So in utility and policy circles, electrification is all the rage. Grid electricity is getting cleaner (i.e. resulting in lower CO2 emissions), on-site renewables are taking off (sometimes even with storage), and heat pump technologies are getting better. More regional and utility initiatives are encouraging building owners/designers/developers to forego onsite fossil fuels entirely (or at least mostly) to help meet CO2 emission reduction goals. But is electricity really more sustainable than natural gas? Is it cheaper? Which is better, really?
No simple answers here. But the rush to electrify makes me nervous. There are situations where all-electric buildings make good sense (lowering CO2 emissions, lowering operating cost, providing good comfort), but I’ve seen plenty of situations where this is not the case. I’ve also seen good opportunities bungled by poor design and execution.
Operating cost is a big issue. Heating with natural gas is generally cheaper than heating with electricity (even with decent heat pumps). Not long ago, I was talking with someone running utility efficiency programs. I asked, “So… you have a strategic electrification program, and you also have a program to expand pipelines to bring ‘affordable natural gas’ to more neighborhoods. What’s up with that?”
The answer: “Yeah, I don’t get it either.”
It’s a strange situation.
So what do the numbers look like? Let’s look at space heating (cooling is pretty much all electric). A space heating load of 50 MMBtu is about what an average, existing, not-very-efficient single family home might use each year in a colder climate. Ballpark costs for heating with gas:
50 MMBtu / 90% eff x 10 therms/MMBtu x $1.27/therm = $705 (556 therms)
Heating with resistance is way more:
50 MMBtu / 100% eff * 293 kWh/MMBtu x $0.19/kWh = $2,780 (14,700 kWh)
…but if you have a good heat pump with a COP of 2.7:
50 MMBtu / 2.7 * 293 kWh/MMBtu x $0.19/kWh = $1,030 (5,430 kWh)
So heat pumps still cost $300+ more than gas, but it’s getting closer. Energy rates vary like crazy; I’m just using some typical prices in the Northeast. And yes – these are totally oversimplified calculations, but they’ll do for this example.
What about CO2 emissions? Let’s take the same three examples:
Gas: 556 therms * 11.8 lbm CO2/therm = 6,570 lbm CO2
Resistance: 14,700 kWh * 0.564 lbm CO2/kWh = 8,260 lbm CO2
Heat Pump (2.7 COP): 5,430 kWh * 0.564 lbm CO2/kWh = 3,060 lbm CO2
So from a carbon standpoint, the heat pump comes out way ahead. That’s just using numbers for New England. Some parts of the country have much higher emissions rates (e.g. more coal, fewer renewables) and electrification doesn’t look great. As another example, EPA’s eGRID shows upstate New York emissions at 0.296 lbm CO2/kWh. At this rate, heating with electric resistance results in about 33% lower carbon dioxide emissions than heating with natural gas. So for carbon counters, you can see why electrifying things has appeal. For dollar counters, gas still looks pretty good.
But I think there are situations where going all electric makes sense for all counters. A common feature of these situations is SMALL LOADS. Down south, where space heating loads are modest, heat pumps have been the norm for quite a while. But even in colder climates, heating loads in efficient, new buildings are small. Using a single-family example, an efficient new home might have an annual heating load of 15 MMBtu. Using the same prices and New England CO2 emission numbers from above:
Efficient gas: $212, 1,970 lbm CO2
Resistance: $835, 2,480 lbm CO2
Heat Pump (2.7 COP): $309, 918 lbm CO2
Gas is still cheaper, but only $97 cheaper. I pay my gas company about $15 each month; that’s $180/year (which, for dollar counters, is more than $97).
But what about water heating? That’s a bit trickier. Integrated, tank, heat pump water heaters (HPWHs) can work well in basements up north. They add some heating load in winter, reduce cooling/dehumidification loads in summer, but they don’t work everywhere. You can’t put them in closets, they make some noise, and sometimes they make spaces too cold for comfort (guidelines on how to avoid these issues). There are new HPWH products that go outdoors and take heat from outside air (not inside). These address many concerns, but they’re pricey. Hopefully we’ll see more/better/cheaper HPWH solutions down the road.
But whether or not HPWHs will work in a particular home, electrification may still make sense if there are SMALL LOADS. Use efficient appliances, use low-flow fixtures, insulate tanks and plumbing systems very well, use smart recirculation (if needed). If a home uses 30 gallons of hot water each day, annual cost/emissions would be something like this:
Gas (80%): $101, 942 lbm CO2
Resistance (95%): $374, 1,110 lbm CO2
HPWH (2.4 COP): $148, 440 lbm CO2
Again, gas is cheapest to operate. Resistance results in more CO2 emissions than gas (in New England), but the difference isn’t huge. Even if a HPWH won’t work, the extra 169 lbm of CO2 associated with resistance (over gas) is modest compared to the 1,050 lbm CO2 saved by using a heat pump for space heating. Hopefully HPWH technology continues to become more versatile and affordable; this would make going all-electric even more compelling. (Note that if natural gas isn’t available, and oil or propane are the alternatives, going all electric is more compelling on all fronts).
In a new home, there are also up-front savings if no gas is needed on site (plumbing, venting, trenching – maybe even one less subcontractor). New buildings are also the right focus for electrification because it’s easier to get heat pump systems designed, installed, and commissioned to operate efficiently (and providing good comfort). Heat pumps are a lot more finnicky than boilers or furnaces. You may be able to get a seasonal COP of 3.0 from a great heat pump system, but the same product in a poor application or installation might only get a COP of 1.4. Good systems in new construction are not a given! But chances for good design & installation are better. (See NEEP’s resources for quality ASHPs.)
Another reason going all electric makes sense with SMALL LOADS is equipment cost. A 60,000 Btu/h furnace isn’t much cheaper than a 100,000 Btu/h furnace (and both are too big for most homes). A 2-ton heat pump, however, IS less expensive than a 4-ton heat pump. And small heat pumps can provide both heating and cooling in an efficient home.
I think this is compelling, and there are some builders and developers out there who get it (and don’t even consider using fossil fuels). If programs started to push designers/developers in this direction (cash sometimes helps), I could see all-electric new construction becoming the norm. First costs would be less (without gas), operating costs would be similar, and carbon counters would be thrilled. To me this seems like a much easier sell than electrifying existing buildings. I know electrifying existing buildings is a key part of CO2 reduction plans, but they’re much harder (a key reason is that they don’t have SMALL LOADS). Bigger buildings (multifamily/commercial) are also trickier; more on that to come.
 Unless you duct them creatively, and that has it’s own challenges.
By Robb Aldrich, Principal Mechanical Engineer