How Long Will a 15kWh Battery Power a House?
Quick Answer
A 15kWh LiFePO4 home battery delivers roughly 12.4 kWh to your appliances after depth‑of‑discharge and inverter losses. Divide that by your average load: about 25 hours at 500 W, 12.4 hours at 1 kW, and roughly 10 hours for the average U.S. household. Running only essential loads can stretch it past two days.
Why you keep seeing three different answers
Ask five sources how long a 15kWh battery lasts and you will get five numbers — 3 hours, 12 hours, 24 hours, "2–5 days." None of them are lying. They are answering different questions with different assumptions, and almost none of them show the arithmetic.
Published figures for the same battery currently range across a 3‑hour‑to‑2‑day spread:
| Source type | Stated runtime for 15kWh | Assumption used |
|---|---|---|
| Manufacturer support article | 12 hours | 30 kWh/day household |
| Manufacturer blog | 12–18 hours | 30 kWh/day household |
| Manufacturer blog | 6–9 hours (room AC) | Load‑specific |
| Manufacturer blog | "nearly 24 hours" | 10–15 kWh/day household |
| Retail blog | ~3 hours | 5 kW continuous load |
Two of those sources simply divide 15 kWh by the load and never subtract depth‑of‑discharge or inverter losses — which is why their numbers are optimistic. Others pick a household consumption figure that may be nothing like yours.
The fix is not a better opinion. It is a repeatable calculation. Below is the single method we use at JM Batteries, applied consistently to every number in this article, so you can check our work with a calculator.
The only formula you need
Two steps. That is the whole thing.
Step 1 — Work out your usable energy, not the nameplate number
A battery's label tells you how much energy it stores, not how much reaches your appliances. Two losses sit in between:
- Depth of discharge (DoD) — how much of the pack you are willing to cycle. LiFePO4 is unusual in tolerating deep cycling, so a well‑designed system can use around 90% without materially shortening life. Lead‑acid, by contrast, is typically held to 50%, which is a large part of why lithium packs deliver more usable energy per rated kWh.
- Round‑trip and inverter efficiency — energy lost converting DC to AC and back. Modern lithium home batteries commonly land in the 89–96% range.
For a 51.2V, 300Ah LiFePO4 pack, the nameplate arithmetic is:
51.2 V × 300 Ah = 15,360 Wh ≈ 15.36 kWh (nameplate)
Usable energy = 15.36 × 0.90 (DoD) × 0.90 (round‑trip + inverter)
≈ 12.4 kWh delivered to your loads
That 12.4 kWh figure is the number that matters. Every runtime below is calculated from it.
Rule of thumb: budget about 80–85% of the nameplate figure as genuinely usable energy in a well‑designed residential system. If a source quotes runtime from the full nameplate, it is overstating by roughly 20%.
Step 2 — Work out your real average load
Runtime is usable energy divided by average load, not peak load:
Runtime (hours) = Usable energy (kWh) ÷ Average load (kW)
Two ways to find your average load:
From your utility bill (most accurate):
- take a month's kWh usage and divide by the days, then by 24. A household using 900 kWh in 30 days averages 30 kWh/day, or about 1.25 kW continuous.
From your loads (better during an outage):
- decide which circuits you will actually keep running, add their average draw, and divide. A fridge (150 W), Wi‑Fi and modem (30 W), a few LED lights (200 W) and a couple of fans (200 W) total about 580 W.
The second method produces much longer runtimes — which is why "backup mode" and "whole‑home mode" numbers differ so dramatically.
