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Concentration of CO2 in the Atmosphere

My Zero Energy Retrofit Beats My 401(k)

A DIYer Project

Our new wood-framed triple-glazed windows have almost eight times the insulation of the old single-glazed ones and have transformed the look and feel of our living room. Courtesy photo.

David Green

I own a 5,400 square foot home built in the 1970s. I did a deep-energy retrofit and successfully achieved my net zero carbon goal. The return on investment on my zero-energy retrofit is about 15% per year. No fees, no taxes, no volatility. Sure beats my 401(k)!

I am a physicist by training, so I have a solid understanding of energy saving technologies. I also have an MBA from Harvard Business School, so I also understand how to use standard financial analysis to see which investments would make me money.

For the past two years, I have kept daily records of my heating oil and electricity use, so that I could measure the effect of each improvement. My results are based on real measurements, they are not predictions based on a software model or on manufacturers’ claims.

In going zero, I only took measures that paid for themselves. This excluded doing many things typically done in deep-energy retrofits:

  • I did not remove and replace my siding to add insulation because it is so expensive that the savings on utility bills would never pay for it. Instead, I added 12” of fiberglass to the ceiling of my basement. This cost me $1,000 and paid for itself in four months. I installed it myself. The return on investment is over 100% per year.
  • I did not install geothermal (ground-source) heat pumps, because modern air-source heat pumps are almost as efficient but are about one-quarter the cost. My heat pumps are about four times as efficient as my old oil-fired furnace. Ours cost about $23,000 installed. The savings on the oil bills (after accounting for the extra electricity the heat pumps use) will pay for the heat pumps in about 9 years. That’s a return on investment of about 9% per year after tax. These heat pumps both heat and cool the house. They replaced our old air conditioning units. They cut our house energy use about 55% and are saving us about $3,000 per year.
  • I opted not to install solar hot-water panels, because solar photovoltaic (PV) panels powering a heat-pump water heater are more efficient and more effective. Solar hot-water panels tend to overproduce hot water in the summer and do not make enough hot water in the winter. Net-metering allows us to store credits for excess electricity generated during the summer months and then use those credits when the sun is not shining. Hence, we can use solar PV plus net metering to generate hot water even on cold cloudy days, which is impossible with solar hot-water panels.

Instead of installing solar thermal panels, I covered the roof of my house with solar panels. 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. The panels will pay for themselves in about 7 years. The return on investment is about 13% per year after tax.

Our first solar panel array on the roof of our house as seen from our son Jack’s drone. © Jack Green.

I installed modern low-E triple-glazed windows, which have about eight-times less heat loss than my old single-glazed windows. These do not pay for themselves, at least they won’t for decades. However, the additional cost of installing triple-glazed windows above the cost of installing double-glazed equivalents did pay for itself. In other words, since we were replacing our windows and patio doors anyway, it made sense to install R-5 and R-4 triple-glazed ones rather than R-3 double-glazed ones. The additional cost will pay for itself in about five years. The return on investment on the additional cost is about 19% after tax.

My total investment for the entire zero-energy retrofit was $75,000 (after tax breaks and subsidies), and I am saving over $11,000 a year on heating and electricity bills. The investments pay for themselves in about 6 years. My return on investment is about 15% after tax. That beats the growth of the S&P 500 stock index of 11.7% before tax over the last 43 years. So, I think I’ve found a way to both go zero and make money.

You can read more about my experience, including the low-interest or zero-interest loans that are available to pay for it all at

David Green has a BA in physics from Oxford University and an MBA from Harvard Business School. He founded two biotechnology companies, was CEO of both of them and took both public. He also is the founder of Zero Carbon, LLC. David is the author of Zero Carbon Home and Zero Carbon Pool.

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