Choosing a Home Battery for Power Outages

A reader in upstate New York emailed me last winter after losing power for three days during an ice storm. She had been looking at home batteries online for weeks and was completely paralyzed by the choices. The right battery is not the biggest one or the cheapest one. It is the one matched honestly to what you want to keep running, sized with real numbers, and bought with the warranty fine print already read.

A home battery is a serious purchase, often the most expensive single piece of electrical equipment you will ever buy outside of major appliances or an EV. The marketing for these units has gotten very polished over the last few years, with sleek wall-mounted photos and promises of energy freedom. What gets glossed over is the basic shopping homework. How much do you actually need to power? For how long? Which chemistry will still be healthy in ten years? What does the warranty really cover when something goes wrong in year seven?

This guide walks through the questions in the order that matters, so by the time you start collecting quotes you can read them critically instead of trusting the salesperson to translate. I am going to skip brand names entirely and focus on the categories and specs that apply across the market, because the brands shuffle every couple of years but the underlying engineering does not.

Start by defining what you actually want to power

Before any sizing math makes sense, sit down for ten minutes and write two short lists. The first is what you absolutely need to keep running during an outage. The second is what would be nice to have but you could live without for a day or two. These two lists drive every other decision in the buying process, and most people skip them entirely and just buy the biggest battery they can afford.

Households generally fall into one of three backup goals:

• Essentials only: a few lights, the fridge, phone and laptop charging, the internet router, and any medical equipment like a CPAP. Total daily energy use is around 2 to 5 kWh, and peak draw rarely exceeds 1,500 watts at any one moment.
• Essentials plus comfort: everything above plus a window AC or small space heater, the microwave, and the washing machine when needed. Daily energy climbs to 8 to 15 kWh, and peak draw can briefly touch 3,000 watts.
• Whole-home or close to it: central HVAC, the electric range, electric water heater, well pump, and everything else on a normal day. Daily energy use can exceed 30 kWh, with peak draws of 8,000 watts or more during startup events.

These three goals map roughly onto small, medium, and large battery systems. If you are sizing for essentials only and someone is quoting you a whole-home stack, you are about to spend two or three times what you actually need. Knowing your goal first lets you push back on quotes that do not match it.

The sizing math: kWh versus kW, in plain language

Every battery spec sheet shows two numbers that look similar but mean very different things. Getting them mixed up is the single most common shopping mistake I see in reader emails.

kWh (kilowatt-hours) is how much energy the battery stores in total. Think of it as the size of the gas tank. A 10 kWh battery holds 10 kWh of usable electricity, which is enough to run a 1,000 watt load for about 10 hours, or a 500 watt load for about 20 hours. This number determines how long your backup lasts.

kW (kilowatts) is how much power the battery can deliver at any one moment. Think of it as the size of the fuel line. A battery rated at 5 kW continuous output cannot run loads totaling more than 5,000 watts at the same time, no matter how much energy is stored inside it. This number determines what you can run.

Both numbers matter, and they fail in different ways. A battery with plenty of kWh but low kW will run out of muscle when your AC kicks on, even though it still has hours of stored energy. A battery with high kW but low kWh will start every appliance fine but die after two hours. You need both numbers in the right range for your goal.

A quick rule of thumb for sizing:

• Essentials goal: 5 to 10 kWh of storage and 3 to 5 kW continuous output.
• Essentials plus comfort: 10 to 20 kWh and 5 to 7 kW continuous.
• Whole-home goal: 20 to 40 kWh and 7 to 10 kW continuous, usually achieved by stacking two or three batteries.

The actual dollar cost behind these tiers varies widely by region and installer. For honest numbers and what drives quotes up or down, our deep dive on a realistic budget for a home battery backup system walks through real installed prices by tier.

LFP versus NMC chemistry, explained simply

Almost every home battery on the market today uses one of two lithium chemistries, and the difference matters more than the brand on the box. You will see these abbreviations on every spec sheet, so it pays to know what they mean.

LFP (lithium iron phosphate), sometimes written as LiFePO4, is the chemistry most newer home batteries use. It is heavier per kWh and slightly bulkier, but it has three big advantages for stationary backup use. It lasts roughly 4,000 to 7,000 full charge cycles before noticeable capacity loss, which translates to 12 to 18 years in typical residential use. It is much more thermally stable, meaning it almost never enters thermal runaway even if punctured or shorted. And it does not contain cobalt, which has both ethical and supply chain implications.

