Hybrid HVAC with Solar Energy and an EV

Our home electric and gas utility bills arrived in late October.
Total: $38.67

Image: Electric bills for 12 month period = $290.44
$24.20 average per month includes EV charging

It’s been over a year since we upgraded our home heating and cooling system from an old central air conditioning unit to a MERV 15 heat pump, and from an 85% efficient gas furnace to a 96% high-efficiency gas furnace. Dual fuel.
Heat pump in Pennsylvania?

Most people north of the Mason-Dixon line aren’t that familiar with heat pumps, which both heat and cool, since they are more common in southern areas with milder winters.

The first thing that became most evident about our new heat pump is how efficient it was handling its air conditioning duties, using less electricity. Combined with higher solar energy production from our rooftop solar panels during summer, that’s a real Win-Win!

The second thing was finding out how different it is when heating our home with a heat pump, since it’s a different kind of heat, being a milder and more gradual sort of heat, when compared to the blast of hot air from our old gas furnace.

Alternate heat

The installer said our model of heat pump could heat down to 30-degrees F. so he set it that way initially. When our thermostat detected temperatures below 30, the HVAC system would switch to “Alternate Heat” from our furnace.

If we had gone the “all electric” heating route, the alternate heat would come from an electric furnace, which in a sense operates like an electric blow dryer for your hair, and according to a friend in Pittsburgh who did go that route, he got some pretty high electric bills during a recent long, cold winter. His rooftop solar panels didn’t make-up for enough of his increased energy usage, especially during prolonged snow cover.

With that in mind, while also wishing to reduce our natural gas usage, we upgraded to a high-efficiency gas furnace, where the new set-up required running two plastic pipes across the basement ceiling, and through a concrete block wall to the outside, instead of venting exhaust up the chimney, as was the case with our previous two gas furnaces.

Thermostat settings

One recent change we made to the initial HVAC settings was raising the temperature for when the alternate heat would kick-in, from 30 to 40 degrees F. We did that for several reasons.

The first reason was to conserve home battery backup storage on our Tesla Powerwall2, since the heat pump can drain the battery 4-times faster than if electricity is only being used for the 2-speed electric fan on the high-efficiency gas furnace.

The second reason was the thermostat was detecting an outdoor temperature that was up to 7-degrees F. warmer than it actually was here in our cold valley, obviously drawing its temperature data from a warmer location in our area.

The third reason was that it seemed to be more energy efficient overall, when having to raise our home temperature several degrees first thing in the morning, from the lower nighttime temperature settings.

Headed into winter

So this is how everything is set heading into winter, which for the past week around southwestern Pennsylvania, has been more like summer or early-fall, temperature-wise.

What I’ll call the “middle months” –both Spring and Fall– help conserve energy, not only at home, but also when driving the Chevy Bolt EV.

Running the heater full-time in the Bolt EV can mean the difference of getting 5 miles per 1 kWh (without any heat or AC turned on) or 3 miles per 1 kWh while using the heater, heated seats and occasionally, heated steering wheel. The factory manual recommends using the heated seats as much as possible over using the car heater, to help conserve energy. The lifetime efficiency is 3.5 miles per 1 kWh.

The bigger home energy picture

While we’ve achieved our goal of reducing our dependence on natural gas, we haven’t eliminated it, since it also remains as the energy source for our water heater and kitchen range. Switching both of those to electricity is quite logical, for many reasons, would be next.

Our original step to reduce electricity usage was swapping-out all our light bulbs with LED’s, even including the 4-foot fluorescent shop lights. Other than being far more energy efficient, it’s a real joy not to be standing on ladders (and chairs) to change burned-out incandescent light so often!

Chevy Bolt EV charging

Since trips in our Bolt EV are all within full-charge range, car charging is all done at home, using a Level 2 charger in the garage, which adds 25-miles of range for every hour of charging. From everything I’ve read and learned about what’s best for lithium-ion batteries, our charge level is maintained in the ideal bracket of 30% to 80%, unless preparing for a longer trip, when the battery is fully charged to 100% and usually indicates a range of around 300 miles.

Home solar energy

Our 8.97 kW rooftop solar energy system (29 solar panels – 300W and 315W) has done a beautiful job of greatly reducing our electric bills, even though none of the panels face due south, which is the ideal direction for maximum solar energy production. In fact, the 18 panel array faces northwest, yet it does benefit from being mounted on a low-pitch (22-degree) roof, which allows the panels to collect more solar energy early in the day.

Tesla Powerwall2 home backup battery

The PW2 has worked as promised, providing access to energy during grid outages, which I feel is the biggest reason to have a home backup battery. If the battery has any charge on it and/or the solar panels are producing energy, your lights stay on, while if you have solar panels alone, your house will be dark, since that solar energy cannot go into your home and onto the grid for safety reasons. A battery system isolates your home during an outage, off-grid.

The PW2 has a great, free app for iPhones, where you control multiple settings, as well as monitor an extensive variety of information. Since our electricity is still the same price 24-7, I haven’t used any of the time-based settings, but do use the app extensively for monitoring. We keep the battery reserve set at 35% for emergencies. A person can also choose to reserve up to 100% of the battery for backup emergencies only.

With that 35% setting, most of our home electricity throughout the evening, and at least part of the night, comes from the PW2. And of course, this is where the amount of electricity drawn by a heat pump comes into play. Say for example, the heat pump ran nonstop, entirely on battery backup power, it could drain a 100% fully-charged PW2 in about 5 to 6 hours (the PW2 has 13.5 kWh of usable storage, with a 14 kWh overall rating). That sort of storage limitation is why Tesla recommends you install multiple PW2’s with your solar energy system, for greater battery capacity.

The future of battery storage

Keeping in mind that the PW2 can store 14 kWh, the Bolt EV battery can store 66 kWh, which is 4.7-times as much. While our 2019 Bolt EV isn’t set-up to do double-duty as a home battery backup, future cars and trucks are being built with that additional feature in mind. Electric vehicles like pickup trucks, with larger batteries, can serve as home backup batteries for even longer periods of time. Beyond these capacity considerations, one could get into time-based storage and usage, to save on the time-based grid cost variation in some states.


This has been a roundabout way to finally reach the breakdown of our latest utility bills, keeping everything written above in mind.

  • 10/27/22 Electric bill: $6.63
  • 10/20/22 Gas bill $32.04
  • Total October energy bills: $38.67

Keep in mind that the electric bill also included charging the Bolt EV for nearly 500 miles of travel. Also, the electric bill indicated that we still have 933 kWh banked as credits from net metering, which will be used in upcoming winter months, when solar energy production typically drops off.

Please leave any questions or comments in the Comment section. Bob


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