A 20 Year Old Energy Efficient House Goes to Net Zero in Florida
Steve Larson, a builder and home energy rater in Florida, recently sent me an email with his energy bills for February through July of this year. When you subtract out the monthly service charges, he paid only $5.35 for electricity during those six months. That's right — less than a dollar a month for electricty...and then $9.88 a month for the service charge. "Saving energy has been my passion for a long time," Larson wrote. Since he added the solar electric system to the house in 2008, producing it has become a passion, too.
He didn't start with a net zero energy home, which has enough onsite power production to offset the energy consumption, but he did start with a mighty efficient one. Larson built the house in 1991-92 (sounds almost like my green home building project, 2001-03). Here are some of the spec's that make it so energy efficient:
- 2144 square feet of conditioned floor area
- Walls: R-19 fiberglass batt cavity insulation with R-3 continuous foamboard on the outside
- Advanced framing: 2x6 studs 24" on center with insulated corners and T-walls (intersections)
- Ceiling: ~R-50 insulation (~R-30 blown fiberglass on top of R-19 fiberglass batts)
- Windows: none on West side, only one on East side, South-facing windows shaded by 6' porch
- 18 SEER 3 ton air conditioner
- Air leakage: 3.4 ACH50
- Duct leakage: 2.7 cfm25 per 100 square feet of conditioned floor area
- Water heaing: solar
- Orientation: long axis along East-West line
If you have a feel for those numbers, you can tell it's a pretty good house. Without the photovoltaic modules, the HERS Index for this house is 65. Remember - this house was built 20 years ago! A HERS Index of 65 means it's 35% more efficient than the same home built to the 2006 International Energy Conservation Code (IECC), so it was 35% better than a code that came 15 years after the house was built!
In 2008, Larson added a 5 kilowatt (kW) photovoltaic (PV) system to the house. That didn't take them all the way to net zero, though because he also has a huge garage and air-conditioned shop. So he added a second 5 kW system in 2010, and that got them to where they are today, paying an electricity bill that's just $9.88 per month in service charges and rarely any charge for actual net electricity consumption.
In case you're wondering, the first 5 kW PV system cost $40 thousand, and they got $23 thousand dollars in federal, state, and utility tax credits and rebates. $20 thousand of the incentives came from the state of Florida. The second 5 kW system cost only $28 thousand dollars, a 30% drop in just two years.
The HERS Index of the home including the energy production from the solar modules is an impressive -14. A HERS Index of zero means that the home produces just as much energy as it uses, no more and no less. According to the HERS Index, their home exceeds the net zero energy threshold. "It has been a very comfortable home to live in for the last 20+ years, but the $9.88 electric bills have us in a very nice position. My wife is a very happy camper," said Larson.
According to the elecricity bills he sent me, Larson's electricity usage is generally in the range of 500 to nearly 1000 kilowatt-hours, mostly toward the lower end. Some months they produce more electricity than they use, as you see in the meter readings to the left, and other months they use more than they produce. The snippet at left is from their May electricity bill, showing that they used 490 kWh that month while producing 965 kWh. They generally run a surplus, as you can see in the bottom number, which is the running total of their electricity production versus consumption.
Steve and his wife are helping to make Florida's nickname—the Sunshine State—true in more ways than one. "We are pretty happy how far out in front of the running of the bulls we are," Steve said. Of course, someone who's been a HERS rater since 1994 is used to being out in front.
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