After this thread about how to build a net-zero house, I was asked to do one on the cost / benefit of energy efficiency work. I’ve got good data on this from our project, so here goes. Caution – nerd thread ahead.
cc: @energysmartohio @EfficiencyLast @EsTresidder
cc: @energysmartohio @EfficiencyLast @EsTresidder
@energysmartohio @EfficiencyLast @EsTresidder 2/ The point of energy efficiency work is to cut energy use, which results in lower energy costs. If it doesn't, then you're not helping the environment or your wallet.
The discussion about cost effectiveness has been going on in this good thread.
The discussion about cost effectiveness has been going on in this good thread.
@energysmartohio @EfficiencyLast @EsTresidder 3/ Here’s the curve of diminishing returns. This shows construction cost and energy use for four new-construction homes. To cut energy use, you’re generally spending money on insulation, windows, and air sealing. Energy Use Intensity has dumb units. 1 kBTU/yr/sq-ft = 0.36 W/sq-m 
@energysmartohio @EfficiencyLast @EsTresidder 4/ The four houses are:
My house built to code minimum standard - “bad”
Nate’s project - “good”
My house - “very good”
PassivHaus standard - “excellent”
We can argue about how much it costs to hit Passivhaus standard, but it won’t change the conclusions.
My house built to code minimum standard - “bad”
Nate’s project - “good”
My house - “very good”
PassivHaus standard - “excellent”
We can argue about how much it costs to hit Passivhaus standard, but it won’t change the conclusions.
@energysmartohio @EfficiencyLast @EsTresidder 5/ But wait. We can just convert energy use to solar panels and compare upfront cost. From “bad” to “good” it’s a 1:1 trade. Beyond that, it’s cheaper to just buy more solar panels, as long as you have room for them.
@energysmartohio @EfficiencyLast @EsTresidder 6/ If you’re buying power, we should compare costs as monthly payments. At my mortgage rate $1,000 of upfront cost becomes a payment of $4.70/mo. In reality, I’d *much* rather pay a mortgage than an energy bill. But most people just look at their monthly expenses.
@energysmartohio @EfficiencyLast @EsTresidder 7/ Where I live, it costs less to heat with gas than with a heat pump. It stinks that polluting is free, but that’s the way it is. The sweet spot of the curve is where Nate’s project is. If you spend more on the house, then the total monthly cost goes up.
@energysmartohio @EfficiencyLast @EsTresidder 8/ But… it costs less to buy solar panels than it does to pay for electricity. So things shift if you’re generating the electricity yourself. Now the heat pump is almost level on costs with gas.
@energysmartohio @EfficiencyLast @EsTresidder 9/ And it shifts still more if we get to trade electrons for gasoline. If we can add more solar panels, we get to run an EV with them. This is what it looks like in my case. My house is equal on costs to conventional, and Nate’s project would be almost $100/mo cheaper.
@energysmartohio @EfficiencyLast @EsTresidder 10/ Assumptions:
Solar generation rate: 3.16 kWh/day per kW of capacity
Solar cost: $1.92/W after tax credits.
Mortgage rate: 3.875% 30-year fixed
Electricity cost: 11.7 c/kWh
Gas cost: $9.59/DTH
EV mileage: 0.32 kWh/mi
Gasoline mileage: 22 mpg
Gasoline cost: $3.10/gal
Solar generation rate: 3.16 kWh/day per kW of capacity
Solar cost: $1.92/W after tax credits.
Mortgage rate: 3.875% 30-year fixed
Electricity cost: 11.7 c/kWh
Gas cost: $9.59/DTH
EV mileage: 0.32 kWh/mi
Gasoline mileage: 22 mpg
Gasoline cost: $3.10/gal
@energysmartohio @EfficiencyLast @EsTresidder 11/ I’m happy with where we ended up. We get a comfortable, resilient house with no indoor air pollution and no emissions, for the same cost as a conventional house. We're past the low point on the curve, but we needed to in order to be able to fit enough solar on the roof.
@energysmartohio @EfficiencyLast @EsTresidder 12/ I’ll finish with the best EE project I’ve worked on. This was a retrocommissioning project at The McGillis School. For an upfront cost of $0.33/sq-ft, we cut the building’s energy use (and bills) by 40%, with a payback period of less than two years.
archive.sltrib.com/article.php?id…
archive.sltrib.com/article.php?id…
@energysmartohio @EfficiencyLast @EsTresidder 13/ If you want to know the energy use intensity of your house, you can use @energysmartohio's excellent calculator.
energysmartohio.com/all-electric/e…
energysmartohio.com/all-electric/e…
@energysmartohio @EfficiencyLast @EsTresidder This is super cool. Here’s what the conventional utilities curve looks like for fuel cost increases of 5% p.a. - after 10 years (1.63x), 20 years (2.65x), and 30 years (4.32x). The low point of the curve shifts right, and EE becomes a better investment.
@energysmartohio @EfficiencyLast @EsTresidder I've appended this cost / benefit thread, in pdf form, to the one about the house construction at tinyurl.com/w2439yr
