kW vs kWh: What Your Home Battery Spec Sheet Actually Means

A home battery spec sheet gives you two numbers that look almost identical and answer completely different questions. One is measured in kW. The other is measured in kWh. Mix them up and you can buy a battery that lasts all night but cannot start your air conditioner, or one that starts everything and runs dry before dinner.

Here is the short version. kW tells you how much the battery can deliver at one moment. kWh tells you how much total energy it holds. The rest of this guide shows how to read both, why the spec sheet lists a "nominal" and a "usable" kWh, and how to match the numbers to a real Texas house.

Key Takeaways

• kW is power, the rate of delivery at a given moment. kWh is energy, the total amount stored (EIA).
• Power answers "will it start my AC?" Energy answers "how long will it last?"
• Usable kWh, not nominal kWh, is the number to divide by your load.
• Runtime in hours is roughly usable kWh divided by the kW you are actually drawing.

What is the difference between kW and kWh?

The U.S. Energy Information Administration defines a kilowatt as one thousand watts and power as "the rate of producing, transferring, or using energy". A kilowatthour is 1 kilowatt of power expended for 1 hour. One is a rate. The other is a quantity.

The U.S. Department of Energy draws the same line for batteries. It describes energy capacity as the total amount of energy that can be stored, and power capacity as the amount of energy that can be released at a given time. The EIA puts it in similar terms: power capacity is the maximum amount of power a battery can provide at a given moment, and energy capacity is the total amount of energy it can store.

Think of a water tank. The kWh is the size of the tank. The kW is the width of the pipe coming out of it. A huge tank with a thin pipe fills a bucket slowly. A fat pipe on a small tank empties fast. You need both numbers to know what you are buying.

Why does a spec sheet list both numbers?

Because they fail in different ways. Run out of kW and the battery cannot carry the loads you ask for at once, so it trips or sheds circuits even with a full charge. Run out of kWh and everything works perfectly until the moment it stops.

In a Texas outage, those two failures show up in two very different moments. The first happens at the instant the AC compressor tries to start. The second happens at 3 a.m. when the battery reaches empty. If you want the outage-end scenario in detail, read what happens when a home battery runs out mid-outage.

So the spec sheet gives you a kW rating to answer "can it carry my loads right now," and a kWh rating to answer "how long can it keep carrying them." Neither number substitutes for the other.

What do continuous and surge kW mean?

Continuous kW is the output a battery system can sustain indefinitely. Surge kW is a higher figure it can hold only briefly, which exists for motor startup. Many appliances with a compressor or a motor pull several times their running power for a moment when they start.

For Eos systems, the controller is rated at 11.5 kW continuous and 17.1 kW surge, the same on every residential tier. The Plus plan page lists the surge figure as a short peak, about 10 seconds, intended to get an AC compressor started. That is the number that matters when someone asks whether a battery can run a central AC unit.

Surge figures differ by appliance and are printed on the equipment nameplate or in the manufacturer's data, so we will not guess a universal number here. For the central AC case in particular, this breakdown of running watts, startup surge and duty cycle walks through it.

One more point. Adding battery modules adds kWh. It does not add kW, because the controller sets the power ceiling. A single controller delivers the same 11.5 kW whether it sits on one battery or five. That is why kW and kWh are specified separately on the specifications page.

What is the difference between nominal and usable kWh?

Nominal kWh is the full rated capacity of the cells. Usable kWh is the portion the battery management system lets you actually draw, because a battery is not run to absolute zero or absolute full. The usable figure is always the smaller one, and it is the one to use in your math.

Eos publishes both on each plan. A single battery module is 9 kWh nominal and 8.76 kWh usable, which is about 97% of nominal. Two modules are 18 kWh nominal and 17.52 kWh usable. The ratio is high for this product, but it is worth checking on any spec sheet you compare. Some products publish only the nominal number in headlines and bury the usable figure in a footnote.

Here is a quick comparison of how the same two numbers read across Eos residential tiers, taken from the plan data:

Plan	Modules	Nominal kWh	Usable kWh	Continuous kW	Surge kW
Essential	1	9	8.76	11.5	17.1
Plus	2	18	17.52	11.5	17.1
Pro	3	27	26.28	11.5	17.1

Notice what changes down the table and what does not. Energy triples. Power stays put. If you are comparing quotes, line up usable kWh against usable kWh and continuous kW against continuous kW, and ignore any headline that mixes them.

How do you convert kW to hours of backup?

The core formula is simple. Hours of backup equal usable kWh divided by the kW you are drawing. The Department of Energy gives the matching conversion for appliances: watts times hours used per day, divided by 1,000, gives daily kWh.

Using the Essential battery's 8.76 kWh usable:

• At a 1.5 kW draw (a refrigerator, Wi-Fi, lights and a few chargers), 8.76 divided by 1.5 is about 5.8 hours. That lines up with the roughly 6 hours the Essential plan page quotes at minimal load.
• At a 5 kW draw (a summer evening with the AC cycling and the kitchen in use), 8.76 divided by 5 is about 1.75 hours.

