How to size a generator: rated watts, starting loads and inverters

Buying a generator that is too small means tripped overloads and tools that won’t start. Buying one that is far too big means carrying, fuelling and listening to more engine than you need. The makers give you the numbers to get it right — you just need to know which ones to use.

Two power figures: maximum and rated

Almost every generator spec sheet has two output figures:

• Honda EU22i: 2,200 W maximum, 1,800 W rated.
• Draper Expert 87059 (open frame): 2,200 W maximum (2.75 kVA), 2,000 W rated (2.2 kVA).
• Draper 95197 (inverter): 2,500 W maximum, 2,300 W rated.

The model name often uses the bigger number. Plan your everyday load around the rated figure and treat the maximum as headroom, not as something to run at for hours.

Step 1: add up what you’ll run at the same time

Look at the rating plate or manual of each thing you want to power and note its wattage. Honda’s advice on its EU22i page is to check the actual product rating of whatever you plan to power rather than relying on typical figures. Add up only the things that will run at the same time — a light and a charger and a drill, say — not everything you own.

Step 2: allow for starting loads

Honda warns that some appliances require much more power at start-up than when they are running. Tools and machines with motors are the usual culprits. Honda describes its intelligent AVR (i-AVR) generators as giving high start-up load performance and as well suited to welders and compressors.

The best source is the tool’s own manual. If it gives a minimum generator size, use that; if it doesn’t, ask the maker before you buy. Don’t assume that a 2,000 W tool will start on a generator rated at 2,000 W.

Step 3: choose the type of output

Honda’s page on power outputs describes several ways of regulating a generator’s voltage:

• Condenser / inductive: simple and economical. Honda says they are best for resistive loads, giving circular saws, angle grinders and hammer drills as examples.
• AVR (automatic voltage regulator): better voltage and frequency stability, with fewer spikes and dips.
• Digital AVR and i-AVR: smoother again; Honda describes i-AVR as giving high start-up load performance.
• Inverter: Honda says inverter generators produce electricity as good as that in your home, making them the choice for sensitive electronics, and that they are more fuel-efficient and quieter.

All three Honda models in this guide — the EU10i, EU22i and EU32i — are inverters, as is the Draper 95197. The Draper Expert 87059 is an open-frame generator; Draper’s page lists low-oil shutdown as its protection feature but doesn’t mention an AVR, so check with the seller if you need one.

Step 4: check the sockets

Sockets matter as much as watts:

• The Draper Expert 87059 has one 110 V 16 A socket and one 230 V 16 A socket, plus 12 V DC for charging — useful on building sites that use 110 V tools.
• The Draper 95197 inverter has two 230 V sockets and a 12 V DC outlet.

Check the plug on every tool you intend to use against the generator’s sockets.

Step 5: run time and fuel

Makers quote run time at a stated load, so read the small print:

• Draper Expert 87059: a 3.6 litre tank and about 4.2 hours at 75% load.
• Draper 95197: a 7.5 litre tank and 5 hours at 100% load.

More fuel on hand means more petrol to store safely — see storing petrol at home.

Step 6: noise and weight

Draper gives 95 dB(A) at 4 m for the open-frame 87059 and 62 dB(A) at 7 m for the 95197 inverter. Because the distances differ, the two figures can’t be compared directly — always check the distance a maker measured at. Honda says the EU22i’s casing and acoustic panelling greatly reduce operating noise.

Weight decides whether you will actually carry it: Honda lists the EU10i at 13 kg, the EU22i at about 21 kg and the EU32i at 26.5 kg, while the two Draper machines weigh 33 kg and 35.5 kg.

A worked example (using only the makers’ numbers)

Say you want to run site lights and charge batteries, with the occasional power tool. Add up the wattages from the rating plates. If the total, plus the start-up need the tool’s manual gives, sits comfortably inside a generator’s rated output, that generator is in the right range. If the tool’s manual recommends a bigger generator, believe the manual.

Safety first

However you size it, a generator only ever runs outdoors, at least a metre from buildings, and never connected to house wiring without an isolation switch fitted by a qualified electrician (Honda’s rules). Read generators and carbon monoxide before first use.