So you've decided to invest in a generator to protect your home or business from long power outages. You have chosen whether it will be petrol, diesel or Inverter. Now comes the most critical question: "How big should it be?"
Many consumers make the following simple (and disastrous) mistake: They add up the Watts of the devices they want to turn on, find e.g. 2,000W, they go to the store and buy a 2,000W generator. When the power goes out, they put in front of the generator, turn on the refrigerator and... the generator "coughs" and shuts down due to overload.
As engineers, we see this scenario all the time. The mistake lies in ignorance of starting currents. Let's see how to do the right math so that the generator you buy will "lift" your equipment without collapsing.
During a power outage, the goal is not to throw a party by turning on all the air conditioners, the furnace, and the washing machine. The goal is survival and basic comfort. You need to make a list of "Absolutely Must Haves".
A typical emergency list for a home includes:
To calculate power, you need to understand the difference between two types of devices. Here's the secret to keeping your generator from shutting down:
These devices are "honest". As many Watts as they write on their nameplate, that's exactly how much they draw, both when they're turned on and when they're working. Examples: Lamps, toasters, electric oven, hair dryer, coffee machine. The Rule: Starting Power = Running Power. (If a stove is 1,000W, it needs 1,000W from the generator).
These are the devices that hide the trap. When an electric motor starts from standstill, it needs a huge, instantaneous "kick" of energy (Inrush Current) to overcome inertia, which lasts 1-2 seconds. After taking a break, the consumption drops to normal. Examples: Refrigerator, freezer, air conditioner (old type non-inverter), water pump (pressure), washing machine. The Rule: Starting Power = 2 to 3 times the Operating Power!
Let's take the list of absolute musts we made in Step 1 and do the math:
| Device | Load Type | Operating Power (Watts) | Starting Power (Watts) |
|---|---|---|---|
| 5 LED lamps | Ohmic | 50W | 50W |
| 55" TV | ohmic (electronic) | 150W | 150W |
| Router & Laptop | Ohmic | 100W | 100W |
| Refrigerator | Inductive | 250W | 750W (x3) |
| Radiator pump | Inductive | 150W | 450W (x3) |
| TOTALS | 700 Watts | 1,500 Watts |
How we read the table: If you go to buy an 800W generator because the "Total Duty" is 700W, by the time the refrigerator compressor kicks in, the system will momentarily demand over 1,000W. The 800W generator will not withstand the "kick" and will shut down. The generator must cover the Starting Total!
Small Engineering Tip: In practice, not all devices will go forward in the exact same millisecond. The safest formula is to take the Total Sum of Operating Powers (700W) and add to it ONLY the starting difference of the largest motor you have (ie the radiator: 750W - 250W = 500W). So, 700W + 500W = 1,200W. This is the absolute minimum number (Instantaneous Maximum Load).
Now that you've found that you need about 1,200 Watts (1.2 kW), you need to translate that into the "language" of generator manufacturers.
First, generators (as we wrote in a previous article) are measured in kVA (Apparent Power). The power factor (cosphi) for most single-phase generators is 1.0 (so 1 kVA = 1 kW), but for three-phase (and larger diesel) generators the factor is usually 0.8.
The formal conversion relationship is: S = fracPcosphi Where S is Apparent Power in kVA and P is Active Power in kW.
Second – and more importantly – no generator should work at 100% capacity continuously. If you buy a 1200W generator and "force" it to 1150W all the time, it will overheat, make terrible noise, burn too much fuel and break down quickly.
The Golden Rule (The Margin of Safety): Multiply the maximum load you find by 1.2 (or else, add 20% to 25% margin).
So, to meet the very basic needs of a small house with absolute safety and engine longevity, you should look in the market for a generator around 1.5 to 2.0 kVA.
1,200W x 1.2 = 1,440W.
Buying the right generator is a matter of math, not guesswork. Make your list, find the "sneaky" inductive loads (motors) and multiply their power by 3 to boot. Add a healthy margin of 20% to keep the engine from "hustling", and you have a system that will never let you down at the critical moment.
Next Step: You have purchased the generator. Now the big question: When the power goes out, will you go outside in the rain with your flashlight to pull the starter cord, or would you want it to start on its own? Continue to our technical guide: Automatic Transfer Switch (ATS): How the generator starts itself when the power goes out.
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