My Electricity Use for my Heat Pump

[dreancestere]

Since we're in the middle of our annual cold-snap down here in north central Texas, I thought I'd stick a chart of my 3-ton Bryant heat pump's daily electricity use over the last 1-1/2 years. In general, it's fine. But, when it gets cold, it really starts to suck down power (especially when it drops below 20 degrees F when the supplemental electric heating circuits start kicking in). You can see the huge spike in power use last year at about this time (when it got down to about 5 degrees F). Last night, since it was in the 20s for most of the night, it drained my 19.2 kWh battery in less than 3 hours.

EDIT: I've updated this chart to include data from the February 2025 cold snap. Also, just for grins, I calculated average and standard deviation for this data. The heat pump averages 21 kWh with a standard deviation of 21 kWh. So, about 68% of the time the heat pump uses between 0 and 42 kWh per day.

 

[dreancestere]

And, let me throw in yesterday's chart of energy use from my Sol-Ark 15k. From that, you can see my actual baseline energy use from early in the morning when it was still fairly warm and see what happens later. Especially when the periodic defrost cycle kicks in on the heat pump and spikes electrical use up to over 11kW for a short period (and I had to have the installer disconnect 1 of the 3 electric lines powering the backup/emergency heating circuits to avoid 16kW spikes).

 

[Supervstech]

Since we're in the middle of our annual cold-snap down here in north central Texas, I thought I'd stick a chart of my 3-ton Bryant heat pump's electricity use over the last 1-1/2 years. In general, it's fine. But, when it gets cold, it really starts to suck down power (especially when it drops below 20 degrees F when the supplemental electric heating circuits start kicking in). You can see the huge spike in power use last year at about this time (when it got down to about 5 degrees F). Last night, since it was in the 20s for most of the night, it drained my 19.2 kWh battery in less than 3 hours.

View attachment 268116

Aux strip heating SERIOUSLY drains solar.
As rare as cold snaps are by you, i would consider disabling them. Or in some cases, if there are multiple strips, disable one or more. I like the way aquatherm does it, they use a heat pump with a hot water coil from the water heater as aux heat.
Minisplits are more efficient, and dont use resistive strips for aux heat or defrost, so that is another option, add a minisplit to provide aux heat.

 

[BarracudaBob]

Minisplit in the master bedroom start up slow so no surge current, slow and steady, and the best thing I ever did for heat and cooling.

18 K BTU but exceeded my expectations. I can't speak how a heat pump would work in Canada but down to 32F zero issues.

Runs around 800W with occasional 1k spike. The wire resistant heater that we had would pull 1500 continuous until the room temp stabilized and then keep going on/off with 1500 spikes making the lights blink on the inverter.

 

[Quattrohead]

Minisplits are more efficient, and dont use resistive strips for aux heat or defrost

Some of them have the heating element in the compressor itself or the pan that it sits in.

 

[Drunkin Solar]

Minisplit in the master bedroom start up slow so no surge current, slow and steady, and the best thing I ever did for heat and cooling.

18 K BTU but exceeded my expectations. I can't speak how a heat pump would work in Canada but down to 32F zero issues.

Runs around 800W with occasional 1k spike. The wire resistant heater that we had would pull 1500 continuous until the room temp stabilized and then keep going on/off with 1500 spikes making the lights blink on the inverter.

Interesting consumption numbers for an 18K BTU unit.
What's the consumption when it's cooling?

 

[BarracudaBob]

Interesting consumption numbers for an 18K BTU unit.
What's the consumption when it's cooling?

I went back to Late July evening when and took this screen shot of Solar Assistant. There is a fridge, deep freezer, and other loads like TV's that are turned off so these numbers are higher than just the Mini-Split. If is in the master bedroom in S Florida and we keep it 73-74F. I am usually freezing with a blanket on. Wife sleeps like a rock.

the peeks could be a fridge doing defrost but they are very short. Biggest Peek was 2.14 KW. Running it on a single EG4-3K at the time. Now I have two an a lot more loads.

