Choosing a Backup Generator: Petrol vs Diesel and Sizing It Right

July 6, 2026

A backup generator is one of those purchases you only really appreciate the first time the power goes out. Getting the fuel type and size right up front saves a lot of grief later — here's how to think it through.

Petrol vs diesel: what actually matters

Petrol generators are generally cheaper upfront and lighter, which suits occasional home backup or worksite use where portability matters. Diesel generators cost more initially but are more fuel-efficient over long running hours, handle continuous heavy loads better, and diesel fuel stores for longer without degrading — a real advantage if it's sitting unused for months waiting for an outage.

Sizing: rated vs peak power

Every generator lists two numbers: peak (surge) and rated (continuous) power. Size against rated power for anything you'll run continuously, but check peak power covers the startup surge of motors — fridges, pumps, and air conditioners can draw 2–3x their running wattage for a second or two on startup.

A rough guide for home backup

Essentials only (fridge, some lights, phone charging): a 3–4kW rated petrol unit is usually enough.
Whole small household (fridge, lights, some power circuits): 5–6kW rated, petrol or diesel.
Worksite or heavier home use (power tools, larger appliances, 3-phase gear): 6kW+ diesel, ideally with electric start.

Don't skip the safety features

Look for an RCD safety switch and electric key start if you're buying for home backup — you don't want to be pull-starting a generator in the dark during a storm.