Watts to Kilowatts

Adding Up Camp Loads Before You Buy a Generator

Everything you plug into an off-grid setup is labelled in watts — the fridge, the water pump, the lights, the coffee machine. Generators are not: the box, the spec sheet and the dealer all talk in kilowatts. Sizing a genset is really one long addition in watts followed by a single move of the decimal point.

Conversion factor: 1 W = 0.001 kW, so shift the decimal point three places left. A camper running a 150 W fridge, 40 W of LED lighting, a 65 W laptop, a 90 W water pump and a 700 W microwave draws 1 045 W, which is 1.045 kW — comfortably inside a 1.8 kW suitcase inverter, but only if nothing else runs while the microwave does.

What Makes the Load List Bigger Than It Looks

Compressors cycle, they do not idle

A compressor fridge might average 40 W over a day, but while the compressor runs it pulls its full 150 W — and the genset has to cover the running figure, not the daily average.

One motor sets the surge ceiling

Induction motors draw two to six times their running watts for a second or two at start-up. The largest single motor on the list, not the sum of them, is what the set must ride through.

Labels in watts, nameplates in kilowatts

Appliance rating plates almost never print kilowatts, and generator model names almost never print watts. The 3 500 W total you calculated is the 3.5 kW class you should be shopping in.

Altitude and heat quietly shrink the plate

A petrol engine at a mountain campsite makes noticeably less than its sea-level rating, so the kilowatt figure you compute is a floor to build headroom on, not a target to hit exactly.

From Load List to Genset Nameplate

Work through the list the way a dealer would size the machine for you, one number at a time.

Total the running watts off the rating plates

Add up everything that can be switched on at once and type the sum in the watt field. The kilowatt value appears as you type. A comma works as a decimal separator and spaces inside the number are ignored, so 1 045 pastes in fine.

Add the single biggest start-up surge

Take the highest starting watt figure on the list — usually a fridge, a well pump or an air conditioner — and add it once to the running total. Convert that second, larger number too: it is what the set's surge rating has to beat.

Carry the kilowatt figure into the comparison

The copy button hands over the bare number with no unit and no spaces, ready to drop into a spreadsheet column or a search box next to model names like 3.5 kW and 7.5 kW. Ctrl + C inside a field does the same.

Turn a machine's kilowatts back into spendable watts

Shopping the other way round? Press the swap button (↔) to run kW → W and see what a 2.8 kW inverter set actually gives you: 2 800 W to share among appliances, minus whatever headroom you keep for surges.

The big number on the box is usually the surge: marketing prints peak watts in the largest type and running watts in the small print. Size against the running figure, and treat the peak only as the margin that gets a motor spinning.

Portable Generator Classes and What the Box Says

Portable sets cluster into recognisable classes. Here is what each one holds continuously, what it can surge to for a couple of seconds, and the kilowatt figure that ends up in the model name.

Generator class	Running watts	Starting / peak watts	Running rating (kW)
Ultra-compact inverter	900 W	1 000 W	0.9 kW
Suitcase inverter, 2 kW class	1 600 W	2 000 W	1.6 kW
Recreational inverter, 2.2 kW class	1 800 W	2 200 W	1.8 kW
Mid-size inverter, roof air-con capable	2 800 W	3 000 W	2.8 kW
Open-frame contractor set	3 500 W	4 375 W	3.5 kW
Dual-fuel wheeled portable	7 500 W	9 500 W	7.5 kW
Large wheeled portable	9 500 W	12 000 W	9.5 kW
Trailer-mounted petrol set	12 000 W	15 000 W	12 kW

Notice how tight the margin is on inverter sets — 1 800 W running against 2 200 W peak leaves only 400 W of surge room — while open-frame machines are usually specified with a peak around 25 % above the running figure. That margin, not the headline kilowatts, decides whether the fridge restarts without tripping anything.

What Helps When You Are Standing in the Aisle

Both boxes live while you tot up the load list

Type in either field and the other follows immediately, so you can run the same list twice — once for running watts, once with the surge added — without clearing anything.

Swap to see what a genset's kilowatts buy

One press reverses the direction, turning a 2.8 kW or 7.5 kW nameplate into the watts you have to allocate across the van, the cabin or the job site.

Engine horsepower sits on the same dropdown

Searchable unit lists on both sides cover hp, PS and BTU/h as well, which is handy when a second-hand set is advertised by its engine rating instead of its output.

Figures that paste into a sizing sheet

Results carry up to eight decimals and thousands are spaced for readability, while the copy button strips all of that back to a plain number.

Off-Grid Generator Sizing Questions

The box says 2 200 W but the manual says 1 800 W — which figure do I size against?

The smaller one. 1 800 W (1.8 kW) is the continuous or rated output the engine and alternator will hold hour after hour; 2 200 W (2.2 kW) is a surge the set can supply for a second or two while a motor spins up. Keep your steady load under the rated figure and leave the peak entirely for start-ups.

How much extra should I allow for a fridge or freezer kicking in?

A fridge that runs at 150 W typically wants somewhere around 600 W to break the compressor free — roughly four times its running draw, with two to six times being the usual spread across motor appliances. Add that surge once, on top of the running total: 150 W running inside a 1 200 W (1.2 kW) list plus a 600 W surge means the set briefly sees 1 650 W, or 1.65 kW.

Is an inverter set's kilowatt worth more than a conventional one's?

Not in raw output — a kilowatt is a kilowatt either way. What differs is the quality and the behaviour: an inverter set rectifies and re-synthesises the waveform, giving clean power for laptops and battery chargers and letting the engine throttle back at light load, which saves fuel and noise. Conventional open-frame sets tend to offer more surge room per kilowatt and cost less, which is why they dominate the 3.5 kW-and-up job-site classes.

My generator is advertised as 5 kVA — how many kilowatts is that?

kVA is apparent power and kW is real power; the bridge between them is the power factor. Larger sets are conventionally rated at 0.8, so 5 kVA corresponds to 4 kW, or 4 000 W of usable output. Small single-phase portables are more often quoted near a power factor of 1.0, where the two numbers are effectively the same. Because kVA is not a unit of power it is not on the dropdown here — multiply by the power factor first, then convert the watts.

Do I lose output running the set at a mountain campsite or in a heatwave?

Yes. Thinner air means less oxygen per stroke, and manufacturers commonly quote a loss of roughly 3 to 3.5 % of output for every 300 m (1 000 ft) of elevation, with a further deduction once ambient temperature climbs above the rating conditions. On a 3 500 W set at 1 500 m that is around 500 W gone, leaving about 3 000 W or 3 kW. Check your own manual — the exact percentages and any high-altitude jet kit are model specific.