NMC (nickel manganese cobalt) is the older chemistry that dominated EV batteries and the first generation of home storage. It packs more energy into a smaller package, which is why EVs still favor it. For home backup, the tradeoffs are less attractive. NMC typically lasts 1,500 to 3,000 cycles before significant degradation, which means 6 to 10 years of useful life. It is more thermally reactive and requires more sophisticated cooling and safety systems. And the cobalt content makes it pricier and more controversial.

For a battery that will sit on a wall in your garage cycling once a day for the next decade, LFP is almost always the better choice. The size and weight penalty does not matter when nothing has to move. The cycle life and safety advantages compound year over year. If a quote includes an NMC battery for a residential install, ask why. The answer is sometimes legitimate (an older inverter that requires a specific battery type) and sometimes just a leftover from inventory the installer wants to clear.

Modular stacks versus single units

Home batteries come in two basic physical formats, and the choice affects both your upfront budget and your future flexibility.

A single unit is exactly what it sounds like. One sealed enclosure holds the battery cells, inverter, and electronics in a single wall-mounted package. Typical capacities run 10 to 15 kWh per unit. Installation is straightforward because one box bolts to one wall with one set of conduit runs. The downside is that if you want more capacity later, you usually need to add a second complete unit, repeating most of the install cost.

A modular stack separates the brain from the muscle. A central inverter or controller handles the smart functions, and individual battery modules plug into it to provide storage. Typical modules are 3 to 5 kWh each. You buy the controller plus as many modules as your budget allows, and you can add more modules later without major rewiring. Stacks tend to look like a small refrigerator or a column of stacked drawers in your garage or utility room.

For households that know exactly what they need and will not change their setup, a single unit is often cheaper and simpler. For households that want to start smaller and grow into the system as budget allows, or who expect their electrical needs to change (an EV purchase, a home addition, a new heat pump), the modular approach pays off over time. Ask the installer what the cost of adding capacity later actually looks like for each option. The honest answer reveals which platform locks you in versus keeps your options open.

Partial-home backup with a critical-loads panel

Most homes do not actually need every circuit running during an outage. The HVAC, the electric range, the dryer, and the water heater together draw more power than most batteries can deliver, and trying to run them all on backup will drain even a large stack in a few hours. The smarter approach for many households is partial-home backup, where only selected circuits stay powered when the grid goes down.

This is done with a critical-loads subpanel, a smaller electrical panel installed alongside your main panel that holds only the circuits you want protected. During normal operation, both panels run on grid power. During an outage, the main panel goes dark and the subpanel stays alive, fed by the battery. The lights, fridge, internet, and a few outlets keep working. The big-draw appliances do not, which is exactly the point because nothing on backup needs to fight the AC for power.

The advantages of partial-home backup:

• A smaller, cheaper battery handles your real needs comfortably.
• Battery runtime extends from hours to days because you are not powering high-draw equipment.
• The install is often simpler because you do not need a whole-home transfer switch.
• You can stage the cost, starting with critical loads and adding whole-home backup later if you want.

The disadvantages are mostly behavioral. You have to remember which circuits are on backup and which are not. If you want to do laundry during an outage, you cannot. For most households, that tradeoff is well worth the savings. For households with critical needs on many circuits (a home office across two rooms, multiple medical devices, a basement freezer separate from the kitchen fridge), the wiring map gets fiddly enough that whole-home backup starts to make sense.

Integrating a battery with solar panels

A battery paired with solar is a fundamentally different system from a battery alone. With solar, the battery can recharge during daylight hours even when the grid is down, which can extend outage runtime from hours to days or even indefinitely depending on weather and load. Without solar, the battery is purely a finite reservoir that drains until the grid returns.

If you already have solar panels installed, adding a battery requires a compatible inverter or gateway. Older grid-tied solar systems automatically shut down during outages for safety reasons, even on sunny days, because they cannot tell whether a utility worker is repairing the line they would be back-feeding. A battery and the right inverter create an island that lets your panels keep producing into the battery during an outage, while still isolating from the grid.

If you are planning solar and storage together from scratch, you have more flexibility. You can choose a hybrid inverter that handles both panels and battery in one device, which is usually cheaper than two separate systems. You can size the panels to recharge the battery comfortably during daylight even on cloudy days. And you qualify for the federal Residential Clean Energy Credit on the full installed cost, which currently covers 30 percent of solar plus storage projects through 2032.