Runtime table: 15kWh at every common load level
All figures use the 12.4 kWh usable baseline from Step 1. Divide 12,400 Wh by the load in watts and you will reproduce every row.
| Average load | Typical situation | Runtime from 12.4 kWh usable |
|---|---|---|
| 300 W | Fridge, internet, a few lights | ~41 hours |
| 500 W | Essentials plus TV and laptops | ~25 hours |
| 800 W | Essentials plus fans and a small pump | ~15.5 hours |
| 1,000 W | Light‑to‑moderate household use | ~12.4 hours |
| 1,200 W | U.S. average household draw over 24 h | ~10.3 hours |
| 1,500 W | Moderate use with some large appliances | ~8.3 hours |
| 2,000 W | Heavy use, one large appliance running | ~6.2 hours |
| 3,000 W | Central AC cycling, or water heating | ~4.1 hours |
| 5,000 W | Multiple heavy loads at once | ~2.5 hours |
| 8,000 W | Near‑maximum continuous draw | ~1.6 hours |
The row that surprises most homeowners is the 1,200 W one. If you keep living exactly as you normally do during an outage, a 15kWh battery gives you about ten hours — not two days. The two‑day figures you read elsewhere assume you shut things off.
What each appliance can actually run — and for how long
This table is calculated the same way: 12,400 Wh ÷ the appliance's typical continuous draw. Wattage ranges reflect commonly published U.S. residential figures.
| Appliance | Typical continuous draw | Runtime from 12.4 kWh usable |
|---|---|---|
| Wi‑Fi router + modem | 20–40 W | 310–620 hours |
| Laptop | 50–100 W | 124–248 hours |
| Whole‑home LED lighting | 100–300 W | 41–124 hours |
| Energy‑efficient refrigerator | 150–300 W | 41–83 hours (1.7–3.4 days) |
| LED TV | 100–200 W | 62–124 hours |
| Gas furnace blower | 400–600 W | 21–31 hours |
| Washing machine (cold wash) | 500–800 W | 15.5–25 hours |
| Window AC, 8,000–10,000 BTU | 700–900 W | 13.8–17.7 hours |
| Microwave | 1,000–1,500 W | 8–12 hours of runtime |
| Dishwasher | 1,200–1,800 W | 7–10 hours |
| Room air conditioner | 1,500–2,000 W | 6.2–8.3 hours |
| Central air conditioning | 3,000–5,000 W | 2.5–4.1 hours |
| Electric water heater | ~4,000 W | ~3.1 hours |
| EV charging (Level 2) | ~7,200 W | ~1.7 hours |
Read this table as a menu, not a total. You cannot run every row at once — the figures assume each appliance is the only thing drawing power. In practice you combine two or three, and the runtimes halve or third accordingly. This is also why a battery's continuous power output rating in kW matters as much as its kWh: a 15kWh pack with a small inverter can hold plenty of energy yet still refuse to start a compressor.
Can a 15kWh battery run your air conditioner?
This is the single most common question we see — and it has a real answer rather than a yes/no.
Air conditioning is hard on batteries for two separate reasons:
- Continuous draw. A typical room AC pulls 1,500–2,000 W, a central system 3,000–5,000 W.
- Inrush current. The compressor's startup surge can briefly demand several times the running wattage. If the inverter's surge rating is not sufficient, the system trips even though energy remains in the pack.
Working from 12.4 kWh usable:
One efficient window unit (700–900 W):
- roughly 14–18 hours of continuous running.
One typical room AC (1,500–2,000 W):
- roughly 6–8 hours.
Central AC (3,000–5,000 W):
- roughly 2.5–4 hours.
Four window units at once (~3,000 W combined):
- roughly 4 hours, and only if the inverter can carry the load.
Two factors routinely make real‑world results better than these continuous figures:
- Duty cycle. A correctly sized AC does not run at full power continuously — it cycles. In mild weather, actual runtime can be 50–100% longer than the continuous estimate.
- Inverter‑driven (variable‑speed) systems. These ramp rather than switching on hard, which reduces both average draw and surge.
Practical takeaway: size the inverter for surge first, then size the battery for energy. If whole‑home AC backup is the goal, a 15kWh pack will cover you for hours, not days — plan on 20kWh+ or pair the pack with solar so it recharges while the sun is up.