 

[Drunkin Solar]

I went back to Late July when and took this screen shot of Solar Assistant. There is a fridge, deep freezer, and other loads like TV's that are turned off so these numbers are higher than just the Mini-Split. If is in the master bedroom in S Floria and we keep it 73-74F. I am usually freezing with a blanket on. Wife sleeps like a rock.

the peeks cold be a fridge doing defrost but they are very short. Bigest Peek was 2.14 KW. Running it on a single EG4-3K at the time. Now I have two an a lot more loads.

View attachment 268140

Thanks for that!
I am really liking the idea of a smaller mini-split, maybe a 9K BTU unit, for myself next year mainly for cooling. Maybe a little heating at nights in spring and fall months. I leave in the winter so don't have to heat in extremes.

 

[Supervstech]

Some of them have the heating element in the compressor itself or the pan that it sits in.

I would think that is for compressor oil protection, not heating the space.

 

[BarracudaBob]

we used to cool the hole house down and that was $300 Plus per summer months on our electric bill. Now we leave the house at 79F with the 5 Ton HVAC and cool the bedroom down off inverter/solar. So I get 3x on the solar running the Mini-Split. The match on electricity generated vs what it would have cost to cool the entire house down is an absolute no brainer.

I even use a fan to blow the cool air at the bottom of the door way that leave the master and blows down the hall and it will keep the kitchen and TV room cool so I don't have to turn on the main HVAC on. It exceed all expectations and I am amazed that I haven't killed it yet because it ran 24/7 all summer.

It also has a DRY mode that keeps the South Florida humidity down.

 

[Quattrohead]

I would think that 8s for compressor oil protection, not heating the space.

They run a defrost cycle too and it surprises a lot of new users when they see huge clouds of steam pouring off of their compressor for the first time LOL

 

[BarracudaBob]

They run a defrost cycle too and it surprises a lot of new users when they see huge clouds of steam pouring off of their compressor for the first time LOL

I would be freaking out thinking all of my Freon is escaping !!

 

[Madcodger]

The term "heat pump" is overly generic, sort of like saying, "car". If your heat pump is designed for the climate and space, and you have a well-sealed, well-insulated building, they don't use excessive electricity and in fact, are the single best source of active heating widely available today.

We live in Maine. It's cold here. Sunday saw a high of 19F and a low of 3.7F. The rest of the week has been similar. The graph below shows our actual usage for the past week, as the sole source of heat for a 910 SF space, using a 2024 model Mitsubishi 12K mini-split (with hyper heat, and no electric strips for auxillary heat. Those are horrible). If we were connected to the grid we would be spending well under $1 per day to heat this space (off grid, we spend nothing once our capital cost is recovered). BTW, we also have a 6K unit in that space (for the bedroom) and haven't had to turn it on all winter, with temps below 0F on at least one night.

Our local town govt wisely spent the money to install grid-tied solar and Mitsubishi heat pumps in our town office, community hall, and fire station this past year. Last month (Dec) we saw an 85% reduction in the town's utility bill. Wow!

If you live in an area where it gets cold, even for just a few weeks each winter, install a heat pump that can handle it (which may not be the one every installer recommends if they're trying to win a bid based on price with an uninformed buyer). That same heat pump will likely find summer heat a walk in the park.

 

[Supervstech]

The term "heat pump" is overly generic, sort of like saying, "car". If your heat pump is designed for the climate and space, and you have a well-sealed, well-insulated building, they don't use excessive electricity and in fact, are the single best source of active heating widely available today.

We live in Maine. It's cold here. Sunday saw a high of 19F and a low of 3.7F. The rest of the week has been similar. The graph below shows our actual usage for the past week, as the sole source of heat for a 910 SF space, using a 2024 model Mitsubishi 12K mini-split (with hyper heat, and no electric strips for auxillary heat. Those are horrible). If we were connected to the grid we would be spending well under $1 per day to heat this space (off grid, we spend nothing once our capital cost is recovered). BTW, we also have a 6K unit in that space (for the bedroom) and haven't had to turn it on all winter, with temps below 0F on at least one night.

Our local town govt wisely spent the money to install grid-tied solar and Mitsubishi heat pumps in our town office, community hall, and fire station this past year. Last month (Dec) we saw an 85% reduction in the town's utility bill. Wow!

If you live in an area where it gets cold, even for just a few weeks each winter, install a heat pump that can handle it (which may not be the one every installer recommends if they're trying to win a bid based on price with an uninformed buyer). That same heat pump will likely find summer heat a walk in the park.

