How Much Additional Electricity Will a Heat Pump Use?
Nicole Grillo1, Kelsey Flores1, Matthew Hartt1, Savannah Hustus1, Thomas E. Stone1,2*
1. Husson University, Bangor ME., 04401
2. University of Maine, Orono ME., 04469
*Corresponding author: thomas.e.stone@maine.edu, 207-581-1237
Abstract
In order to meet decarbonization goals associated with mitigating climate change, many states and the federal government offer a variety of rebates and tax incentives for heat pump installation. Heat pumps are generally more efficient than other heat sources and can maintain the same temperature in a home or building with lower overall greenhouse gas emissions. While the climate change implications are of crucial importance, electric ratepayers have an immediate question: how much will my electric bill go up if I install a heat pump? With thousands of heat pumps being added to the electric grid each year, electric utilities have questions related to grid reliability, grid capacity, and preparing for more electrification. Here we analyze monthly electrical loading data for 155 homes in the northeastern United States who installed a single heat pump. We find that the most common result among households is an increase of ~25% in electricity use after the installation of a heat pump. However, there is very wide variability in the electrical loading changes that makes defining a typical home challenging, and we recommend treating this ~25% result as an order of magnitude estimate only.
I. Introduction
The energy necessary to heat, cool, light, and otherwise make buildings function accounts for 18% of global greenhouse gas (GHG) emissions, and 13% of the United States’ GHG emissions [1, 2]. 1 In the state of Maine, located in the cold northeastern corner of the United States, ~33% of GHG emissions are from the heating, cooling, and lighting of buildings [3]. As a part of their portfolio of efforts to mitigate the effects of climate change by reducing GHG emissions, many states (and the federal government) incentivize the installation of heat pumps [4-7]. Heat pumps use the thermodynamic properties of a refrigerant, combined with electricity to run the necessary mechanical components, to move thermal energy from a region of cooler temperature to a region of warmer temperature—this is opposite the natural direction of heat flow [8]. In the winter, a heat pump moves thermal energy (or colloquially, heat) from the colder outside air to the warmer inside air in order to heat a building. The opposite happens while air conditioning in the summer; the heat pump moves the heat from inside the cooler building to the warmer outside. While operating, a heat pump’s GHG emissions derive only from the emissions associated with producing the electricity needed to run it. As with any device purporting beneficial electrification (switching from fossil fuels to electricity for the same benefit but with less environmental degradation), a key parameter is how much GHGs are emitted by the electric grid [9, 10]. Maine’s electric grid generates 0.481 pounds of carbon dioxide per kilowatt-hour of energy (0.481 lb/kWh), which is one of the lowest carbon intensities in the country and makes beneficial electrification very attractive [11].2 In fact, Maine has installed 115,442 heat pumps since 2019 and has reset its installation goal to 275,000 heat pumps by 2027 [3, 12].
Besides the environmental benefits of beneficial electrification3 and the practical benefit of being able to both heat and cool a building, heat pumps have other appealing features. Heat pumps can often heat a home more cheaply than by other means, offer finer temperature control by heating in zones, and are quiet [13, 14]. However, heat pumps have large upfront costs, though rebate programs often mitigate these. Heat pumps do not work during power outages unless an alternative source of power is procured, and they can substantially increase a building’s electric bill (even if overall heating costs decrease).
In this work we seek to quantify the last point—how much does a residential building’s electrical usage typically increase after installing a single heat pump? Homeowners can use this information, along with their local price of electricity, to estimate the increase in their monthly electric bill. Though switching to a heat pump will often save homeowners on their overall heating bill, they can still be surprised by the increase in their electric bill. The analysis here gives homeowners another piece of information to aid their financial decision-making. Furthermore, with thousands of heat pumps being added to the grid, electric utilities can use this information to help plan for the additional loading. Utilities that offer rebates for customers purchasing heat pumps can use this analysis to calculate an average payback time on their investment.
II. Methods
Stowe Electric (SE), located close to Maine in northern Vermont, provided anonymized monthly electrical energy loading data (in kilowatt-hours, kWh) for all of their residential and commercial customers who installed one or more heat pumps between 2018 and 2021.4 They provided monthly electrical loading data from ~2 years prior to each heat pump installation until present day. SE also provided the installation date, heat pump information (manufacturer, model, unit type—ducted or ductless, number of units), financial information (heat pump cost, rebate given), and type of customer (residential or commercial) for all installations. Where known, SE provided the type of heating system being replaced by the heat pump as well as any other known home electrification efforts (electric vehicle charging, solar array present). In this study, we only use the installation date and monthly electrical loading for each customer. Though a further analysis of the financial data would be very interesting (calculating a typical payback time, for example), we would need to know additional information about customer’s previous heating systems and costs that were not widely reported.
