Build a Battery Backup for Home Heating for Under $600

This affordable homemade system activates instantly when the power goes out, and can be set up using a combination of new and old 12 volt car batteries.

Before I get into this article, let me issue a caveat. This was an experimental project, so if you try it on your own, do so at your own risk. I’m not an electrician, but I like to play with gadgets. And, being a natural born Yankee, I can’t resist doing something on my own, if I think I can do it cheaper, and get the same results.

Up here in Maine, when the power goes out in winter, the clock starts ticking. In mid-January, you’ve got a few hours before your water lines (and boiler lines) begin to freeze. Some people have wood stoves for a backup. But these won’t always do the trick. 

Not only are they dangerous to your health, they don’t distribute heat to every bathroom to keep pipes and drains from freezing. They also don’t start themselves if you’re away.

Many of us have mixed modes of heating. For example, I have two electric heat pumps, plus a whole-house propane boiler that serves as a backup. The problem with gas boilers is that most require AC power to operate. They have an igniter and “smart” controls that fire up the gas. The thermostat may need power too.

Choosing the Right Inverter-Charger

In an ideal world, batteries would be cheap and sustainable, and I’d be able to run my electric heat pumps with battery power. But neither of those things is yet true, although hopeful technology is on the horizon.

So for now, my most affordable path to keeping my house from freezing in a power outage is to keep my propane boiler operational. Doing some rough calculations, I figured I’d need about 800 watts to power the boiler and the water pumps that move fluid through the baseboards. 

I based my inverter size on that benchmark. I later discovered that I overshot my actual power draw, but I’ll get to that.

The 1500-watt inverter I purchased on Amazon for $449 bucks doesn’t just convert battery power to AC voltage. It’s also a battery charger, and allows the constant throughput of AC power when battery juice is not needed. It's described as a “pure sine wave inverter charger.” 

These are the things I looked for before buying:

  •       Quiet operation
  •       Non-continuous fan operation
  •       Made for constant throughput
  •       12 volt charging
  •       Automatic transfer switch
  •       Automatic shutoff and overload safety features

You might be wondering why I didn’t just install an automatic whole-house gas generator. I’ll be honest, I didn’t want to spend $3,000 or more on an emergency-only fossil fuel-powered generator. I don’t like the cost, the energy source, or the noise.

Of course, I could also have purchased a portable “power station,” which is essentially a large battery pack. Goal Zero, for instance, makes this 1500-watt model that sells for about $1,799. However, based on the product literature, it does not appear to have a built-in transfer switch. You’d have to throw the circuit manually to switch to battery backup. Automatic transfer to DC is a key perk of my home-built system.

plug-and-play battery backup

If you don’t mind spending two or three times more for your setup, you can skip the handyman aspects of my project and buy a plug-and-play battery backup that has some of the same capabilities.

Making Use of Old Car Batteries

To make my system even more affordable, I wondered if I could string together auto batteries of various sizes and ages, and build a decent power capacity up without spending much money. I asked around and found friends with 12-volt batteries lingering in their garage or basement. I had one newish battery and a couple of older ones.

I did a bunch of research. The initial consensus from the Internet wonks was that I shouldn’t combine 12 volt batteries in this way, but their primary pushback was reduced performance as the batteries “equalized,” not safety. My priorities were different. I wanted a safe system first, sure, but I could sacrifice performance for overall capacity.

Digging deeper, I found battery experts such as Barry Hunter, who explain that as long as the batteries are connected properly in parallel, and they’re all 12-volt batteries, there’s no real problem with mixing and matching batteries of different ages and capacities, with one caveat:

“Don’t mix FLA, AGM, or gel batteries in a parallel set up,” he says. “They have different charge and discharge profiles.”

Other than that, however, Hunter says “they will charge and discharge according to capacities. A far as charging is concerned, there is no problem, because batteries are controlled by their voltage--which will always be equal in a parallel set up. The same with discharge. The charger and the load don’t count the number of cells.”


The key to safety and performance of your homemade battery bank in parallel is following this format for tying the 12-volt batteries together. My experiment used the “better” method initially, which is slightly different, but this “best” method is how the final connections should work. Order the battery cables online or get them at an auto parts store. Source

An Emergency Only Checklist

The key to my backup system is that it doesn’t try to keep the whole house running—just the baseboard heating. To do so, I needed to intercept the power for my boiler, the small pump that moves fluid through the baseboards, and also keep my wifi up and running, so I could get alerts and know the system is running by accessing my smart thermostat.

To do this, I needed to create a “loop,” where my inverter charger plugs into the grid-based AC power for the boiler, and the boiler, pump, and wifi plug into the inverter charger. To do this I put in a new outlet midstream between the emergency power shutoff for the boiler and my system. That way in the summer I can simply unplug and shut down the inverter backup and put the batteries on a trickle charge for the off season.

Before running my first “live” test of the backup, I wanted a better sense of the total load for my backup devices. I contacted the local library and asked if they had a “Kill-a-Watt” device to loan out. They did, so I brought it home and tested each item. 

I was pleasantly surprised to learn the propane boiler only called for 200 watts. The pump was 25 watts. My modem and router draw another 25. So my total load was just 250 watts to keep the heat on.


First Test. The initial test of the heating backup with one newer battery and two used ones provided 2.5 hours of operation. Adding one new battery more than doubled the run time. Optimized parallel wiring of the batteries will look slightly different.

Testing and Tweaking

For my first test of the backup (shown above), I hooked up two old batteries and one new one, plugged a radio in, and shut off the circuit. The inverter kicked in almost instantly, so that the radio never noticeably stuttered. I turned up the heat, so the boiler would run continuously, as it likely would in a deep freeze outage.

About 2.5 hours later, the radio shut off. I decided I needed more backup time, so I bought one new 12 volt battery for $89 at my local auto parts store. I tried a second test. This time, the system stayed up and running for 5.5 hours.

Would this be long enough? I looked at some statistics. Maine has some of the longest power outages in the country, and they’re getting longer—about 14 hours of downtime in a typical year. 

Average restore time varies by town. My town was on the lower side of those stats, so I figured even at single-digit temperatures, my backup would buy me at least 8 hours of freeze protection, more if the temperatures outside were warmer. 

By maintaining WiFi access (assuming that service stays operational), I’d also have several hours of monitoring the temperature inside the home, to decide whether more drastic action was needed.

On the other hand, I can easily add more batteries to my parallel battery bank this year, and double or triple my backup capacity. As I do so, I’ll probably remove the more tired cells and gradually add newer batteries.

If you build a system of your own, send me a photo and let me know how it works.

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