Generator sizing seems straightforward until a motor tries to start. A 5 HP well pump draws about 2,500 watts while running, so a 5,000-watt generator should handle it with room to spare. Except the moment that motor starts, it pulls 15,000 to 17,000 watts for 3 to 8 seconds. The generator sags, the voltage drops below 80%, and the motor stalls or the breaker trips. That scenario plays out every year during storm season when homeowners discover their "perfectly sized" generator cannot start their well pump or AC compressor.
The difference between running watts and starting watts is the single most important concept in generator sizing. Running watts represent the continuous load the generator can sustain. Starting watts represent the brief overload capacity, typically 10% to 25% above rated watts for conventional generators. Motor starting loads can exceed running watts by 5 to 7 times, which means the starting surge, not the running load, determines the minimum generator size. This guide covers the math, the NEC requirements, transfer switch considerations, and the real-world consequences of getting it wrong.
Running Watts vs Starting Watts
Every motor-driven load has two power demands: the running load (steady-state power consumption while operating) and the starting load (the surge of current required to accelerate the rotor from zero to full speed). The starting surge exists because a motor at rest looks like a near-short-circuit to the power source. The rotor has no back-EMF to oppose current flow, so the inrush current is limited only by the winding resistance, which is very low. As the motor accelerates, back-EMF builds and current drops to the normal running level.
For standard induction motors, the starting current is typically 5 to 7 times the full-load running current. A motor drawing 20 amps at full load will pull 100 to 140 amps during the first few seconds of startup. In watt terms, a 3 HP single-phase motor running at about 3,450 watts will require 17,000 to 24,000 starting watts depending on the load type and motor design. Capacitor-start motors (common in compressors and pumps) tend toward the higher end of this range.
The duration of the starting surge matters too. Most motors reach full speed in 2 to 8 seconds under normal load. Hard-to-start loads like reciprocating compressors, loaded conveyors, and submersible pumps can take 10 to 15 seconds because the load torque is high from the first revolution. During that entire startup window, the generator must supply the surge current without dropping voltage below the motor's minimum operating threshold, typically 80% of nominal (96V on a 120V circuit).
Resistive loads (heaters, incandescent lights, toasters) have no starting surge. Their running watts equal their starting watts. This is why a generator that runs six space heaters perfectly well cannot start a single well pump that draws fewer running watts than those heaters combined. The heaters never surge; the motor surges hard.
Starting Watts = Running Watts × LR Multiplier
Typical LR (Locked Rotor) Multipliers:
Refrigerator/freezer compressor: 5×
Well pump (submersible): 5–7×
AC compressor: 5–6×
Sump pump: 3–5×
Furnace blower: 3–4×
Garage door opener: 3–4×
The Correct Sizing Method
The correct way to size a generator is not to add up all the running watts. It is to identify the single largest starting load, add it to the running load of everything else that will be on at the same time, and use that total as your minimum generator requirement. The formula: Generator Size (watts) ≥ Total Running Watts of All Loads + Largest Single Motor Starting Surge − That Motor's Running Watts. The subtraction is because you already counted that motor's running watts in the total.
Example: You want to run a well pump (1,000W running, 6,000W starting), a refrigerator (200W running, 1,200W starting), a furnace blower (800W running, 2,400W starting), and lights/electronics (500W running, 500W starting). Total running watts: 2,500W. The largest starting surge is the well pump at 6,000W. Generator minimum = 2,500 + (6,000 − 1,000) = 7,500W. You need a 7,500-watt generator minimum, even though the continuous running load is only 2,500 watts.
This method assumes you are not starting two large motors simultaneously. If the well pump and AC compressor could start at the same time (both on thermostatic or pressure-switch control), you need to account for both starting surges simultaneously, which typically means a much larger generator or a load management system that sequences the starts. Most transfer switches with load management will start large motors one at a time with a 5 to 10 second delay between them.
Add a 20% to 25% safety margin to your calculated minimum. Generators lose capacity at high altitude (about 3.5% per 1,000 feet above 500 feet elevation) and at high ambient temperature (about 1% per 10°F above 77°F). A generator rated at 10,000 watts at sea level delivers about 8,600 watts at 5,000 feet elevation. Fuel quality, age, and maintenance condition also reduce actual output below nameplate ratings.
