kW vs kWh: The Difference Between Power and Energy

Reading a bill, sizing a battery, choosing a solar system, comparing EV chargers: most of the confusion in home energy comes from mixing up two units, the kilowatt (kW) and the kilowatt-hour (kWh). They look alike, and news reports and sales brochures swap them all the time, yet they measure different physical quantities. Learn to tell them apart and much of energy arithmetic turns into simple multiplication.

The short answer

kW (kilowatt) measures power, the rate at which energy flows at a given instant. Think of the speed of a car (km/h) or the flow rate of a tap (litres per minute).
kWh (kilowatt-hour) measures energy, the amount delivered over a period. Think of the distance driven (km) or the volume of water in a bucket (litres).

Time links the two:

Energy (kWh) = Power (kW) × Time (h) Power (kW) = Energy (kWh) ÷ Time (h) Time (h) = Energy (kWh) ÷ Power (kW)

A 2 kW kettle running for 3 minutes (0.05 h) uses 2 × 0.05 = 0.1 kWh. A 10 W LED bulb (0.01 kW) running for 10 hours also uses 0.1 kWh. Same energy, very different power.

Where the units come from

The watt (W) is the SI unit of power, equal to one joule per second (1 W = 1 J/s), according to the BIPM’s SI Brochure. A kilowatt is 1,000 W. A kilowatt-hour is the energy of 1 kW sustained for 1 hour:

1 kWh = 1,000 W × 3,600 s = 3,600,000 J = 3.6 MJ

The joule is the “proper” SI energy unit. Electricity uses kWh because the numbers are easier to handle: a household uses thousands of kWh a year, not billions of joules. For conversions to BTU, therms, toe, calories and more, use the energy unit converter or the energy units cheat sheet.

The prefix ladder

Power	Energy	Typical scale
W (watt)	Wh	Phone charger (5–20 W); phone battery (10–20 Wh)
kW = 1,000 W	kWh	Kettle (2–3 kW); daily home use (5–30 kWh)
MW = 1,000 kW	MWh	Large wind turbine (3–15 MW); small town per hour
GW = 1,000 MW	GWh	Large nuclear reactor (~1 GW); annual output of a mid-size solar farm (~100–200 GWh)
TW = 1,000 GW	TWh	National annual demand (e.g. tens to hundreds of TWh)

National electricity statistics are reported in TWh per year. You can compare nations on our country profiles and rankings.

Common mistakes (and how to spot them)

“kW per hour.” Almost always wrong. kW is already a rate, so “kW per hour” describes how fast power is changing (a ramp rate). Grid engineers use that; households almost never do. What people usually mean is kWh.
“A 5 kWh solar system.” Solar systems are sized in kW (strictly kWp). What they produce is kWh. A 5 kWp system might produce 4,500–8,000 kWh a year depending on location.
“The battery is 5 kW” when it means capacity. A battery has both: a power rating (kW: how fast it can charge or discharge) and an energy capacity (kWh: how much it holds). A 13.5 kWh battery with a 5 kW inverter can run a 5 kW load for about 2.7 hours, but cannot run a 7 kW load at all.
“A 1 GW solar farm powers X homes.” Homes consume energy (kWh), so you need the farm’s annual energy (GWh), which depends on its capacity factor (see what capacity factor means).
Confusing kWh with kWh/kWp or kWh/m². Specific yield (kWh/kWp) and irradiation (kWh/m²) are ratios, normalised by system size or by area.

kW and kWh in everyday situations

Reading your electricity bill

Your meter counts kWh. If your tariff is €0.25/kWh and you used 300 kWh in a month, the energy part of your bill is €75. Many bills also have a fixed daily standing charge and, in some countries, a charge based on your connection capacity in kW (or kVA): the maximum power your supply can deliver before the breaker trips. Commercial customers often pay demand charges on their peak 15-minute kW. That is why a business can cut its bill by spreading loads out, without using a single kWh less.

Appliances

An appliance label gives power in W or kW. For energy use, multiply by hours of use and, for appliances that cycle on and off, by the duty cycle:

Appliance	Typical power	Typical use	Energy
LED bulb	8–12 W	5 h/day	0.04–0.06 kWh/day
Fridge-freezer	100–200 W when compressor runs	~30–40% duty	0.6–1.5 kWh/day
Electric oven	2–3.5 kW	1 h	1–2 kWh (thermostat cycles)
Heat pump (space heating)	1–4 kW electrical	winter day	10–40 kWh/day
Electric shower	7–10 kW	10 min	1.2–1.7 kWh
EV charging at home	7–11 kW	4 h	28–44 kWh

The electric shower shows the difference well: very high power, short duration, moderate energy. The fridge is the reverse. Its power is low, but it never switches off, so it adds up to a lot of energy over a year. The US Department of Energy’s appliance guide uses the same wattage × hours × days method.

Solar panels

A solar system’s kWp is its peak power. Its annual kWh is what saves you money:

Annual kWh ≈ kWp × specific yield (kWh/kWp) 6 kWp × 1,200 kWh/kWp ≈ 7,200 kWh per year

Specific yield runs from about 800–1,000 kWh/kWp in the UK and northern Germany to 1,400–1,700 in Spain or the southern US. Get a site-specific figure from the solar panel calculator, or check your location’s sun resource in the Energy Explorer.

Home batteries

A battery comes with two numbers, and you need both:

Usable capacity (kWh) sets how long it can run your loads.
Continuous power (kW) sets how many loads it can run at once. Peak (surge) power for a few seconds is higher, which matters for motors and pumps.

