When a severe storm hits and the power grid goes down, a backup generator can be a lifesaver—keeping your food from spoiling, your house warm (or cool), and your essential medical devices running. However, buying a generator isn't as simple as picking one off the shelf. If you buy one that is too small, it will overload and shut down; if you buy one that is too large, you will waste money on the purchase price and excess fuel consumption.
To size a generator correctly, you must navigate a landscape of different power units: your appliances are rated in Watts, the generator's electrical output is usually rated in Kilowatts (kW), and the generator's physical engine might be rated in Horsepower (HP).
In this guide, we will walk you through the essential steps to size a generator for your needs and demonstrate how our Power Unit Converter can simplify the complex unit conversions involved.
Step 1: Running Watts vs. Starting Watts
The most critical concept in generator sizing is understanding the difference between Running (Continuous) Watts and Starting (Surge) Watts.
- Running Watts: The continuous power required to keep an appliance running. For example, a refrigerator might need 200 Watts to stay running.
- Starting Watts: Many appliances with electric motors (refrigerators, AC units, well pumps, sump pumps) require a massive, momentary surge of power just to start the motor. This surge lasts only a few seconds but can be 3 to 5 times higher than the running watts. That same refrigerator might require 1,200 Starting Watts to kick on.
If your generator can handle the running watts but cannot handle the starting watts, the appliance will fail to turn on, and the generator's breaker will trip.
Step 2: Calculate Your Total Power Needs
To size your generator, you need to perform an energy audit of the essential items you want to run simultaneously during a power outage.
Create a list and find the Wattage for each item. (Remember, if the label only shows Volts and Amps, multiply them together: Volts × Amps = Watts).
Example Essential Load List:
- Lights (5 LED bulbs): 50W Running / 50W Starting
- Laptop & Router: 100W Running / 100W Starting
- Refrigerator: 200W Running / 1,200W Starting
- Sump Pump (1/3 HP): 800W Running / 1,300W Starting
- Small Space Heater: 1,500W Running / 1,500W Starting
The Math:
- Total Running Watts: 50 + 100 + 200 + 800 + 1,500 = 2,650 Running Watts
- Total Starting Watts Needed: Take your total Running Watts (2,650) and add the single highest Starting Wattage requirement from your list. In this case, the Sump Pump needs an extra 500 Watts above its running wattage to start.
(2,650 + 500 = 3,150 Starting Watts)
You need a generator that can supply at least 2,650 Continuous Watts and handle a surge of 3,150 Starting Watts.
Step 3: Understanding Generator Ratings (kW vs. Watts)
Most portable generators are marketed based on their Starting Watts. For example, a "Generac 4000" likely means it provides 4,000 Starting Watts, but perhaps only 3,200 Running Watts. Always read the fine print.
Furthermore, standby home generators are usually rated in Kilowatts (kW).
Using the math from our example (3,150 peak watts), and knowing that 1 kW = 1,000 Watts, you would need a minimum of a 3.5 kW or 4 kW generator.
You can easily verify these conversions by plugging your total wattage into the Power Unit Converter to see the exact kW equivalent.
Step 4: The Engine Horsepower (HP) Connection
Sometimes, particularly with smaller portable or PTO (Power Take-Off) tractor generators, the marketing focuses on the engine's mechanical power, listed in Horsepower (HP). How does engine HP translate to electrical kW?
By definition, 1 Mechanical HP = 0.746 kW (746 Watts).
However, a generator's alternator is not 100% efficient at converting mechanical engine power into electrical power. There are heat and friction losses. A general rule of thumb in the industry is that it takes about 2 HP of engine power to produce 1 kW of electrical power under continuous heavy load.
So, if you are looking at a generator boasting a 10 HP engine:
- Mathematical conversion: 10 HP * 0.746 = 7.46 kW of mechanical power.
- Real-world electrical output: Due to efficiency losses, a 10 HP engine will typically be paired with an alternator that produces roughly 5 kW to 5.5 kW of continuous electrical power.
If you are dealing with metric horsepower (PS) on a European generator, the conversion is slightly different (1 PS = 0.735 kW). To avoid confusion, you can use our Power Unit Converter to instantly switch between Mechanical HP, Metric HP (PS), and kW.
Conclusion
Sizing a generator correctly is a vital safety and financial decision. By carefully adding up your Running and Starting Watts, adding a 20% safety margin so the engine doesn't run at maximum capacity constantly, and understanding how to read generator specifications in kW and HP, you can ensure your home stays powered through the worst storms. Whenever you are in doubt about the math, rely on our Power Unit Converter to translate the manufacturer's specs into numbers you can trust. A well-sized generator not only protects your family and property but operates more efficiently, consuming less fuel and requiring less maintenance over its lifespan.