In the south at least, codes require Heatpump’s be sized to the cooling load, not the heating load.
So, in cold weather, additional heating is required.

 

[Madcodger]

In the south at least, codes require Heatpump’s be sized to the cooling load, not the heating load.
So, in cold weather, additional heating is required.

I don't doubt or dispute that. Still,

1) Why is it that so often, "code" means both the least quality thing you can legally do, AND a sub-optimal outcome.
2) That wise tiny town government noted above has wisely decided to not impose codes for HVAC units on its residents, assuming that said residents have functioning organs above their necks in a free-market society, so that's that (a big part of why we live here). BTW, several of us local residents encouraged town leaders to learn a bit more about heat pumps and encouraged them to ask questions of installers rather than just bite at the lowest bid. They ended up purchasing Mitsubishi even though it cost a few pennies more, and the local residents supported that "spend a little more for quality" approach in one of our famous New England town meetings (every resident gets to speak and has a vote), so now we're all happy with the town's decision. Makes for a pretty good place to live. The state, of course, has its own set of requirements but they at least tie enforcement to the rebates they offer (more carrot than stick). Most of their requirements make sense. Which might explain why Maine has had one of the highest rates of heat pump adoption in the country, and is well ahead of its goals in that area. The government even raised its goals for heat pump adoption rather than simply rest on its laurels, which I thought was impressive (especially for government).
3) My guess is that too many people think all heat pumps are the same, largely out of ignorance. For example, any time I see a unit being quoted with heat strips, I automatically think, "questionable". We've used heat pumps without such monstrosities down to -17F over the past five years, stayed toasty warm, and avoided watching the electric meter spin out of control (and now, we don't even have a meter!).

Anyway, we can't solve everything. But hopefully we can help people learn more, and make better decisions.

 

[bhbaker220]

We're going on our 4th year in a new house we built that is fairly well insulated. The only thing I regret is putting in the heat pump rather than an AC and propane furnace. I thought it was about a wash economically and it likely is but presents challenges on the electrical load side as mentioned above. I had to disable one of my 5kW heat strips so that I could power the heat pump in the winter with my EG4 6000XP if we were to lose the grid and needed some supplemental heat. I'm very glad we put in a direct vent propane fireplace ... just wish we had one in the bedroom too. Would love to have the mini split in our bedroom but I need them to come up with a cheap DIY one that has a ceiling cassette.

 

[Supervstech]

I don't doubt or dispute that. Still,

1) Why is it that so often, "code" means both the least quality thing you can legally do, AND a sub-optimal outcome.
2) That wise tiny town government noted above has wisely decided to not impose codes for HVAC units on its residents, assuming that said residents have functioning organs above their necks in a free-market society, so that's that (a big part of why we live here). BTW, several of us local residents encouraged town leaders to learn a bit more about heat pumps and encouraged them to ask questions of installers rather than just bite at the lowest bid. They ended up purchasing Mitsubishi even though it cost a few pennies more, and the local residents supported that "spend a little more for quality" approach in one of our famous New England town meetings (every resident gets to speak and has a vote), so now we're all happy with the town's decision. Makes for a pretty good place to live. The state, of course, has its own set of requirements but they at least tie enforcement to the rebates they offer (more carrot than stick). Most of their requirements make sense. Which might explain why Maine has had one of the highest rates of heat pump adoption in the country, and is well ahead of its goals in that area. The government even raised its goals for heat pump adoption rather than simply rest on its laurels, which I thought was impressive (especially for government).
3) My guess is that too many people think all heat pumps are the same, largely out of ignorance. For example, any time I see a unit being quoted with heat strips, I automatically think, "questionable". We've used heat pumps without such monstrosities down to -17F over the past five years, stayed toasty warm, and avoided watching the electric meter spin out of control (and now, we don't even have a meter!).

Anyway, we can't solve everything. But hopefully we can help people learn more, and make better decisions.

In the south, we have a goofy thing called humidity...
If the heat pump was sized for the heating load, in air conditioning mode, the air would cool so fast that humidity would rise into the 90+ range, and everything in the house would mold.
So, our areas need to be sized for cooling loads only.

