A home battery stores electricity so you can use it later. Most people buy one for two reasons: to keep the lights on during an outage, or to use more of their own solar power instead of selling it to the utility for a low price. Whether a battery pays off depends mostly on your utility's rules, not on the battery itself.
This guide explains the two numbers that matter on every spec sheet, the main battery chemistries, what a typical system costs, and how to size one for your home.
What do kWh and kW mean on a battery?
Every home battery has two ratings, and they measure different things.
- Capacity (kWh) is how much energy the battery holds. Think of it as the size of the fuel tank.
- Power (kW) is how fast the battery can deliver that energy. Think of it as the width of the hose.
A battery with 13.5 kWh of capacity and 5 kW of continuous power could run a 5 kW load for about 2.7 hours (13.5 ÷ 5 = 2.7). It could also run a 500-watt load for about 27 hours. The capacity sets how long you last. The power rating sets how many things you can run at once.
Power matters more than people expect. A central air conditioner, well pump or electric range can pull 3 to 7 kW, and motors need a short burst of extra power to start. If the battery's power rating is too low, it will trip even when it is nearly full. Many spec sheets list a separate "peak" or "surge" rating that lasts a few seconds for this reason.
Most home batteries sold today hold about 10 to 15 kWh per unit. Many systems let you stack two or three units for more capacity and more power. For quick definitions of terms like these, see the glossary.
LFP or NMC: which battery chemistry is better?
Nearly all home batteries use one of two lithium-ion chemistries.
| Feature | LFP (lithium iron phosphate) | NMC (nickel manganese cobalt) |
|---|---|---|
| Typical cycle life | 4,000–6,000+ cycles | 1,000–3,000 cycles |
| Fire risk | Lower; more stable when hot | Higher, though still rare |
| Weight for same capacity | Heavier | Lighter |
| Common in home units | Most new models | Some older and compact models |
For a battery bolted to a garage wall, weight hardly matters. That is why most newer home batteries use LFP. It lasts longer and handles heat better. NMC is not unsafe, but if you are choosing between two similar quotes, LFP is usually the better long-term pick.
Check the warranty too. Many home batteries carry a 10-year warranty that promises the battery will still hold about 60% to 70% of its original capacity at the end. Some warranties also cap total energy throughput, so a battery that cycles hard every day may reach the limit before 10 years.
How much energy do you lose in a battery?
No battery gives back all the energy you put in. The share you get back is called round-trip efficiency. For most home lithium batteries it is about 85% to 92%.
Here is what that means in practice. If your panels send 10 kWh into a battery with 90% round-trip efficiency, you get about 9 kWh back out. The missing 1 kWh is lost as heat in the battery and inverter. Batteries also use a small amount of power just to stay on standby, often 20 to 50 watts.
These losses are small, but they add up over thousands of cycles. They are one reason a battery rarely makes sense purely as a money-saving device unless your utility rates give it a reason to work.
Should you back up essential loads or the whole home?
When you install a battery for backup, you choose what it powers in an outage. There are two common setups.
Essential loads backup
An electrician moves a handful of important circuits into a separate "backup" or "critical loads" panel. Typical choices are the fridge, some lights, Wi-Fi, outlets in one or two rooms, the furnace fan and a sump pump. Because these loads are small, one 10 to 15 kWh battery can often run them for a day or more.
Whole-home backup
The battery connects so it can power your entire panel. This is simpler to live with, since everything works. But large loads like central air, an electric dryer or an EV charger can drain a single battery in a few hours. Whole-home backup usually needs two or more units, a higher power rating, or a load controller that sheds big appliances automatically.
Either way, wiring a battery into your home panel is a job for a licensed electrician. The installer will also handle permits and utility approval.
How do net-metering rules change the value of a battery?
This is the biggest factor in whether a battery saves you money. Under classic net metering, your utility credits each kWh you export at the full retail rate. The grid works like a free battery. A real battery adds backup, but it saves you very little extra money.
