The Goldilocks guide to getting your generator size just right
- Greenplant
- 7 days ago
- 4 min read
Before you hire or buy a generator, pretend you are Goldilocks (wig optional). A generator that is too small will trip frequently because it cannot handle the load. On the other hand, a generator that is too large will run inefficiently, wasting fuel and possibly causing unnecessary wear. You're looking for a generator that is just the right size. Our guide below explains how to estimate the size of the generator you need and why it matters.
Why getting the right generator size matters
Choosing the right generator size affects safety, efficiency and cost and also has an impact on the lifespan of the generator itself. Here’s why:
A generator that is too small will overload and trip the circuit breaker, causing power interruptions and potential damage to your appliances.
Oversized generators consume more fuel and run less efficiently because they operate below their optimal load.
Larger generators cost more to hire or buy and have higher operating expenses.
Running a generator at an improper load can shorten its lifespan.
Getting the size right means your generator will provide steady power without interruptions, save money on fuel, and last longer.
How to estimate your power needs
To find the right generator size, you need to calculate the total wattage of the devices you plan to power. Here’s a step-by-step method:
1. List Your Appliances and Devices
Write down every item you want to run on the generator. Include:
Appliances
Air conditioners
Lights
Power tools
Electronics (TV, computer, chargers)
Water pumps
Anything else that will draw power
2. Find the Wattage of Each Device
Check the label or manual for the wattage. If only amps are listed, multiply amps by volts (usually 230V in homes) to get watts.
Example:
A device rated at 5 amps on 230 volts uses 5 × 230 = 1150 watts.
3. Account for Starting Watts
Some devices, especially motors and compressors, require extra power to start. This is called starting or surge wattage. It can be 2 to 3 times the running wattage.
Example:
A refrigerator might run at 700 watts but require 2100 watts to start.
4. Add Running and Starting Watts
Add the running watts of all devices you want to run simultaneously. Then add the highest starting wattage of any device to the total running watts.
Example:
Total running watts: 3000
Highest starting watts: 2100
Generator size needed: 3000 + 2100 = 5100 watts
5. Add a Safety Margin
Add 10-20% extra capacity to handle unexpected loads or future additions.
Example:
5100 watts + 20% = 6120 watts
You would need a generator rated for at least 6,000 watts.
Common Generator Sizes and Their Uses
Here are some typical generator sizes and what kinds of equipment they can power:
1.3 to 2.7kVA generators (1000 to 2200 watts): Small appliances, lights, phone chargers, laptops.
3.4 to 6kVA generators (2700 to 5000 watts): Refrigerators, sump pumps, power tools, small air conditioners.
7.5 to 10kVA generators (6000 to 8000 watts): Multiple appliances, larger air conditioners, well pumps.
12kVA+ generators (10,000 watts and above): Whole-house backup, heavy equipment, multiple large appliances.
Common Construction Site Equipment Power Usage
These examples of construction site equipment are to give you a rough idea of what size generator you might need. Please check the actual wattage for the specific tool you are using as this can vary between brands and models.
Handheld Power Tools
These smaller tools are commonly plugged into mobile, open-frame petrol generators or small site distribution boxes.
Tool | Running Wattage | Starting Wattage |
|---|---|---|
800W to 1200W | 1500W to 2000W | |
1200W to 1600W | 2400W | |
1000W to 2000W | 3000W to 4000W |
Mid-Sized Construction Equipment
These machines rely on electric motors to move heavy materials or compress air, leading to high starting surges.
Machine | Running Wattage | Starting Wattage |
1000W to 1500W | 2000W to 3000W | |
1800W to 2000W | 3500W to 4500W | |
1500W | 3000W to 4000W |
Heavy-Duty Structural Machinery
Large-scale operations require industrial diesel generators (often rated above 15kVA) to manage these continuous, high-draw loads.
Machine | Running Wattage | Starting Wattage |
3000W to 7000W | depends on arc strike intensity | |
Large Site Compressor | 4000W to 15,000W | up to 30,000W |
Tower Crane (Small/Medium) | 20,000W to 50,000 | up to 100,000W |
Site Support & Infrastructure
Generators also power the critical support systems that keep a site safe and functional.
Equipment | Running Wattage | Starting Wattage |
200W to 1000W | (no surge required) | |
Mobile Welfare Cabin / Office Hub (Powers space heaters, kettles, microwaves, and charging banks) | 5000W to 10,000W |
Tips for Hiring a Generator
Know your power needs before renting.
Ask about fuel type: petrol, diesel, or propane.
Check rental terms for duration and delivery.
Inspect the generator for condition and fuel level.
Understand operation instructions to avoid damage.
Common Mistakes to Avoid
Choosing a generator based on price alone
Ignoring starting watts
Underestimating future power needs
Not considering fuel availability
Forgetting to include safety margin
Follow these tips and you'll find the generator that's just right for your needs.
Have a look at our generators available to hire, and let us know how we can help!
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