kWh vs kW: Decoding Home Battery Specs

A home battery spec sheet leads with two numbers that look like siblings — say, 13.5 kWh and 5 kW — and they answer completely different questions. Kilowatt-hours measure how much energy the battery stores. Kilowatts measure how fast it can deliver that energy. The first decides how long your house runs during an outage; the second decides what you can run at once. Blur the two and you end up with a battery that lasts all night but can’t start the well pump, or one that runs everything in the house at once for a grand total of ninety minutes before it’s empty. Neither is a defective product. Both are the wrong product for the outage the homeowner was actually preparing for.

The tank-and-tap analogy holds up unusually well here. Kilowatt-hours are the size of the water tank; kilowatts are the width of the tap. A big tank with a narrow tap holds plenty but flows slowly, while a wide tap on a small tank gushes impressively and then runs dry. A battery needs the right amount of both for your particular loads, and the two are priced and limited separately, which is exactly why reading a spec sheet with the units straight is worth the ten minutes it takes.

Capacity is endurance; power is the ceiling

Capacity, measured in kilowatt-hours, is endurance — how long the battery can keep going. Put it in terms of a real load and the number stops being abstract. A refrigerator drawing about 150 watts on average consumes roughly 3.6 kWh over a day, so a 13.5 kWh battery could in principle run that fridge alone for more than three days. Real backup is never one appliance, though. Load up a realistic critical-circuits setup — fridge, lights, internet router, phone charging, and a furnace fan — and you might average 500 to 800 watts, which drains that same 13.5 kWh battery in something like 17 to 27 hours. Now add central air conditioning at 3,000 watts or more and the math collapses to a few hours, because the air conditioner alone eats more than everything else combined. Backup duration is simply capacity divided by average load, and the average load is the number homeowners almost never have measured, which is why duration estimates so often surprise people after the fact. Two pieces of fine print live under that headline capacity figure and both are worth checking. The first is usable versus nameplate capacity: most modern home batteries quote usable capacity directly, but confirm it, because some chemistries hold back a slice to protect battery life, so a “14 kWh” unit might actually deliver 13 or a little less. The second is degradation. Capacity fades gradually with use, and warranties typically guarantee something on the order of 60 to 70% of original capacity after ten years, which means a battery sized with zero margin today is quietly undersized by year eight — a reason to build in a cushion rather than buying exactly enough for this year’s loads.

Power, measured in kilowatts, is the ceiling on how much you can run simultaneously, and it behaves nothing like capacity. A 5 kW battery can supply 5,000 watts continuously, which comfortably covers lights, electronics, refrigeration, and a microwave running together — but not all of that plus an electric dryer and a level-2 EV charger. Push past the ceiling and the system either sheds load or shuts down, and it does so regardless of how full the battery is, because a full tank behind a narrow tap still can’t flow faster than the tap allows. This is why whole-home backup usually requires either a high-power battery, several units stacked together (their power ratings add), or a critical-loads subpanel that walls off the heavy circuits so they never demand power from the battery in the first place. Spec sheets actually carry two power numbers, and the second one is where backup plans quietly succeed or fail. Continuous power is the sustained ceiling. Peak or surge power is a higher short-burst rating, often lasting only a few seconds, and it exists because motors are greedy at startup: a well pump, sump pump, or air-conditioning compressor can briefly pull two to three times its running wattage in the instant it kicks on. A battery rated 5 kW continuous with 7 kW surge may start your well pump; one rated 5 kW continuous with only 5.5 kW surge may refuse to, even though the pump runs at a placid 1.5 kW once it’s spinning. If anything in your backup plan has a big motor, the surge rating is the first spec to check, along with whether the battery advertises a specific motor-starting capability using a measure like locked-rotor amps. Voltage matters here too, and it’s easy to miss: well pumps, dryers, and central air are typically 240-volt loads, and not every battery backs up 240-volt circuits in every configuration, a detail that determines whether the load can run at all before any wattage math even applies.

Two more realities sit underneath both numbers and are worth knowing before you shop. A battery never returns quite as much energy as you put into it — round-trip efficiency for modern home units typically runs around 90%, so charging with 10 kWh of solar hands back roughly 9 kWh at the outlet, and that small tax quietly shrinks both your usable capacity and the amount of solar a battery can actually bank in a day. Temperature matters too, and it hits both specs at once. Lithium batteries lose usable capacity in the cold and may throttle their power output to protect themselves, so a unit rated 13.5 kWh and 5 kW in a warm room can deliver noticeably less of each in an unheated garage during a winter outage — which is exactly when you’re most likely to be leaning on it. Most home batteries carry a rated operating temperature range, and some include their own heating or thermal management, a spec worth checking if yours will live somewhere that freezes. Neither efficiency nor cold changes the core distinction between energy and power, but both are good reasons to build a margin into whichever number your outage plan depends on hardest, rather than buying exactly enough for a mild day.

