Sizing a backup generator is the process of calculating the total continuous (running) wattage of your essential loads and adding the highest single surge (starting) wattage to determine the minimum kW rating required to power your home or circuit without tripping breakers or stalling the alternator. When you get this calculation right, it changes your installation by ensuring clean voltage delivery during motor startups, preventing the generator's automatic voltage regulator (AVR) from failing under inductive spikes, and stopping your transfer switch from nuisance-tripping. If you undersize the unit, the engine will bog down, voltage will sag below 105V, and sensitive electronics like furnace control boards will brown out or fry.
The Core Math: Running vs. Starting Watts
To properly size a unit, you must separate your loads into two categories: resistive and inductive. Resistive loads (like incandescent lights, electric water heaters, and space heaters) draw the exact same amount of power the moment you flip the switch as they do while running. Inductive loads (anything with an electric motor, like air conditioners, well pumps, and refrigerators) require a massive spike of current to overcome initial inertia and magnetic field establishment.
Generators are rated by their continuous output, but they also have a surge capacity (usually lasting 10 to 30 seconds) provided by the alternator's flywheel effect and over-current windings. Below is a data-dense reference table for common household loads. Always check the specific nameplate data on your equipment, as older or less efficient models will draw significantly more current.
| Load / Appliance | Running Watts (Continuous) | Starting Watts (Surge) | Typical Breaker Size |
|---|---|---|---|
| Central AC (3-Ton, 14 SEER) | 3,500W | 10,500W | 40A (240V) |
| Well Pump (1 HP, Submersible) | 1,500W | 4,500W | 20A (240V) |
| Refrigerator (Modern, 22 cu ft) | 700W | 2,100W | 15A (120V) |
| Sump Pump (1/2 HP, Pedestal) | 1,050W | 2,150W | 15A (120V) |
| Electric Water Heater (40 gal) | 4,500W | 4,500W | 30A (240V) |
| Gas Furnace Blower (1/3 HP) | 800W | 2,350W | 15A (120V) |
For a deeper dive into motor starting currents and locked rotor amps (LRA), the U.S. Department of Energy's generator guidelines provide excellent baseline multipliers for various motor types and compressor configurations.
Worked Example: Sizing a Backup Generator for Essential Loads
Let's walk through a real-world scenario. You are sizing a backup generator for a 2,000 sq ft home with a gas furnace, municipal water (no well pump), and a 3-ton central AC. You want to back up the AC, the refrigerator, the sump pump, the furnace blower, the electric water heater, and a baseline lighting/outlet load of 1,500W.
Step 1: Sum the Running Watts
Add up the continuous wattage of every load that will be on simultaneously:
3,500W (AC) + 700W (Fridge) + 1,050W (Sump) + 800W (Furnace) + 4,500W (Water Heater) + 1,500W (Lights) = 12,050 Running Watts.
Step 2: Identify the Largest Starting Surge
Look at the starting watts column. The 3-ton Central AC has the highest surge at 10,500W.
Step 3: Apply the Sizing Formula
The correct formula for total required generator capacity is: Total Running Watts + (Largest Starting Watts - Running Watts of that same load). You only add the additional surge required by the largest motor, because the other motors will either already be running or will start sequentially via your transfer switch's load management system.
- Largest additional surge = 10,500W (Starting) - 3,500W (Running) = 7,000W
- Total Required Capacity = 12,050W (Running) + 7,000W (Additional Surge) = 19,050 Watts (19.05 kW)
Step 4: Select the Generator
You need a unit rated for at least 19.05 kW continuous. In the standby market, this pushes you out of the 18kW tier and into a 20kW air-cooled standby generator (like the Generac Guardian 20kW or Kohler 20RCL), which typically provides about 22kW of surge capacity to handle the AC compressor's locked rotor amps without tripping the internal alternator breaker.
Where You Meet This in Practice
You will confront these sizing realities physically when installing the Automatic Transfer Switch (ATS) and routing the feeder wires. If you are installing a 20kW generator on a home with a 200A main service panel, the physical connection point dictates your wire sizing and safety margins.
In practice, a 20kW generator produces roughly 83 amps at 240V. According to NEC Article 310 and standard ampacity tables, the feeder wires from the generator to a 100A-rated ATS must be sized for the breaker protecting them, not the generator's max output. You will typically pull 3 AWG copper THHN in conduit, or 2 AWG if using aluminum, to feed a 100A subpanel or a whole-house 100A service-rated ATS.
If your math from the worked example above exceeded 20kW, or if you wanted to downsize to a cheaper 14kW generator, you would meet this constraint by installing a Load Management System. Devices like Generac Smart Management Modules (SMMs) or Kohler Shed Kits use current transformers (CTs) clamped around the AC and water heater feeders. If the generator's CT detects the total current approaching the 14kW limit (approx 58A), the module physically drops the AC compressor contactor for 5 minutes, allowing the water heater to heat up, before rotating the priority. This lets you run a '20kW house' on a 14kW machine by ensuring the two largest inductive surges never overlap.
Common Confusions and Sizing Mistakes
When discussing generator sizing with suppliers or reading spec sheets, three major confusions routinely lead to improperly sized systems.
1. Confusing kW with kVA (Power Factor)
Many industrial and portable generators are rated in kVA (kilovolt-amps), while residential standby units are rated in kW (kilowatts). The relationship is defined by Power Factor (PF): kW = kVA × PF. Residential single-phase generators operate at a 1.0 Power Factor, meaning 20kW equals 20kVA. However, if you are buying a surplus 3-phase industrial generator with a 0.8 PF, a '25kVA' machine only provides 20kW of real, usable work power. Always verify the kW rating, not just the kVA sticker.
2. Sizing for the Main Breaker vs. Essential Loads
Homeowners often ask for a generator that matches their 200A main breaker. A 200A panel at 240V requires 48,000W (48kW) of continuous power—a massive, liquid-cooled commercial unit costing upwards of $25,000. In reality, no home draws 200A continuously. Sizing should always be based on the essential loads calculation (as shown in our worked example), not the physical size of the utility service entrance breaker.
3. Ignoring Total Harmonic Distortion (THD)
By separating your continuous loads from your inductive surges, applying the correct addition formula, and matching your transfer switch to the physical feeder limits, you can specify a backup power system that will reliably carry your home through a multi-day grid failure without stalling the engine or frying your appliances.
