During a Power outage, can you run your furnace with an inverter and an AGM battery bank, then recharge with a generator?

If you’re like me, you want and need heat when the power is out. Are your power outages are too frequent like mine?

My name is Bill Thompson, my answers is “Yes give me the heat in a power outage”. I have a Rheem Furnace which has an emergency heat mode with Natural Gas. It can run off of 120 volts, and doesn’t draw more than 1000 watts running high speed heat mode. The power to the furnace is separate from the heat pump and on its own 20 amp breaker. Most the time, the furnace while in heat maintain mode, the power draw is less than 300 watts allowing me to run a fridge and freezer, some lights, and charge some laptops and cell phones.

This Inverter + Battery solution doesn’t costs very much. It was sub 2000$ at time of purchase in 2018. The simplest answer that I have is get an Inverter generator (300-500$), and get an auto transfer switch for your furnace (70$). See the links further below for affiliate links to generator and transfer switches.. No matter what you think now, when the power outage happens you will want a minimum of an inverter generator.

The generator only solution is sub 600$ solution and will work for 12-18 hours a day, this leaves some time you would NOT have heat. Unless you refuel the generator every 6 to 8 hours.

The batteries and Inverter solution come into play to supplement that generator only solution. Go silent at night with the inverter battery solution is about 1600$. This helps safe fuel, and the nightly visits to refuel a generator that keep you awake. The 5 gallons of gas per day will extend out further.

I live in the suburbs of Woodburn Oregon. I didn’t think power outages would hit our small town, but the more I google, and YouTube(tm) peoples experiences during the past year regarding storms. I realize grid down/power outages isn’t something new to people anywhere, and people in cities, suburbs go without power often enough to want something exactly like what I put together and get installed.

I grew up in the country, and knew power outages happen out in the sticks.

I didn’t anticipate grid down in the city I live for the first few years. If it happened, we were not home, at the office or out for a drive. 2021 was the first time I experienced a city grid down for days.

I sized and made a UPS with an Inverter with built in battery charger and transfer switch, and 5 x 100 Amp AGM batteries that will get me through several hours of ‘no grid power’ situations, and a generator to make up for ‘days’ of outages. The Inverter + Battery solution works for up to 23 hours when it is 32 F outside, this is to full 0% DoD (Depth of Discharge) of the batteries. The DoD target is no more than 50% of that, so run it for a max of 12 hours before recharging with a generator.

Before Fusing the battery.
Added a 300 Amp Fuse to protect the Wires and make the install safer.

My story

2021 February 5 days no power. This was the worst Ice storm I’ve lived through without Power, 4G, 5G LTE, Internet, and Cable. The local businesses, and gas stations were closed down as well for 5 days, in some parts of Woodburn, it was 2 weeks. We had water and natural gas, that was it really. Nearest gas 60+ miles at first, closest cities were closed due to grid down power outages as well all up and down I5. After 2 days we could drive 30 miles to get gas. Being able to listen to radio and occasionally get news from my wife and sister that PGE was working the problem and be online in 2 days. Always 2 days. Then the news changed “It will be at least 5 more days before power is restored”.

Even with Camping gear out to cook with and running water, the only thing that made it better was heating the house with our furnace and the Inverter and Batteries, supplementing with a Generator. We had not needed the generator before on all previous power outages. Power outages are frequent enough but never lasted beyond 10 hours before.

All items in affiliate links are items I’ve personally purchased and used for the past 2 1/2 years. The furnace we’ve had since about 2007.

Disclaimer: If you decide to DIY this system, you do so at your own risk to life and limb, and hold me harmless. 120 volts AC can kill. Working in a breaker panel is very dangerous. Battery banks for 500 amps and 6000 watts potential energy can start fires. AGM Lead Acid batteries off-gas hydrogen when charging and can be explosive. Save yourself lots of issues and hire a professional. The information presented here is for informational purposes only.

DIY: The inverter has a dual receptacle plug on it, you can mitigate the risks by wiring on a male plug from a hardware store on the input side, and plugging your furnace into the receptacle. *Point: This solution offers the most flexibility and is cheapest. I had to re-wire my inverter input to get generator power into it in the middle of the storm. It was too late, the AGM batteries had been at 0% DoD for several days. Longer story about the generator here.