Example: a household uses 8 kWh between sunset and sunrise, with a peak of 4 kW when the oven and kettle overlap. A 10 kWh battery with 5 kW output covers both. A 10 kWh battery limited to 3 kW has enough energy but has to leave the peak to the grid. The solar battery calculator models this hour by hour; the home battery storage guide covers sizing in more detail.

Electric vehicles

EV batteries are rated in kWh (typically 40–100 kWh for cars); chargers in kW.

Charging time (h) ≈ energy to add (kWh) ÷ charger power (kW) ÷ charging efficiency 60 kWh × (80% − 20%) = 36 kWh to add At 7.4 kW and 90% efficiency: 36 ÷ 7.4 ÷ 0.90 ≈ 5.4 h At 50 kW DC: 36 ÷ 50 ≈ 0.7 h (plus taper near full)

Running cost is in kWh: a car using 17 kWh/100 km costs €4.25 per 100 km at €0.25/kWh. Compare tariffs in the EV charging cost calculator.

Worked example: a day in one home

Say your smart meter shows this for one day:

Period	Average power	Duration	Energy
Night (00–07)	0.25 kW	7 h	1.75 kWh
Morning (07–09)	2.0 kW	2 h	4.0 kWh
Day (09–17)	0.4 kW	8 h	3.2 kWh
Evening (17–23)	1.5 kW	6 h	9.0 kWh
Late (23–24)	0.5 kW	1 h	0.5 kWh
Total	0.77 kW average	24 h	18.45 kWh

The daily energy (18.45 kWh) sets the energy bill. Peak power sets the connection size you need and, for a battery or off-grid system, the inverter rating. Here it might reach 7 kW for a few minutes when the oven, kettle and washing machine overlap. The average power (18.45 ÷ 24 = 0.77 kW) is handy for back-of-envelope checks: it works out to about 6,700 kWh per year.

kVA: apparent power, used for transformers, generators and some connection ratings. kW = kVA × power factor; for resistive loads they are equal, for motors kW is lower.
kWh/m²/day: solar irradiation, the “fuel” for solar panels. Multiply by panel area and efficiency to estimate energy.
W/m²: irradiance (instantaneous sunlight power) or wind power density.
MWh/yr and TWh/yr: annual energy from power plants and countries. A figure in GW on its own never tells you annual output.
Primary energy vs electricity: 1 kWh of electricity from a gas plant takes roughly 1.7–2.5 kWh of gas, depending on efficiency. As the EIA’s units guide points out, statistics that mix the two need care.

Key takeaways

kW is power (rate); kWh is energy (amount). kWh = kW × hours.
Bills charge mostly for kWh; connection size, battery inverters and chargers are sized in kW.
Solar systems are rated in kWp but judged by annual kWh; batteries need both a kWh and a kW rating.
When a headline gives only GW, ask for the GWh, and for the capacity factor that links them.

Calculators for this topic

Energy unit converter Convert any amount of energy between joules, watt-hours, calories, BTU, therms, tonnes of oil or coal equivalent, barrels of oil equivalent, electronvolts and foot-pounds. Shows all units at once, with real-world equivalents. Solar battery size & payback Simulates your solar panels, your household’s hourly demand and a home battery for all 8,760 hours of a real year at your location, then finds the battery size that actually pays off. EV charging cost What does it cost to charge an electric car? Enter your battery, efficiency and mix of home, public AC and rapid DC charging to get the cost per charge, per 100 km and per month, including charging losses, compared with a petrol car. Solar panel output & payback Simulates every hour of a full year at your exact location using NASA satellite weather, then turns the energy into savings, payback time and CO₂ avoided.

Frequently asked questions

What is the difference between kW and kWh in simple terms?

A kW (kilowatt) is how fast energy is used or produced at a moment, like the speed of a car. A kWh (kilowatt-hour) is how much energy was used or produced over time, like the distance driven. Running a 1 kW heater for one hour uses 1 kWh.

How many kWh does a 1 kW solar system produce?

It depends on sunshine. A 1 kWp system produces roughly 800–1,000 kWh a year in the UK or northern Germany, 1,100–1,400 kWh in southern Europe or much of the US, and 1,600–2,000 kWh in very sunny deserts. That annual figure divided by 8,760 hours gives the capacity factor.

Is my electricity bill based on kW or kWh?

Household bills in most countries charge mainly for energy in kWh plus a fixed daily charge. Some tariffs, and most commercial and industrial tariffs, also include a demand or capacity charge based on your peak power in kW, or on the size of your connection.

How long will a 10 kWh battery run my house?

Divide usable energy by average power draw. A 10 kWh battery with 9 kWh usable powering a 0.5 kW average evening load lasts about 18 hours; at 2 kW it lasts about 4.5 hours. The battery's kW rating limits how many appliances can run at the same time.

What does kWp mean on a solar panel?

kWp (kilowatt-peak) is the DC power rating of solar panels under standard test conditions: 1,000 W/m² of sunlight at 25 °C cell temperature. Real output is usually below that because the sun is weaker most of the time and panels run hotter.

Why do EV chargers use kW and batteries use kWh?

The charger's kW rating is how fast energy flows into the car; the battery's kWh rating is how much it can store. Charging a 60 kWh battery from 20% to 80% means adding 36 kWh; at 11 kW that takes roughly 3.5 hours, at 150 kW (if the car accepts it) around 15–20 minutes.

Sources & references

Figures in this guide are typical ranges drawn from the sources above; your own numbers depend on location, prices and equipment. Use the linked calculators for site-specific estimates.