How winter changes the answer
Cold weather reduces usable capacity. Two effects stack:
- Capacity derating. Lithium cells deliver less usable energy at low temperature; systems that sit in an unheated garage or outdoor cabinet are affected most. Expect a meaningful reduction in delivered kWh in freezing conditions, and the effect is larger the colder it gets.
- Higher load on the same days. Heating demand rises exactly when the battery's capacity falls — the two curves work against you.
Practical mitigations: keep the pack in a conditioned or insulated space where possible, ensure the BMS supports low‑temperature charge protection so the pack is not charged below freezing, and add a solar array so the pack can top up during daylight outages rather than relying on one stored charge.
15kWh vs 20kWh vs 30kWh: which do you actually need?
Same formula, different nameplate figure. Using the same 90% DoD and 90% round‑trip assumptions:
| System size | Usable energy | Runtime at 1 kW | Runtime at U.S. average (1.2 kW) | Runtime on essentials (580 W) |
|---|---|---|---|---|
| 10 kWh | ~8.1 kWh | ~8.1 hours | ~6.8 hours | ~14 hours |
| 15 kWh | ~12.4 kWh | ~12.4 hours | ~10.3 hours | ~21 hours |
| 20 kWh | ~16.2 kWh | ~16.2 hours | ~13.5 hours | ~28 hours |
| 30 kWh | ~24.3 kWh | ~24.3 hours | ~20.3 hours | ~42 hours |
Pick by the gap you are actually trying to cover:
15kWh — fits most homes that want solid outage coverage of essentials plus a few comforts — and is a common single‑unit size for residential systems.
20kWh — suits homes that want to keep a well pump, some heating, or a larger AC running.
30kWh+ — is for near‑whole‑home backup or off‑grid autonomy across multiple cloudy days.
Because usable energy scales linearly, doubling capacity doubles runtime at a constant load. What it does not do is raise your peak power ceiling — that is the inverter's job.
How long does a 15kWh battery take to recharge?
Recharge time follows the same logic in reverse:
Recharge time (hours) ≈ Usable energy ÷ Charging power
From grid or a 5 kW charger:
- 12.4 ÷ 5 ≈ 2.5 hours
From a 3 kW solar array:
- 12.4 ÷ 3 ≈ 4.1 hours of peak production — realistically a full sunny day, since output is not constant.
Round‑trip cost from the grid:
- 12.4 kWh ÷ 0.90 (round‑trip efficiency) ≈ 13.8 kWh purchased. At typical U.S. rates, about $2–$3.45 for a full charge.
Is 15kWh enough for your house?
It depends on which of two questions you are asking.
| Your goal | Is 15kWh enough? |
|---|---|
| Keep fridge, lights, internet and phones running for 1–2 days | Yes — comfortably |
| Run "normal" whole‑home usage during an outage | Partly — about 10 hours |
| Run central AC plus everything else for a full day | No — plan on 30kWh+ or add solar |
| Shift solar generation to evening peak rates daily | Yes — for most households |
| Go fully off‑grid, year‑round | Unlikely alone — size for multi‑day autonomy |
How to verify a battery supplier before you buy
Runtime figures are only as good as the hardware behind them. Before committing, check these five things — they are the checks buyers most often skip:
- Ask for the usable‑capacity and DoD spec, not just the nameplate. A supplier who cannot state the DoD their warranty assumes is a supplier whose runtime claims you cannot verify.
- Confirm the continuous and surge power ratings. kWh tells you how long; kW tells you whether your AC will start.
- Read the certification documentation. Residential storage should carry recognised international safety and transport certifications.
- Read the warranty terms in full — specifically what cycle life and capacity retention are guaranteed, and what voids coverage.
- Check who actually manufactures. Trading companies and factories make different claims, and the difference shows up in cell grade and after‑sales support.
