can learn more about the combination of photovoltaic panel and heat pump water heaters in this blog post.

Instead of installing solar panels only to power your swimming pool, you are better off installing solar panels for your house and installing a few extra to power your pool. Instead of installing solar hot-water panels, I covered the roof of my house with a 15kW array of 46 Sunpower 345W panels and covered the roof of my garage with a 13kW array of 40 Sunpower 327W panels. That sounds like a lot of solar panels, but I make money on every solar panel. Solar panels are cheaper than they have ever been and are heavily subsidized. My total investment in solar panels, after the tax breaks and subsidies, was about $42,000. My savings are over $5,500 per year on electricity bills. My investment in solar PV panels produces power at about 7 cents per kilowatt-hour (after tax breaks and subsidies). That’s about one-third of what I would have paid my electric utility company here in Massachusetts, which is now 23 cents per kilowatt-hour. The solar panels will pay for themselves in about 7 years. But the most important economic measurement is the internal rate of return (IRR): 13% per year after taxes.

My overall investment (after tax breaks and subsidies on the solar panels) for the entire zero-energy retrofit of my pool was about $10,000 and I am saving about $3,000 a year on propane and electricity bills. The investments pay for themselves in just over 3 years and the IRR is about 44% after tax. That handsomely beats the growth of the Standard & Poor’s 500 stock index of 11.7% over the last 43 years. It also beats the return on investment we achieved by going zero on our house, which was about 15% per year. Additionally, the S&P return is before tax and before fees whereas my return is after tax and with no fees. Finally, financial markets are risky especially when seeking the highest returns. The risk associated with energy efficiency improvements and solar are usually nil and are will increase with inflation.

Much as I found with going zero on our house, the conventional wisdom on going zero on a pool was often wrong and did not make financial sense. I found that using the standard financial evaluation process used by companies and investors together with some simple experiments cut through the myths. So I think I’ve found a way to both go zero and make money. This represents an inflection point for the zero energy movement. Going zero is no longer like buying an expensive lifestyle statement for your house or pool. It’s cold, hard cash in your pocket, proving you can save money and save the planet!

You can read more about my experience and a summary of my books, Zero Carbon Home and Zero Carbon Pool, detailing how I did it all at

David Green lives and works in Dover, Massachusetts.