Many utilities have moved away from that model. Common newer rules include:
- Net billing: exports earn a lower credit than what you pay for imports. If you pay 30 cents per kWh but earn 8 cents for exports, every kWh you store and use yourself is worth about 22 cents more than exporting it.
- Time-of-use rates: power costs more in the late afternoon and evening. A battery charges from solar at midday and runs your home during the expensive hours.
- Demand charges or grid programs: some utilities pay battery owners to share stored power during peak events.
Here is a simple example. Say your battery shifts 8 kWh per day from export to self-use, and the gap between your retail rate and export credit is 20 cents. That is 8 × $0.20 = $1.60 per day, or about $584 a year. Under full retail net metering, the same battery might save close to zero. To see how your rate plan affects solar value, try the savings calculator.
How much does a home battery cost?
Typical installed prices in the US often land somewhere in these ranges. Quotes vary a lot by region, installer, brand and how much electrical work your home needs.
| Setup | Usable capacity | Typical installed range |
|---|---|---|
| One battery, essential loads | 10–15 kWh | $10,000–$16,000 |
| Two batteries, larger backup | 20–30 kWh | $18,000–$30,000 |
| Added to an existing solar system | 10–15 kWh | $11,000–$18,000 |
That works out to roughly $900 to $1,300 per usable kWh installed. Adding a battery to an older solar system can cost more, because you may need a new inverter or panel upgrade. A main panel upgrade alone can add $2,000 to $4,000.
The US 30% homeowner tax credit (25D) ended for systems installed after Dec 31, 2025. State, utility and local programs may still help. Check current incentives in our incentives guide.
A rough payback check: a $13,000 battery that saves $584 a year takes about 22 years to pay for itself (13,000 ÷ 584 ≈ 22). That is longer than most warranties. A battery pays sooner when your export credits are low, your evening rates are high, or a utility program pays you. Otherwise, think of it as backup insurance with some savings on the side.
How do you size a home battery? A worked example
Start with the loads you want to run in an outage and how long each runs per day. Here is an example for an essential-loads setup.
| Load | Watts | Hours per day | Wh per day |
|---|---|---|---|
| Fridge (average draw) | 150 | 24 | 3,600 |
| Furnace fan | 400 | 8 | 3,200 |
| LED lights | 100 | 6 | 600 |
| Wi-Fi and modem | 20 | 24 | 480 |
| Phones, laptop, TV | 150 | 4 | 600 |
| Sump pump | 800 | 2 | 1,600 |
| Total | 10,080 |
That is about 10 kWh per day. Add about 10% for inverter losses, and you need roughly 11 kWh of usable storage for one day. A single 13.5 kWh battery covers it with a small cushion. If your solar panels can recharge the battery during the day, that setup could run these loads for several days in a long outage.
Next, check power. The largest loads that might run at once here are the sump pump, furnace fan and fridge: 800 + 400 + 150 = 1,350 watts running, with a short start-up surge from the pump. A battery rated for 5 kW continuous handles that easily.
To run your own numbers, use the battery sizer. If you are weighing a battery against a generator or portable unit, read backing up your home in a power outage. When you are ready, compare installer offers on our quotes page.
Frequently asked questions
Will my solar panels work in a blackout without a battery?
Usually not. Grid-tied solar shuts off during an outage so it cannot send power onto lines that workers are repairing. A battery with the right inverter lets your home run as an island.
How long does a home battery last?
Most come with a 10-year warranty, and LFP batteries often last longer. Expect capacity to fade slowly, often to about 70% of the original after 10 years.
Can I add a battery to solar panels I already have?
Yes, in most cases. Some batteries connect on the AC side and work with any inverter. Others may need a new hybrid inverter, which raises the cost.
Is one battery enough for my home?
One 10 to 15 kWh battery is usually enough for essential loads for a day. Whole-home backup with central air or electric heat often needs two or more.
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