Due to the small number of commercial customers installing heat pumps, we limited the scope of this study to residential customers who installed a single5 heat pump (either ducted or ductless). Customers had up to 90 days to submit their rebate paperwork, which is a (likely small) source of error. Occasionally a customer would have multiple monthly reads of 0 kWh, indicating they had shut off electrical service for some reason. Since multiple months of 0 kWh electrical usage could potentially skew the data, and because this suggests a non-typical customer, we removed all customers from the data set who had more than two consecutive months with 0 kWh electrical usage (we retained customers with a single monthly read of 0 kWh in the analysis, which did not significantly change our results or conclusions). Finally, some customers did not have 12 months or 24 months of previous electrical data available. For example a homeowner could have bought a home in January and installed a heat pump in June, which would yield only five months (January – May) of pre-installation data. We removed these customers from the data set as well since they did not have adequate pre-installation data to compare with their post-installation data. After we pared these exceptions from the data set, we had 155 residential customers with 12 months of pre- and post-installation electrical loading data available. Of these 155 customers, 123 had 24 months of pre- and post-installation data available. We chose the 12 and 24 month timeframes as the before and after so as to ensure the data reflected an entire heating and cooling season, no matter when the heat pump was installed.
Of course, there are a number of limitations to this data set and subsequent analysis. Perhaps the most glaring is that we cannot attribute the electrical loading changes solely to a customer installing a heat pump [15]. A home is not a laboratory where a single variable can be isolated, and a number of unknown factors could have changed a home’s heating demand and overall electrical usage during the time under study. In fact, we observed some homes with decreased electrical loading after installing a heat pump. Clearly, adding a large electrical load like a heat pump will increase electricity demand so some other changes must have taken place in these cases. In the 24 months under study (12 months pre-installation and 12 months post-installation) for each home (48 months total where data was available), a number of electrical and behavior changes likely took place. In this study, the only change we know for certain is that a homeowner installed a heat pump.
The typical variation in a home’s electrical use over a 2 (or 4) year period points to the next limitation in the data set: the average or typical home in this study may, or may not, represent an individual home well. While one of our intents here is to give homeowners an order of magnitude estimate of what will happen to their electrical usage after installing a heat pump, we emphasize that this is only an estimate and individual homes may differ greatly from the average results presented here (see Table 1 and Figure 1). Our results might be most applicable to electric utilities who are interested in average changes when a large number of heat pumps are added to their territory.
Finally, all of the homes in this study are geographically co-located in northern Vermont, making our results most applicable to regions with similar temperatures (such as the state of Maine, which we have focused on here).
III. Results and Discussion
We calculated each customer’s monthly electrical energy consumption in kWh for the twelve months before and after installing a heat pump. We also calculated their median consumption of the same period. For the 155 customers, we found an average annual change in usage of 22% and a 28% change in median usage.6 For the 123 customers with 24 months of data available, the average usage increased 22% and the median increased 29% (see Table 1).7
| 12 months before/after | 24 months before/after | |
| Number of customers in data set | N = 155 | N = 123 |
| % change in average electric loading | 22% | 22% |
| % change in median electric loading (kWh/month) | 28% | 29% |
Based on this data set and analysis, our primary result is that a customer might expect their electrical loading (and thus electric bill) to increase by ~25% due to the installation of a single heat pump. We stress, however, that there was a tremendous amount of variability in the average and median, including a maximum 12 month average increase of 490% and a minimum 12 month average increase of -69% that was actually a decrease in electrical consumption (see Figure 1 for the distribution of 12 month changes). Though counterintuitive, there are a number of ways that the average electrical demand in a home could decrease after installing a heat pump. For example, the homeowners could have added solar capacity that covered the additional electricity for a heat pump as well as other electrical demand; changed their behaviors with respect to heating and cooling; changed the building envelope (such as by adding insulation); reduced other electrical loads in their home (such as by replacing appliances); or they could have increased their use of non-electrical appliances (such as heating with a woodstove or switching to a propane hot water heater). In any case, other decreases in electrical loading must be greater than the increase due to the heat pump installation in order to see an overall decrease in usage. If we remove the 57 customers who had a decrease in their electricity consumption in the 12 months post-installation, the expected electrical loading increases to ~50%. This ~50% increase might be a more conservative indicator of expected increase if no additional actions are taken by the homeowner (such as adding solar capacity or insulation, for example).
Likewise, some customers saw more than 100% increases in their monthly loading, indicating a greater than doubling of their electric energy consumption (and bill). While adding a heat pump might explain the doubling of small ~ 100 kWh/month accounts, adding a single heat pump is very unlikely to cause a doubling of larger ~ 1000 kWh/month accounts where other factors were likely at play (such as adding a heat pump in conjunction with a home addition, for example).