Now, if you want to install a heat pump dedicated to heating, that is fine, but the cost is up there...
Most homes here need between 2 and 4 ons of cooling, but between 4 and 12 tons of heating...
So...

 

[Madcodger]

In the south, we have a goofy thing called humidity...

The problem, then, is that people aren't doing the appropriate calculations (not you, but many others). A big reason one should perform a Manual J calc is to avoid the very problem you note (not saying that for your benefit, of course, but for that of others reading the thread). I find it hard to imagine one not being able to use a high quality heat pump appropriately sized for cooling, for heating as well, in most parts of the country (maybe most parts of the world). It might spin up a bit faster and certainly use more electricity than it would use at a higher temperature, but it would still provide adequate heat in most situations. A 10-20F day (about as cold as it usually gets in most parts of the south) is not a terrible heating requirement. People buy a "heat pump" that hasn't been designed for the full range of climate typically experienced in a region, and then wonder why it doesn't perform well. That's a problem with the designer, not the equipment.

 

[dreancestere]

Just to be clear, in my case, I'm not saying I'm not happy with the heat pump. In general, I'm actually happy with it. It's a 5 stage heat pump, so it handles the humidity and mild spring/fall weather wonderfully (running for most of the day, but at Stage 1 or 2). Even in the heat of summer (115 degrees for much of this summer), it's fine. It's just in the winter where the cost spikes up (and lot of that is just because of the near-hourly defrost cycles). And, we've never had an issue maintaining our desired temperature. We did have a Manual J done and were right on the 3 ton/3-1/2 ton border. We went with the 3 ton and it looks like it was the right choice.

I'm pretty happy with the sizing of our whole system as well (12.4kW of PV, Sol-Ark 15k passing through the grid, and 19.2kWh of battery). We sell back excess energy for most of the year. But, have to use those "banked" dollars to pay for the December, January, February heat pump electricity. Averaged out for the year, we end up paying about $8/month for electricity. So, our sizing is good. It's just that the difference in heat pump electricity use between cooling and heating (and defrosting) was surprising.

 

[RDuke]

Thanks for the info it is good to know practical experiences.
I converted my house from natural gas heat to a heat pump back around 2007 and upgraded to a 2-stage 4 ton heat pump last year. The frustrating thing about heat pumps is that big spike in current during the defrost cycle. My 6500 watt generator kept us going for 3 days with no utility power after hurricane Helene. It ran the compressor fine in cooling mode, but the generator would not be able to handle the defrost cycle in the event of an ice storm - rare where I live but possible. Then my only heat would be the fireplace. Since the defrost cycle is relatively short, I am thinking I could use a battery storage system to provide that peak power needed during the defrost cycle while charging the batteries off the defrost cycle with an EG4 chargeverter. I am more concerned with self sufficiency than the cost savings. Night time heating with large heat pumps seems to take quite a large system as you figured out. Thanks for sharing.

 

[Quattrohead]

Have you been able to measure the current during your systems defrost cycle?
Depending on the size of your house one or maybe two EG4 18Kpv would very easily cover it I have no doubt

 

[Madcodger]

It's just that the difference in heat pump electricity use between cooling and heating (and defrosting) was surprising.

That's a good point. And, your reaction is very common. So, let's think about what we're asking the device (heat pump) to do. And in doing that I do not mean to pick on you in any way! I had the same reactions until I had to do the math to plan things out for our property and buildings. It's just that your example is a good one to help people re-frame their thinking about heat sources, including heat pumps, and how they present such a good value. Thanks for that.

A quick check of NOAA's "Climate at a Glance City Series" (your tax dollars at work) shows us that the mean average temperature for Witchita Falls in July, averaged over 1991-2023, was 85.3F. So if we're trying to keep a house at about 74F, we're asking the heat pump to deal with a delta of just over 11 degrees.

The mean average temperature for January over that same time period was 43.0F. So if we're trying to keep the house at about 70F, we're asking the heat pump to deal with a delta of just over 27 degrees. That's a 245% difference! So, it makes sense that the heat pump would use a LOT more electricity to get us into our comfort zone.

