Diesel Generator Sizing Calculator
Accurately sizing your diesel generator is the most critical step in ensuring a reliable power supply for your home, business, or job site. An undersized generator can fail to start your equipment and may sustain damage from overloading. Conversely, a significantly oversized generator is inefficient, wastes fuel, and has a higher upfront cost. This calculator helps you determine the correct generator size based on your specific power requirements.
Understanding Generator Sizing Terms
Before using the calculator, it's important to understand the key terms involved in electrical load calculation:
- Running Watts (Continuous Watts): This is the amount of power an appliance or tool consumes to run continuously.
- Starting Watts (Surge Watts): Many appliances with electric motors (like refrigerators, air conditioners, and pumps) require a large burst of extra power for a few seconds to start up. This is known as the starting or surge wattage. It can be 2-3 times higher than the running wattage.
- Kilowatt (kW): A measure of "real power." It represents the power that actually performs work. 1 kW is equal to 1,000 Watts.
- kVA (kilo-volt-ampere): A measure of "apparent power." It is the total power supplied by the generator, including both real power (kW) and reactive power. Diesel generators are commonly rated in kVA.
- Power Factor (PF): The ratio of real power (kW) to apparent power (kVA). For sizing diesel generators, a standard Power Factor of 0.8 is typically used. The relationship is:
kW = kVA × 0.8.
How to Calculate Your Load
To use the calculator effectively, you need to determine two values from the equipment you intend to power:
- Total Running Watts: Add up the running watts for all the appliances and lights you plan to run simultaneously. You can usually find this information on the appliance's identification plate or in its user manual.
- Total Starting Watts: This is the most crucial figure. It's the peak load the generator must handle. To find this, first identify the appliance with the highest starting wattage. Then, calculate the total as: (Total Running Watts) + (Highest Starting Watts – That item's Running Watts). This represents the moment when all your items are running and the most demanding one starts up.
Example Calculation
Let's say you need to power the following items for a small workshop:
| Appliance | Running Watts | Starting Watts |
|---|---|---|
| Air Compressor (1 HP) | 1600 W | 4500 W |
| Work Lights | 1000 W | 1000 W |
| Grinder | 800 W | 1600 W |
1. Calculate Total Running Watts:
1600 W (Compressor) + 1000 W (Lights) + 800 W (Grinder) = 3400 W
2. Calculate Total Starting Watts:
The air compressor has the highest starting watts (4500 W).
(Total Running Watts) + (Highest Starting Watts – Its Running Watts) = 3400 + (4500 – 1600) = 3400 + 2900 = 6300 W
You would enter 3400 for "Total Running Watts" and 6300 for "Total Starting Watts" into the calculator.
Frequently Asked Questions (FAQ)
Why is a safety margin added?
We recommend a generator that is 20-25% larger than your calculated peak load. This safety buffer ensures the generator doesn't run at 100% capacity, which can reduce its lifespan and cause issues during unexpected power surges. It also provides capacity for future additions.
What is the difference between kW and kVA?
kW (Kilowatt) is the actual power an appliance uses, while kVA (kilo-volt-ampere) is the apparent power supplied by the generator. Because of the Power Factor, a generator must produce more kVA than the kW required by the load. For diesel generators, the rule of thumb is kVA = kW / 0.8.
Can I just add up all the starting watts?
No, because appliances do not all start at the exact same time. The correct method is to calculate the peak load that occurs when the most demanding item starts up while others are already running. Our calculation method reflects this standard practice.