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How to Size a Generator

Sizing a generator correctly is vitally important to ensure you can power all your essential equipment during an outage, while keeping your running costs and initial investment at an optimal level.

In order to get an accurate reading of what size generator you need, there are two standard methods:

Method 1: Professional Load Test

Have a qualified electrician perform a physical load test at your premises to measure your exact peak draw and quote on the installation simultaneously.

Method 2: Manual Calculation

Manually add up the wattage of all the various items you need to run off the generator during an outage.

Download Chart

First Time Buyer? What you should consider:

1. Size & Type
  • Phase: Do you require a Single-Phase or Three-Phase generator?
  • Capacity: What size generator do you need? Do you actually need to run everything at the same time, or just essentials?
2. Location & Environment
  • Altitude: Generators must be de-rated for altitude. For every 100m above sea-level, output drops by roughly 1%.
  • Emissions: Generators produce toxic fumes. Ensure the installation location complies with health, safety, and local by-laws.
  • Temperature: If operating at ambient temps consistently above 35°C, you may need to look at our heavier Industrial range.
  • Noise: Do you need it to be quiet? Our closed canopies are designed to minimize noise while maintaining optimal airflow.
  • Ventilation: If installed indoors (like a plant room), the exhaust must be extended outside, and aggressive ventilation is required. We DO NOT recommend garage installations.
3. Running Conditions & Cost
  • Optimal Load: Your generator should run at an average load of 70-80% for the best fuel consumption and engine longevity.
  • Underloading: Running the generator at very low loads (< 40%) can cause damage ("wet stacking") to the engine over time.
  • Overloading: Consistently turning on large appliances simultaneously can overload the alternator. Size appropriately for heavy startup spikes.
  • Motor Startup: Electric motors (pumps, compressors) can use up to 7x their running power just to start up. Identify your motor starting methods.
  • Fuel Costs: Running everything in your home uses more power, which directly translates to burning more diesel or gas.
4. After-care & Maintenance
  • Parts Availability: Are the parts readily available locally for your specific generator model?
  • Support Network: Does the supplier offer robust backup, servicing, and technical support after the sale? (Bundu Power does!)
Sizing Checklist
  • Have an Electrician perform a load test.
  • Determine essential vs non-essential equipment.
  • Get an installation quote simultaneously.
  • Decide if you need an Automatic Transfer Switch (ATS).
  • De-rate the generator calculations for altitude.
  • Find a suitable, safe, and ventilated location.
  • Try to install as close to your Distribution Board (DB) as possible to save on cable costs.