But does that mean that heat pumps aren't still a good value, even for heating? Real world data says they remain a great deal. People in our state (Maine) think a lot about heating costs, so our state government provides a heating cost calculator (I'm fairly certain it's a derivative of one the US Dept of Energy once produced). You can download it here and update prices and other variables as you wish: https://www.maine.gov/energy/sites/maine.gov.energy/files/inline-files/Home-Heating-Calculator.xls

When I ran the numbers, using this and other tools, it convinced me to invest in heat pumps. We continued to use them are our primary heat source when we went off grid, and I haven't regretted that one bit. To me, math is math. I just can't make the numbers work as well for any other heating source I might want to use.



July data above, see: https://www.ncei.noaa.gov/access/mo...se_prd=true&begbaseyear=1991&endbaseyear=2023

January data above, see: https://www.ncei.noaa.gov/access/mo...se_prd=true&begbaseyear=1991&endbaseyear=2023

 

[Zwy]

In January, I'll run heat pumps only when there is excess power from PV and the temp is above 10°F. It is mostly during the day and I make a quick judgement based on charge rate and available sun whether I can run the heat pumps. I will run the heat pumps in the evening if sunny conditions are predicted for the next day or two and recharging the battery bank isn't a problem. When it is cold, heat pumps do like power. When temps are below 10°F and there is or will be excess PV, I will run resistive heaters controlled thru a Shelly Smart Plug. I do not like the defrost occuring often and defrost will pull some heat energy out of a room.

Last February, we heated the house entirely from Feb 1st until spring with just excess PV. Heat pumps and resistive heating.

 

[dreancestere]

Have you been able to measure the current during your systems defrost cycle?
Depending on the size of your house one or maybe two EG4 18Kpv would very easily cover it I have no doubt

I've never actually measured the total power required for a defrost cycle. But, when I had all 3 of those electrical circuits hooked up, the spike was about 16kW for a couple of minutes (so, call that 16/30 = 0.5kWh). Now that I've had them disconnect one of the circuits, it's about 11kW for "more" minutes (I'll say about 5 minutes - so, call that 11/12 = 0.9kWh). Just eyeballing my SOC for my 19.2kWh battery, it looks like I lose about 4% for every defrost cycle (about 19.2 * 0.04 = 0.8kWh). So, an average guess at about .75kWh (EDIT: I also had MySolArk graph Daily Consumption and checked the values just at the start and just at the end of one of those defrost cycles -- 0.8kWh consumed during it). And those defrost cycles start at about 40 degrees F and happen about every hour. So, assuming that's about 10 times/day (mostly at night) when it's cold enough, that's about 7kWh/day. But, there's an awful lot of eyeballing and guessing in there.

With our 5 stage heat pump, I never worried about running the compressor through our Sol-ark 15k. The electrical use slowly ramps up as it progresses through the stages and even with all 5 stages running, that's probably less than 5kWs. But, with the defrost surges, I'm constantly worried about that surge in an outage situation (as @RDuke said, in an icestorm). With all 3 of those electric circuits connected, that was right near what the Sol-ark can handle. With only 2 connected, there's some leeway. But, what happens if I use the toaster, microwave or the dryer (but, besides a coincidence of timing, why I would do that during an outage, I don't know)?

EDIT: And, just in case that bad case happens and it does overload the inverter, we bought some electric space heaters just to heat individual rooms with the heat pump turned off.

 

[Supervstech]

The problem, then, is that people aren't doing the appropriate calculations (not you, but many others). A big reason one should perform a Manual J calc is to avoid the very problem you note (not saying that for your benefit, of course, but for that of others reading the thread). I find it hard to imagine one not being able to use a high quality heat pump appropriately sized for cooling, for heating as well, in most parts of the country (maybe most parts of the world). It might spin up a bit faster and certainly use more electricity than it would use at a higher temperature, but it would still provide adequate heat in most situations. A 10-20F day (about as cold as it usually gets in most parts of the south) is not a terrible heating requirement. People buy a "heat pump" that hasn't been designed for the full range of climate typically experienced in a region, and then wonder why it doesn't perform well. That's a problem with the designer, not the equipment.

No.
The problem is that it is impossible for a heatpump alone in the humid south to be sized for the full heating load of a home.
Impossible.
There will ALWAYS be more heating load than cooling load, so no matter how accurate the load calculation is, a home with ac/heat pump in one system.

A home will ALWAYS require additional heating over the ac load.