I am interested in solar PV. What do you recommend for an old house?


I am interested in solar PV. What do you recommend for an old house?

Asked by Exton Quinn, Santa Monica, CA

I am interested in a solar photovoltaic system. What do you recommend for an old house?

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Paul Rosen's picture

I recommend you first "reduce, then produce" energy for your home. Older homes are generally inefficient, often uncomfortable, and even unhealthy. By improving your home's performance, you can address all three of these issues at the same time, at a lower cost per dollar of energy saved than the cost of producing electricity with a photovoltaic system.

(I provide advice on sizing a solar PV system in a separate Ask A Pro article here.)

To illustrate the advantage of a "reduce, then produce" approach, here's an overview of a home performance project my firm completed last year. The whole project cost approximately $15,000 and the payback should be less than 10 years. By comparison, that same $15,000 could have paid for a 1.7 kW PV system, which would produce about 3,000 kilowatts of electricity worth maybe $750 annually in savings at our high electric rates. That means breaking even on the PV system would take 20 years-twice as long as the 10-year payback on improving the home's performance!

For this project we worked on a beautiful 2,100-square-foot 1908 craftsman house in the Bay Area that had high utility bills, drafts and uneven heating on cold winter days, uncomfortably hot bedrooms during the summer, and a stuffy damp smell after a big storm during the rainy season.

A Home Performance Audit revealed the following issues:

  • A Blower Door test determined the home had a "leakage" rate of 1.2 air changes per hour. The recommended ACH, according to ASHRAE (American Society of Heating, Refrigeration, and Air Conditioning Engineers) is .35 ACH. In other words, the home was almost four times "leakier" than desired. The total of little and big leaks equaled having a two-foot square hole open to the outside.
  • A Duct Blaster test of the natural-gas, forced-air heating system indicated the ducts leaked over 45% of their output. In other words, almost half the heat output never made it into the conditioned living space.
  • Because it was built when energy was abundant, the house had no insulation in the walls; it had a poorly insulated attic and asbestos insulation on the forced-air ducts.
  • The lighting was primarily incandescent lamps and the household appliances were an average of 14 years old.

We took the following steps to improve the home's performance.

  • We reduced leakage by "air sealing" penetrations in the attic and floor, which took care of half the issue. Weather-stripping the windows and doors finished the job.
  • We installed a new duct system, which solved the lost energy and pressure issues, and the uneven heating, by supplying the correct air flow volume and velocity to each of the rooms. The existing furnace was only eight years old and did not need to be replaced.
  • Cellulose insulation (made from 100% recycled material) was blown into the attic to a level of R 38 (exceeding the current R 30 code requirement). In this Mediterranean climate, insulating the walls would have only increased the energy efficiency an additional 5% and was not cost effective or necessary. The air sealing of the attic and floor stopped most of the heat loss through the walls by preventing the flow of air currents that "suck" the heat out via conduction.
  • The crawlspace floor and foundation walls were "vapor proofed" with a special fabric, and the vents were closed off. A mechanical vent fan brings in fresh air when needed. This not only stopped the musty damp smells, but also made the floors warmer and improved efficiency.
  • Most of the lightbulbs were changed to CFLs (compact fluorescent lamps); a new Energy Star refrigerator was installed; and the big-screen TV and entertainment center were put on a master switch that can be turned off when the area is not being used, eliminating the significant standby power loss.

The outcome of this project was:

  • The natural-gas consumption for heating was reduced by 60%.
  • The electric bill was cut in half.
  • The home is now very comfortable, and the space evenly conditioned year-round. There are maybe five or six hot days when it would be nice to have air conditioning, but it's not really necessary.
  • The air quality is significantly improved and all unpleasant smells have been eliminated.

The whole project cost approximately $15,000 and the simple payback should be less than 10 years. Breaking even on a new PV system would take 20 years-twice as long!

Remember the best first step before adding a solar system is "reduce, then produce!"

For more information:

I provide advice on sizing a solar PV system in a separate Ask A Pro article here, and architect Cynthia Phakos has also given good advice on converting a home to solar energy.

Two great sites to learn more about home performance are the EPA's Energy Star website and, in California, the utility-funded Green Home Energy Upgrade program provided by California Building Performance Contractors Association.