If you also have or plan to have an EV, the integration gets one layer more interesting. The car becomes a second large electrical load that the battery and panels need to account for, and some newer systems can even pull energy from the car back into the house in an emergency. For the basics of how home charging fits into your panel and load planning, our explainer on how home EV charging actually works walks through the panel side carefully.

Cycle life expectations and real-world degradation

Battery cycle life is the number of full charge and discharge cycles a battery can complete before its usable capacity drops below 70 to 80 percent of the original. After that point the battery still works, it just stores less energy than it used to. Cycle life is one of the most important specs on any battery you buy, and one of the easiest to misread.

What spec sheets say versus what you actually get:

• A 6,000 cycle rating on LFP chemistry assumes lab conditions, full depth of discharge each cycle, and moderate temperatures. In real residential use, you probably will not cycle the battery to empty every day, which extends the practical life.
• Most home batteries cycle 0.7 to 1.5 times per day on average, depending on whether you use them for daily solar self-consumption or only for backup. A backup-only battery may cycle just 20 to 50 times per year and last 20 years or more.
• Temperature matters more than people realize. A battery installed in an unconditioned garage that hits 100 degrees in summer will degrade faster than one in a conditioned utility room. Cold extremes hurt too, but most batteries have built-in heaters to compensate.
• Depth of discharge affects life. Cycling between 20 percent and 90 percent (a 70 percent depth) is gentler than cycling between 0 percent and 100 percent. Many batteries reserve some capacity automatically for exactly this reason.

The warranty fine print to read before signing

Battery warranties are where the marketing meets the legal department, and the gap between what the brochure implies and what the warranty actually covers can be significant. Before you sign a quote, ask for the actual warranty document (not the summary) and read it carefully. Here is what to look for.

Calendar versus throughput limits: as noted above, most warranties end at whichever expires first. Make sure you understand both numbers and how your expected usage stacks up against them.

Capacity retention guarantees: a typical warranty promises 60 to 70 percent of original capacity at the end of year 10. That means a 13 kWh battery could legitimately drop to 8 to 9 kWh of usable storage and still be considered “in warranty.” If that drop would leave you under-sized for your needs, plan accordingly.

Labor coverage: many warranties cover the cost of a replacement battery but exclude the labor to remove the old one and install the new one. Replacement labor on a wall-mounted battery can run $1,500 to $3,500. Some installers offer extended labor warranties separately. Ask.

Installation requirements: warranties often require professional installation by a certified installer and may be voided if the battery is moved, modified, or installed in conditions outside spec (extreme heat, flooding risk, vibration). Keep the original installer documents and any monitoring records.

Transfer terms when you sell the house: some warranties transfer to a new homeowner automatically, others require a transfer fee, and a few do not transfer at all. If you might sell within the warranty period, this matters for both your resale value and the buyer’s confidence.

What voids coverage: common voiders include using non-approved firmware, connecting the battery to non-listed inverters, allowing third-party service technicians to open the unit, and failing to maintain a working internet connection for remote monitoring. The last one surprises people. If your wifi goes down for three months and the battery cannot phone home, some warranties consider that grounds for denial.

The customers who end up happiest with their batteries are not the ones who bought the biggest or the latest. They are the ones who matched the system honestly to their backup goal, chose chemistry for longevity, and read the warranty before signing the check. The right battery for your home is the one still doing its job quietly in year twelve, not the one that looked most impressive on day one.

Once you have your sizing goal, your chemistry preference, and your understanding of partial versus whole-home backup clear, the quote conversations get much shorter. You stop being a person who needs a sales pitch and become a person buying a clearly specified piece of equipment. That shift in posture is worth more than any feature on the spec sheet. For the full cost picture once you know what you want, the budgeting for a home backup energy system walkthrough covers three honest spending tiers and the line items quotes often leave off.

Frequently asked questions

Read next in EV & Battery Basics

If this guide was useful, these two neighbouring pieces will fill in the surrounding context:

• A Realistic Budget for a Home Battery Backup System
• An Honest Look at Third-Party EV Charging Cables

Sources and further reading

For the underlying data behind the numbers in this guide, and for the standards, incentive programs, and safety rules referenced throughout, see:

• SAE J1772 charging connector standard
• FuelEconomy.gov: official EV mileage and cost comparisons
• DOE: batteries for hybrid and plug-in vehicles