Because of the wide variability seen in Figure 1, it is challenging to define what a ‘typical’ homeowner might expect for a change in their electrical energy consumption which is why we only offer an order of magnitude estimate of ~25%. Compounding the variability issue is that the data can be parsed in a number of ways. For example, if we exclude an outlier at the high (489%) end the expected change shifts closer to ~20%, but then increases if we also exclude customers who had a negative change. If we choose to calculate the median (instead of the average) change in each Table 1 statistic we find a change of ~10%, which then returns to ~25% if we again exclude customers with a negative change. Perhaps the best conclusion we can draw from this data is as follows: if you do not make efforts to decrease your grid-produced electrical energy production (such as installing a solar array or lowering your usage with behavioral changes), your electrical loading will likely increase between 5 and 50% upon installing a single residential heat pump, but you cannot exclude higher increases.
Our ~25% order of magnitude result might be most applicable for utility-scale planning, where averages are more relevant. Homeowners can use this estimate, coupled with the full distributions of changes in Figure 1 (which may be more illuminating for an individual customer), as a starting point for their financial decision-making process regarding a heat pump.
IV. Conclusion
In this paper we have analyzed pre- and post-installation electrical loading data for 155 homes that installed a single heat pump, finding that a typical home might expect a ~25% increase in their electric bill as a result of installing a heat pump (although their overall heating/cooling costs might have decreased). We stress that this ~25% is an order of magnitude estimate that cannot be attributed solely to the installation of a heat pump since a residential setting does not allow us to isolate a single variable (heat pump installation) for study. Other behavioral and structural changes almost assuredly took place in each home over the 24 or 48 months of study, which affects electrical loading in unknowable ways. We also stress the wide variability in the data set, as seen in Figure 1, which makes drawing conclusions about a ‘typical’ home very challenging. Even given these qualifications, we hope that homeowners can use these results to better inform their financial decision-making process when considering heat pump installations. Electric utilities can use this information as they prepare for more electrification in the future.
Finally, we conclude with reminders from Wendell Berry that “if we are not in favor of limiting the use of energy, starting with our own use of it, we are not serious” and “if we are not in favor of rationing energy, starting with the fossil fuels, we are not serious” [16]. Conservation, or source reduction, will always yield the greatest environmental benefit. While converting from heating with fossil fuel to heating with an electric heat pump might generate a number of environmental benefits, true environmental stewardship asks us to reconsider and minimize how much heating is truly necessary in the first place. Deeply engaging with all of our energy use will likely be uncomfortable, especially as it becomes clear that we will have to relinquish some of our behaviors and comforts [17]. Nevertheless, the climate emergency requires us to take on this engagement and it is our hope that the work presented here will add, in a small way, to the larger conversation.
V. Footnotes
1Different studies rarely consider exactly the same inputs for the energy associated with buildings, so the GHG emissions stated are best thought of as order of magnitude estimates and not direct comparisons.
2Reducing the quality of Maine’s electric grid to a single number (CO2 emitted per unit of energy produced) is a gross oversimplification. We must note that a full accounting of the environmental quality of Maine’s electric grid would not only involve all of the GHGs emitted in the production of electricity, but other ecological and social ramifications of producing that electricity.
3It is beyond the scope of this paper to do a complete life cycle analysis comparison between heat pumps and the many other heating options available, which others have done elsewhere. We note that heat pumps generally exhibit lower life cycle emissions than other heating options, though the GHG emissions of the local electric grid can make them less beneficial in some cases.
4Husson University Institutional Review Board Approval #22SH02.
5In most instances it was clear when a residential customer had installed two or more heat pumps, and those were immediately excluded from analysis. However, there were instances where it was not clear if multiple buildings had each received a heat pump or if one building had received multiple heat pumps. We excluded these from the results presented in Table 1, but if we had included them our results, discussion, and conclusions remain essentially unchanged: Average12 = 26%, Median12= 31%, Average24 = 24%, Median24 = 30%.
6Let Abefore,i and Aafter,i be the ith customer’s average monthly electrical energy consumption (in kWh) in the twelve months before (-12 ≤ t ≤ -1) and after (1 ≤ t ≤ 12) installing a single heat pump; we excluded the installation month (t=0) from our analysis as that month contained both days with and without a heat pump. Similarly, let Mbefore,i and Mafter,i be the ith customer’s median monthly electrical energy consumption in the twelve months before and after installing a heat pump. The percentage change in the ith customer’s average energy usage can then be calculated as 
and the percentage change in the ith customer’s median energy usage can be calculated as 
. For the N = 155 customers with 12 months of before and after data, we find an average percentage change in average energy usage 
(significant using a paired sample t-test, p = 0.00022). We also find an average percentage change in median energy usage 
(significant, p = 0.00726). Performing similar calculations for the N = 123 customers with twenty four months of data available, we find 
and 
for that timeframe (p = 0.00254 and p = 0.00758, respectively).
7As indicate in Figure 1, there was wide variability in the electrical loading data. Here, we list the standard deviation and range for each summary statistic presented in Table 1. 
(12 months)
(12 months)
(24 months)
(24 months)
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