Sizing with both numbers, and what the spec sheet won’t tell you

The clean way to shop is to answer two questions in a deliberate order, because they filter the market in different ways and getting the order wrong wastes time. The first question is what must run at the same time. Sum the running watts of your must-have simultaneous loads, and separately identify the single largest motor start among them, because together those set the required continuous kW and surge kW. This question eliminates specific products outright — a battery that can’t start your well pump is off the list no matter how much energy it stores, and no amount of capacity fixes a power shortfall. The second question is for how long, and it uses a different number: multiply your average load, which is much lower than the simultaneous peak because appliances cycle on and off rather than all running flat-out, by the outage duration you’re planning for. That sets the required kWh, which in turn sets how many units or which capacity tier you need. Working an example with labeled assumptions makes the two-step filter concrete. Take a critical-loads setup averaging 700 watts, including a fridge with a roughly 1,200-watt surge and a gas furnace fan, and target a 24-hour outage. The duration math is 0.7 kW times 24 hours, about 17 kWh, which likely means two stacked units of a typical 10-to-13.5 kWh product. The power math is a peak simultaneous load of maybe 2.5 to 3 kW, comfortably inside a single unit’s 5 kW rating. This household is capacity-constrained rather than power-constrained, which is the common pattern for modest backup goals — you run out of stored energy long before you run out of delivery capacity — whereas whole-home ambitions flip the constraint entirely and make power the binding limit. The solar battery calculator runs both sides of this from your own load list, which beats guessing at the average load that duration math depends on.

There is a third kilowatt hiding on the spec sheet that rarely drives a purchase but occasionally matters: the charging rate. The same power electronics that limit how fast the battery discharges also cap how fast it refills from solar, and on short winter days a low charge rate can mean the battery doesn’t reach full before sunset, so it enters the evening — and any overnight outage — only partly charged. It’s a second-order concern for most homeowners, but worth a glance if you live somewhere with genuinely short winter daylight. For the full picture of how energy actually travels from the panels, through the battery, and out to your loads, how home solar batteries work traces the whole path, and it’s useful context for why these two numbers are engineered and limited separately in the first place rather than being two views of one thing.

Scaling up changes both numbers together, which is where whole-home ambitions collide with the two-question method. Backing up an entire house rather than a handful of circuits raises the simultaneous load — now the air conditioner, the electric range, and the dryer might all want to run at once — so the required continuous and surge kW climb, and it raises the energy total too, because you’re carrying every load through the night instead of a curated few. Meeting both usually means either stacking multiple units, since their power ratings and their capacities add together, or installing a critical-loads subpanel that deliberately keeps the heavy circuits off the battery so a smaller unit can still do useful work. There is also a quiet dependency on the solar side: a battery only delivers its rated endurance day after day if the array can actually refill it, so pairing a large battery with an undersized array in a short-winter climate can leave you with capacity you can’t keep full. Sizing storage in isolation from the panels that feed it is one of the more common ways a spec-sheet-perfect battery still underperforms once it’s on the wall.

Kilowatts and kilowatt-hours also anchor how batteries get priced, which is the last place the distinction earns its keep. Batteries are frequently compared in dollars per kWh of usable capacity, and that comparison is fair only between products with similar power ratings. A unit with double the continuous kW is a different class of machine, not an overpriced tank, so ranking it against a lower-power battery purely on dollars-per-kWh will make the more capable product look like a bad deal when it is simply a different one. Reading the two numbers together stops that mistake. Whether a battery pencils out financially at all is a separate question entirely, one that turns on your rate structure, your export rules, and how much you value backup, and it’s taken up in is a solar battery worth it rather than here. What spec-sheet fluency buys you is narrower but essential: the confidence that whatever you buy is matched to the outage you’re actually preparing for, with enough energy to last as long as you need and enough power to run what you need while it lasts. Get one of those two numbers wrong and the battery disappoints in a way no amount of the other number can rescue — which is the whole reason the spec sheet leads with both.