Home battery backup systems are getting a lot of attention as an alternative to generators that run on fossil fuel.

This is especially true for homes with solar panels that can easily and more cheaply keep the batteries charged. More-frequent and longer-lasting power outages also make the technology attractive.

Power outages are the primary reason New Englanders install battery backup systems, said Greg Blake, who owns South Pack Solar in Peterborough. In other regions in the United States with high electric costs, people might at times run their household on battery, especially during more-expensive time-of-use periods.

Prices are coming down and performance is improving, but that doesn't mean that battery backup makes sense for everybody today. The decision becomes clear once you understand the benefits, costs and limitations of the technology. Here's what what you need to consider when evaluating battery backup systems.

What do I need to keep running during an outage?

The typical battery backup system is not a replacement for those large whole-house backup systems that run on propane or another fuel and automatically provide full power when the grid is disrupted. You need to know what needs to stay running during an outage and the electrical load they will place on the battery.

Refrigerators and freezers, home heating systems and well pumps are examples of items that people most want to keep running. The electricity these important items draw is referred to as critical load, and they should be isolated from other loads via a critical-load panel. If your home has an outlet to plug in a generator, then you likely already have a critical-load panel.

How big and what kind of a battery system do I need?

The critical load of the items you want running during an outage will determine the capacity of the battery system you buy. However, you also need to consider whether the battery system can handle short-burst demands that some devices can draw.

For example, a well pump might draw 30 to 40 amps when it comes on, according to Blake, and some battery backup systems aren't designed to accommodate sudden loads like this. A qualified installer will work with you to evaluate the types of loads each system might draw from the battery and make a recommendation for capacity.

The installer will also likely ask about future investments in your home that require electricity. You don't want to buy a battery system that works fine with what you have today, only to have it struggle after you install, say, a heat pump. Even if you don't expect to add load to your electrical system, installing battery backup that can be expanded provides flexibility should plans change.

Battery capacity is measured in kilowatt hours (kWh), and the higher the capacity, the more the battery costs. The average home in the Northeast uses about 22.5 kWh per day. The most-popular battery backup models range from 6 kWh to 15 kWh, though most have a modular design that allows you combine multiple battery units. This underscores the importance of being selective about what items you connect to the backup system.

Battery backup for off-grid homes presents the biggest challenge, as the storage system needs to power the home when the sun doesn't shine. That means a battery with substantially higher capacity and more cost. Again, work with a professional who can accurately evaluate your needs.

Do I really need a professional to install a battery backup system?

Short answer – yes. Blake said that while any "reasonably intelligent" person could install a battery system, it likely would not perform optimally. "[Battery technology] has its own idiosyncrasies," he said.

A self-installed system "will work but not meet its full potential." For example, it matters where you install inverters between the backup system and the solar panels, and length of wire runs might affect performance.

It can also get tricky designing the battery system to work properly with your solar power system. Solar panels are designed to shut down with a power outage, as they will have no place to send the electricity they generate. You want them to continue to work during an outage to charge your batteries. That requires isolating the system correctly.

How long will the battery system keep my critical items running?

How long the battery backup power will last depends on a lot of factors in addition to the types of systems it keeps running. If you don't have solar panels and have a standalone battery backup system, then you have no way to recharge the batteries during the outage. Your uptime for electricity will depend on the capacity of the battery and the load placed on it.

Solar panels will give you more time, depending, of course, on the availability of sunlight. A system that is well-designed for the home's load should be able to deliver about 20 hours of power. That can vary if, for example, the outage happens in the early evening when the sun is down or during an extended cloudy period.

It also depends on how careful you are during an outage. "If you are frugal, you can make it go longer," said Blake. "Don't bake a turkey in the oven. That 20 hours will go to one hour."

Do I have the right space to install a battery backup system?

Battery backup systems are usually installed indoors and must meet building and fire code standards. Space needed varies depending on the brand and capacity, and that space needs to be dry to avoid corrosion. Outdoor models need to be out of direct sunlight to avoid overheating.

How much can I expect to spend on a battery backup system?

Battery backup systems have become less expensive, even though both the batteries themselves and the electronics used to manage them have become more advanced. However, the price to purchase and have a professional install a battery system capable of supporting all your critical items might give some people pause.

Prices for the battery systems themselves can range from around $8,000 to $20,000 or more depending on how much capacity you want. Batteries that can power everything in the typical home could cost $30,000.

Then you need to add installation costs on top of that. You might need to pay an electrician as well as an expert to install the battery. An electrician might need to install a critical-load panel in addition to its whole-house electrical panel if your home does not already have one.

These prices seem high, especially when compared against the cost of generators, but government incentives cut that cost down significantly. The Inflation Reduction Act offers a 30% uncapped tax credit, bringing a $20,000 system down to $14,000, for example.

Once you've installed battery backup, your operating costs are nil. The power is free, assuming you have solar panels, so no need to buy and store gasoline or have propane delivered for the generator.

Is battery backup right for you?

I wrote this article because I want a backup battery system to complement my solar panels. However, I decided to wait a year or two to see if prices come down further. I also plan to install a heat pump, which would do more to reduce my energy costs and allow me to stop burning oil for heat. That's a higher priority for me. The battery backup would also eliminate fossil-fuel consumption, just not as much.

A battery backup would be a huge improvement over my gas generator in terms of the climate, convenience, reliability and cost to operate. It would also be noiseless and can add to the value of your home. Those are all compelling reasons to consider a battery backup system for your home.

Michael Nadeau is a member of the Community Power Committee.