The Rating That Isn't a Promise
A 2,000 W kettle draws 2,000 W the whole time it's on. Straightforward.
A 2,000 W air conditioner does not. It cycles — running hard to reach temperature, then idling, then running again. Its actual average draw over an hour might be 600 W.
A 900 W microwave doesn't draw 900 W either. That's its output power delivered to the food. Its input draw is typically 1,400 W or more, because the magnetron isn't efficient.
A laptop charger rated 65 W draws 65 W only when charging a depleted battery at full rate. Most of the time it draws far less.
A power rating is a maximum, a nominal, or an output figure depending on the device — and knowing which determines whether your calculations are useful.
Three Different Things Called "Watts"
Rated input power
What the device draws from the supply at maximum. This is what matters for circuit sizing, fuse selection and generator capacity.
Usually what's on the nameplate for heating appliances, motors and most electronics.
Rated output power
What the device delivers. Microwave ovens, audio amplifiers, motors and power supplies are commonly rated this way.
The gap between input and output is efficiency:
input = output / efficiency
A motor rated 750 W output at 85% efficiency draws about 880 W.
This catches people out most often with microwaves. An "800 W microwave" needs around 1,200–1,300 W of supply capacity.
Average consumption
What it actually uses over time, accounting for cycling, standby and duty cycle.
This is the figure that determines your electricity bill, and it's almost never printed on the device.
The Power Converter handles the unit conversions:
- Enter the value.
- Select the source unit — watts, kilowatts, horsepower, BTU/hr.
- Read the equivalents.
Just be clear about which watts you're converting.
Duty Cycle: The Missing Multiplier
For anything that cycles, average power is rating times duty cycle.
average power = rated power × fraction of time running
A 2,000 W air conditioner running 30% of the time averages 600 W. A 150 W fridge compressor running 25% of the time averages about 38 W.
This produces a genuinely counterintuitive result: a low-power device that runs constantly can cost more than a high-power device used briefly.
Fridge: 38 W average × 24 h = 0.91 kWh/day
Electric shower: 8,500 W × 0.17 h = 1.45 kWh/day
Comparable, despite a 200-fold difference in rating. And a device drawing 5 W in standby, continuously, uses 44 kWh a year — more than many appliances that seem far more significant.
Rated power tells you about the circuit. Duty cycle tells you about the bill.
Apparent Power and Power Factor
For AC circuits with anything other than a pure resistive load, there are two power figures.
Real power (watts) does actual work — heating, motion, computation.
Apparent power (volt-amps, VA) is voltage times current, which is what the wiring and supply equipment actually have to carry.
power factor = real power (W) / apparent power (VA)
A purely resistive load — a heater, an incandescent bulb — has a power factor of 1. Motors, transformers and switching power supplies have power factors below 1, sometimes well below.
The practical consequences:
UPS sizing. A UPS rated 1,000 VA does not deliver 1,000 W. With a power factor of 0.6, it delivers 600 W. Sizing a UPS by VA against a load in watts is a common and expensive mistake. Always compare like with like.
Generator sizing. Same issue. A generator's VA rating and its watt rating differ.
Circuit capacity. Wiring and breakers carry current, so apparent power determines whether a circuit is adequate — not real power.
Commercial billing. Many commercial tariffs penalise poor power factor, because the utility has to supply the current regardless of how much of it does useful work.
Modern equipment increasingly includes power factor correction, bringing it close to 1. Older equipment and cheap power supplies often don't.
Horsepower Has Three Definitions
Converting horsepower requires knowing which one:
Mechanical (imperial) hp: 745.7 W
Metric hp (PS, CV, pk): 735.5 W
Electrical hp: 746 W
Metric horsepower is about 1.4% smaller than mechanical. European vehicle specifications typically use PS; UK and US specifications typically use mechanical hp. The difference is small but real, and it shows up when comparing figures across markets.
There's a further complication with engine ratings. Gross figures, historically used in some markets, measured an engine on a test stand without accessories — alternator, water pump, exhaust system. Net figures measure it as installed. Gross figures are substantially higher for the same engine, which is why some historical power figures look implausible against modern equivalents.
BTU/hr and Cooling Capacity
Air conditioning and heating equipment is frequently rated in BTU per hour:
1 BTU/hr ≈ 0.293 W
1 kW ≈ 3,412 BTU/hr
A "12,000 BTU" air conditioner has roughly 3.5 kW of cooling capacity. That's output — the heat it moves. Its electrical input is considerably less, because it's a heat pump moving heat rather than generating it.
The ratio between them is the efficiency rating:
- EER — cooling output in BTU/hr divided by input in watts, at a specific test condition
- SEER — a seasonal average across a range of conditions
- COP — the same ratio expressed dimensionlessly
A unit with an EER of 12 delivers 12,000 BTU/hr of cooling for 1,000 W of electrical input. The cooling output exceeds the electrical input because it's transporting heat, not creating it.
Never compare a cooling BTU figure to an electrical watt figure directly. They're measuring different sides of the same device.
Converting Between Them Correctly
A short checklist before any power conversion:
1. Is this input or output? Check the context. Motors, amplifiers and microwaves are usually output-rated.
2. Is it real power or apparent power? W or VA. If it's VA, you need the power factor to get watts.
3. Which horsepower? Mechanical, metric, or electrical.
4. Is it a peak, a nominal, or an average? Peak for circuit sizing, average for running cost.
5. Is it a duty-cycled device? If so, the rating overstates consumption substantially.
Practical Tips
Size circuits on peak input power, never on average consumption.
Size UPS units on watts, not VA, and check the unit's power factor rating.
Estimate running costs from duty cycle, not from the nameplate rating.
Measure rather than calculate where it matters. A plug-in energy meter costs little and gives you actual consumption including standby, which no nameplate does.
Watch standby loads. Individually trivial, collectively significant across a house full of devices.
Check whether a motor rating is input or output before sizing a supply. The efficiency gap is substantial.
FAQ
Why does my 800 W microwave need more than 800 W? That's its cooking output. The electrical input is typically 1,200–1,400 W, because the magnetron and electronics aren't fully efficient.
What's the difference between watts and VA? Watts measure real power doing useful work. VA measures apparent power — what the supply and wiring must carry. They're equal only for purely resistive loads.
How do I convert a UPS VA rating to watts? Multiply by the power factor, which the manufacturer should specify. Many UPS units have a power factor around 0.6–0.9.
Which horsepower should I use? Depends on the source. European car specs usually use metric hp (PS); UK and US specs usually use mechanical hp. They differ by about 1.4%.
How do I estimate an appliance's running cost? Average power × hours of use, converted to kWh, multiplied by your tariff. Use average consumption rather than the nameplate rating for cycling devices.
Is a 12,000 BTU air conditioner using 3.5 kW of electricity? No. That's its cooling output. Electrical input is far lower, because it moves heat rather than generating it.
The Takeaway
The number on a nameplate is answering a specific question — usually "how much supply capacity do I need" — and that isn't the same question as "what will this cost to run." Checking whether a rating is input or output, real or apparent, peak or average, is what makes a power conversion mean anything.
Convert between watts, kilowatts, horsepower and BTU/hr free with the Power Converter at sadiqbd.com — no sign-up, instant results.