The system I own and present to you has the following;

Alternatives I don’t own;

Choosing what to run in an emergency power outage;

  • Refrigerator
  • Furnace
  • Water pump
  • Charging Cell Phones, Computers
  • Internet Cable Modem and Router (My dads fiber optic internet stayed online during the entire Ice storm. Wow!)

What I didn’t or couldn’t run;

  • Hot water heater – 240 volts x 30 amps = 7200 watts.
  • Stove + Oven
  • Heat Pump – 240 volts x 45 amps
  • Washer
  • Dryer (natural gas)
  • Other items really lead up to ‘capacity’ of the battery bank. Add more batteries to handle those items.

Q. What if my furnace is ‘heat strip’ only?
A. My answer is to check into emergency heat with propane, maybe there is a retrofit for your system to add something like that, but this adds complexity and a ‘tank’. As a side note I have a pellet stove, but it doesn’t promote the kind of comfort my furnace does. In my case I have 3 heating options for home heat before I need a “Mr Heater” propane camp heater that I purchased. Heat Pump, Emergency Natural Gas, Pellet Stove.

Split Heat Pumps that heat a “Room” of the house are a great new option. They consume 700 watts of AC electrical to produce up to 4x as much heat pumped from outside. There are window based Heat pumps that heat and cool. Most of those options are beyond this document.

To help save some people money honorable mentions for heating; fireplaces, passive ‘fire stoves’ are probably the best options. There are reasons I didn’t go with those options. Heating a room, leaving the rest of the home colder. And of course heat control are little more difficult.

Pellet stoves require ‘power’ to run their fans. It was a nice idea, it does warm the home when it is ‘cool’ out while the rest of my home stays cold, and that is one of the reasons I use it in rare emergencies (no natural gas).

Why a generator and an inverter?

Most inexpensive sub 600$ generators in 2018 were not ‘electronics safe’. Charging batteries could be done from the generator while the inverter runs. Eventually I wanted solar panels to do it, but those were very expensive in 2018 as well, and I had a lot to learn about solar MPPT, winter charging conditions and lack of sunlight during Oregon winters. Even now in 2021 it still makes sense to run a generator. Using the ‘inverter generator’ it is far safer to power my furnace and charge the batteries. My dad has electronics die when running yea old non inverter generators, so they choose not to run electronics in most circumstances.

I still choose this solution, running quiet all night with no need to re-fuel the generator is nice. I have the option to add multiple charging solutions and change the batteries to lithium including larger banks for increased capacity and run time. The sizing is perfect for 12 hours no generator, then 12 hours generator. The generator won’t waste fuel while the fridge, freezer and furnace are idle and will always be charging the batteries during the day..

What other battery Alternatives are there (2021)?

I purchased some LionEnergy Safari UT 1300 watt batteries (Quantity x 2) from Costco in 2021 April. I have yet to install these. The Setting on the AIM’s inverter for the charger turns out to be “SLA” mode for these batteries. I will double check with the OEM and write a blog entry about it. The max run time I would get out of them is roughly 12 hours run time. They will also last for 10+ years enduring many 100% discharge cycles. Still the question remains if this Inverter’s charger will be ok to use and not ruin the batteries.

Why not a Power Wall with Solar Panels?

The world is rapidly changing. Covid time here in 2021 isn’t over. I ‘hear’ news that Power Walls must now go with the Solar Roof solution. This is what prompted me to post about my solution. Tesla’s Power Wall and Solar roof are a great solution together, and I’m sure there are reasons for the sudden change. I have wanted a power wall and a critical loads panel from it. There are more options being developed all the time, and I’m curious to see where it goes. Another reasoning behind “no Power Wall” is that Lithium Iron Phosphate has possibly a superior life to Lithium Ion/Lithium polymer battery solutions. I have a Lithium Iron Phosphate battery that still works purchased around 2008.

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Bill Thompson

I am a mobile smartphone and embedded Linux engineer. I love to tinker with electronics and fly drones in spare time when not geeking out in a developers world!