Same battery, same kWh, a threefold difference in runtime. Runtime is not a property of the battery alone. It is a property of the battery and the load together. That is why a single "hours of backup" figure on a brochure always comes with a load assumption attached, and why you should ask what load it assumes.

Real systems lose a little energy in conversion and runtime shifts as your load changes minute to minute, so treat the formula as a planning estimate rather than a promise.

A worked example from a utility bill (illustrative)

The following numbers are illustrative, not a measurement of any customer. Start with a bill. The EIA reports that the average U.S. residential customer bought 10,791 kWh in 2022, about 899 kWh a month. It notes that this counts electricity purchased from utilities, so homes with their own solar may use more than they buy. We did not find a Texas-specific per-customer figure we could verify against a primary source for this post, so we use the national average only to set the scale.

Take a home that uses 900 kWh in a month.

1. Divide by about 30 days. That is roughly 30 kWh per day.
2. Divide 30 kWh by 24 hours. That is an average of about 1.25 kW, all day, every day.

That 1.25 kW is an average. Your real draw swings far above and below it. Overnight it might sit near 0.5 kW. On a hot afternoon with the AC running it might hit 5 kW or more. The average tells you about energy (kWh over a month). It tells you almost nothing about the peak (kW at one moment), and the peak is what the controller has to handle.

This is the practical reason utility bills are a good starting point for kWh but a poor guide to kW. Your bill is an energy statement. It never shows how hard your house pulls at 5 p.m. on a Houston August afternoon. How to size a home battery in Texas covers the full sizing method, including how to list the circuits you want covered.

Air conditioning is part of why Texas peaks run high. The EIA reports that 93% of households in the South Census Region used air conditioning in 2020, and that two-thirds of U.S. households use central AC or a central heat pump as their main equipment.

Which number matters more for your home?

It depends on what you are protecting. Use this as a rule of thumb.

Power (kW) matters more when you want to run a central AC, an electric range, a well pump or an EV charger. These are the heavy and motor-driven loads. If the controller cannot carry them together, the battery capacity behind it barely matters.

Energy (kWh) matters more when your outages are long, or when your priority is overnight medical devices, a refrigerator and communications. Light loads over many hours is a kWh problem.

Most Houston households have both problems on different days. A short afternoon outage in July stresses kW. A multi-day event after a hurricane stresses kWh. For how long a system can realistically carry a house, see how long a home battery backup lasts in Texas.

A practical approach to the decision:

• List the loads you must run at the same time. Add up their running watts. That is your kW target.
• Note which of those can be turned off or deferred. A smart panel that sheds nonessential circuits lowers the kW you need.
• Decide how many hours you want covered. Multiply your average load in kW by those hours. That is your kWh target, before accounting for usable versus nominal.

Common mistakes when reading a spec sheet

1. Comparing nominal to usable. One quote says 10 kWh and another says 9.7 usable. They may be the same battery. Compare like with like.
2. Treating kW and kWh as interchangeable. "A 10 kW battery" is ambiguous. Ask which one the number is.
3. Ignoring surge. A system rated for your running load can still fail to start the compressor if surge is too low.
4. Assuming runtime without a load. "Up to 24 hours" is only meaningful with the load it assumes. Ask for the kW behind it.
5. Forgetting that more modules do not add kW. In a single-controller design, capacity grows but the power ceiling holds.
6. Using the monthly bill as a power number. It is an energy total. It hides your peaks.

To build your load list, check the wattage printed on each appliance's nameplate, or start from our wattage chart of common household loads, then compare the total against the continuous and surge ratings on the specifications page.

Or call Eos at 833-989-3737 to talk through your setup with an installer.

Frequently Asked Questions

Is a higher kW or a higher kWh better for a home battery?

Neither is better on its own. Higher kW means the battery can run more loads at once and start motors. Higher kWh means it runs those loads longer. You need enough of each for your house, so size kW to your peak and kWh to your outage length.

How many kWh is a typical home battery?

Residential batteries are commonly sold in modules or single units from under 10 kWh to several tens of kWh. For scale, the EIA reports an average U.S. home uses about 899 kWh a month, roughly 30 kWh a day, though backup usually covers only your priority circuits.

Does adding more batteries increase kW?

Usually not. Extra modules add kWh, so you get more hours. The power ceiling is set by the controller, so a single 11.5 kW controller stays at 11.5 kW no matter how many modules sit behind it. Raising kW generally means adding controller capacity.

What does usable capacity mean?

Usable capacity is the part of the rated kWh you can actually draw. The management system keeps a small reserve unused to protect the cells. On Eos plans a 9 kWh module delivers 8.76 kWh usable. Always use the usable number when you calculate runtime.

Why does my utility bill show kWh but not kW?

Residential bills charge for energy used over a month, which is kWh. Peak power draw is not billed for most homes, so the bill hides it. To estimate kW, add up the running watts of the loads you would operate at once.

The short version

kW is the pipe, kWh is the tank. Read the continuous kW and surge kW to learn what the battery can run at once. Read the usable kWh to learn how long it can keep running it. Divide one by the other, with an honest load assumption, and a brochure number becomes a real answer for your house.