How to Calculate Starting Watts When Hiring a Generator
G’day! If you’ve spent any time on a job site or planning a massive backyard event, you’ve probably heard the dreaded sound of a generator cutting out the moment someone hits the "on" switch for a circular saw or an air compressor. It’s a rookie mistake that happens at every hire desk, but after 12 years behind the counter, I’m here to make sure it doesn't happen to you.

The secret isn’t just picking the biggest unit in the shed; it’s understanding the difference between starting vs running watts. https://shedblog.com.au/what-to-consider-when-hiring-a-generator/ If you get this wrong, you’re looking at a stalled project, tripped breakers, and potentially damaged equipment. Let’s dive into how to size your hire correctly so you can avoid generator overload and get the job done right.
The Golden Rule: Starting vs. Running Watts
Most folks look at the power rating of a tool and think, "That’s it, that’s all I need." Not so fast. To calculate your needs, you have to account for two different types of power draw:
Running Watts (Rated Watts): This is the continuous power your device needs to keep humming along once it’s already up and running. Starting Watts (Surge Watts): Many motors—especially those in pumps, saws, and refrigeration—require a massive "kick" of power to get moving from a dead stop. This surge can be 2 to 3 times the running wattage.
If you hire a generator that only covers the running watts, the moment you squeeze that trigger, the generator will choke. You need to ensure your generator can handle the surge without breaking a sweat.
The Sizing Math: How to Calculate Your Load
Before you commit to a hire, you need to do a quick audit of what you're plugging in. I always tell my customers to list their gear and use a calculator. You need a generator sizing buffer to ensure you aren't running the unit at 100% capacity—that’s a recipe for burnout.
Here is a quick reference table to help you estimate:
Appliance/Tool Running Watts (Approx) Starting Watts (Approx) Circular Saw 1400 2300 Portable Air Compressor 1500 4500 Fridge/Freezer 700 2200 Halogen Work Light 500 500 Bench Grinder 1000 2500
The "Buffer" Formula
To avoid generator overload, never size your generator to the exact limit. A healthy generator sizing buffer is usually 25-30%. If your total starting load is 5,000 watts, aim for a generator that provides at least 6,500 watts of surge power. This keeps the engine cool, the fuel consumption stable, and your gear safe from voltage fluctuations.
Fuel Choice and Run Time Planning
When you're looking at a place like Wenbro Hire, you’ll see options for petrol, diesel, and even dual-fuel units. Your choice depends on the site:
Petrol: Great for portability and lighter-duty work. Easy to refuel, but they burn through fuel faster under heavy loads. Diesel: The workhorse of the construction world. Better fuel efficiency over long hours and generally more durable for site-wide power distribution. Run Time: Always check the fuel tank capacity against your expected daily usage. Don't be the person having to stop work every three hours to refuel. If you're running lights for a night event or power tools for a full shift, opt for a unit with an "extended run" fuel tank.
Noise and Local Considerations
There’s nothing worse than getting a complaint from the neighbors or council because your generator sounds like a jet engine. In Australia, we have to keep an eye on environmental regulations. You should always check the guidelines provided by the Australian Government Department of Climate Change, Energy, the Environment and Water (dcceew.gov.au) regarding emissions and noise ordinances, especially in residential zones.
If you’re working in a built-up area, consider an "Inverter" generator. They are significantly quieter, produce cleaner power (safer for laptops and sensitive electronics), and throttle their engine down when the demand is low. It’s the "pro move" for residential renovations.
Expert Tips for a Stress-Free Hire
Having worked the counter for 12 years, here’s my pro-advice for your next visit to the hire desk:
The "One-at-a-Time" Rule: Even if you have a big unit, don't start every high-draw tool at the same time. Start the biggest one first, let it reach full speed, then start the others. Use Good Extension Leads: Cheap, thin leads cause voltage drop. This makes your tools work harder, which forces the generator to work harder, which leads back to—you guessed it—overload. Use heavy-duty, rated outdoor leads. Check the Connections: Ensure the socket type matches your site requirements (10A vs 15A plugs).
Need Help Deciding?
If you're still scratching your head, don't just guess. Most reputable hire shops use tools like Chatlio to provide instant support to their customers. If you see a chat icon on a hire website, use it! Tell them exactly what tools you have, and they can check the specs for you in seconds. It’s better to ask a "silly" question now than to drag a heavy generator back to the shop because it kept tripping.
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