For our own 15kWh‑class packs, the published specification is 48V/51.2V at 300Ah/314Ah with a 10‑year warranty and a 6,000‑cycle rating.
Frequently asked questions
Is 15 kWh enough for a house?
For essential loads, yes — a 15kWh LiFePO4 battery gives roughly 21 hours running a 580 W essentials profile (fridge, internet, lighting, fans), and can exceed two days if you cut loads further. For normal whole‑home consumption at the U.S. average of about 1.2 kW, expect around 10 hours. Central air conditioning on its own reduces that to 2.5–4 hours.
How long will a 15kWh lithium battery last?
Two different questions hide behind that wording. Per charge, a 15kWh battery delivers about 12.4 kWh usable — 12.4 hours at a 1 kW load. Over its service life, a LiFePO4 pack rated at 6,000 cycles with roughly one cycle per day typically corresponds to well over a decade of service, though calendar ageing and temperature affect the final figure.
How many air conditioners can a 15kWh lithium battery run?
It depends on inverter power as much as energy. Assuming 12.4 kWh usable: one efficient window unit (700–900 W) runs about 14–18 hours; a typical 1,500–2,000 W room AC runs 6–8 hours; four window units drawing ~3,000 W combined run about 4 hours, provided the inverter can handle the compressor startup surge.
How long will a 20kWh battery last?
Using the same method: 20 kWh × 0.90 × 0.90 ≈ 16.2 kWh usable. That is 16.2 hours at 1 kW, 13.5 hours at the U.S. average household draw, and about 28 hours on an essentials profile. On the same continuous AC load, a 20kWh pack lasts roughly a third longer than a 15kWh unit.
How big of a battery bank do I need to power a house?
Work backwards from the outage you want to survive. Multiply your essentials load in kW by the hours you need, then divide by about 0.81 (0.90 DoD × 0.90 efficiency) to get the nameplate size you must buy. Covering a 580 W essentials profile for 24 hours needs about 14 kWh nameplate; covering the full U.S. average household for 24 hours needs roughly 36 kWh.
How many solar panels do I need to charge a 15kWh battery?
You need to replace about 12.4 kWh, plus losses — call it 13.8 kWh. In a location with roughly 4–5 peak sun hours per day, that is a 3 kW array in ideal conditions; 4–5 kW gives realistic headroom for cloudy days and winter. Whether you need solar at all depends on your goal — daily bill reduction needs it, outage backup does not.
How much does a 15kWh home battery cost?
Cell chemistry, cell grade, inverter integration, certification scope and warranty length drive the price far more than the kWh figure. Be cautious of quotes that undercut the market substantially — that gap is usually paid for in cell grade or after‑sales support. Request a current quotation for your configuration rather than comparing headline kWh prices.
Sources and methodology
How every number in this article was produced. All runtimes use one formula chain, applied consistently:
Usable energy (kWh) = Nameplate (kWh) × Depth of Discharge × Round‑trip efficiency
Runtime (hours) = Usable energy (kWh) ÷ Average load (kW)
= (Usable energy × 1000) ÷ Appliance draw (W)
Assumptions used throughout: nameplate 15.36 kWh (51.2V × 300Ah), DoD 90%, round‑trip + inverter efficiency 90%, therefore 12.4 kWh usable. Change any input and recompute with the formulas above — the arithmetic is deliberately transparent so you can.
Reference points used: U.S. household consumption of roughly 29–30 kWh/day (≈10,500–10,800 kWh/year, U.S. Energy Information Administration); the NREL definition of storage duration as the time a battery discharges before usable energy is depleted; and typical U.S. residential appliance wattage ranges. Appliance figures are continuous‑draw estimates and will vary with model, age and duty cycle — treat them as planning values, not measured results.
What this article does not claim. No runtime figure here is a guarantee of performance in your home. Actual results depend on your load profile, inverter, ambient temperature, battery age and how deeply you cycle the pack.