Generator Sizing Calculator
What size generator do you need? Add your appliances and loads to calculate total running watts and starting surge. Get a recommended generator size with built-in headroom.
NEC Sizing Rules for Generators
The National Electrical Code addresses generator sizing primarily in Article 445 (Generators) and Article 702 (Optional Standby Systems). For legally required standby systems (hospitals, fire pumps), Article 701 applies with stricter requirements. The NEC does not tell you exactly what size generator to buy, but it sets minimum standards for the electrical installation that effectively constrain the sizing decision.
NEC 445.13 requires that the generator nameplate include the rated power output in watts or kVA, the power factor, the voltage, the frequency, and the number of phases. When sizing a generator for a known load, the generator's continuous rating must equal or exceed the calculated load per Article 220 load calculations. For motor loads, NEC 430.24 requires that the largest motor in the circuit be calculated at 125% of its full-load current, which adds a built-in margin for motor starting.
For optional standby systems, NEC 702.4 requires a transfer switch or other means to prevent backfeed to the utility. The transfer switch must be rated for the available fault current and the load to be transferred. This means the transfer switch rating often sets a practical ceiling on the generator size: a 200-amp transfer switch pairs with generators up to about 48 kW (single-phase) or 60 kW (three-phase).
One frequently overlooked NEC requirement is 702.5, which states that the optional standby system must have adequate capacity for all equipment connected to it. You cannot connect a 30,000-watt load panel to a 7,500-watt generator and call it compliant. Either the generator must be sized for the full connected load, or you must use a load management system or sub-panel that limits the connected load to the generator's capacity. Many inspectors require a load calculation worksheet as part of the permit documentation.
Transfer Switch Requirements and Backfeed Safety
A transfer switch disconnects your building from the utility grid before connecting it to the generator. Without a transfer switch, generator power can backfeed through the utility transformer, step up to distribution voltage (typically 7,200V or 14,400V), and energize the power lines that utility crews believe are de-energized. Backfeed kills linemen. It is the single most dangerous mistake in generator installation, and it is a criminal offense in most jurisdictions.
There are three types of transfer switches: manual, automatic (ATS), and interlock kits. Manual transfer switches require someone to physically flip the switch when the power goes out. Automatic transfer switches detect the outage, start the generator, wait for it to stabilize, and transfer the load automatically within 10 to 30 seconds. Interlock kits are mechanical devices that retrofit onto the main breaker panel, using a sliding plate to ensure the main breaker and generator breaker cannot both be on simultaneously.
The transfer switch must be rated for the generator's output current and the available fault current at the installation point. Residential transfer switches are typically rated for 100, 200, or 400 amps and 10,000 to 22,000 amps of fault current. The fault current rating must meet or exceed the available fault current from the utility. Under-rated fault current capacity means the transfer switch may not safely interrupt a fault, creating a fire or explosion hazard.
For portable generators connected through an inlet box and interlock kit, the inlet box must be a listed device with a flanged inlet that prevents accidental contact with energized pins. Extension cords run through windows to individual appliances bypass all safety mechanisms and violate the NEC. The interlock kit or transfer switch eliminates backfeed risk by making it mechanically impossible.
What Happens When You Undersize
An undersized generator does not simply run slower. It fails in ways that damage both the generator and the connected equipment. When a motor starting surge exceeds the generator's capacity, the engine bogs down, the frequency drops below 60 Hz, and the voltage sags. If the voltage drops below about 80% of nominal, motors stall mid-start, drawing locked-rotor current continuously. This prolonged high current overheats the motor windings and can trip the generator's overload protection.
Voltage sag also damages sensitive electronics. Modern refrigerators, furnaces, and HVAC systems have electronic control boards that malfunction or fail when voltage drops below their minimum operating range. A $15,000 standby generator that cannot start the AC compressor cleanly can destroy a $500 control board with repeated low-voltage starts.
Chronic overloading shortens generator life dramatically. A generator running continuously at 95% load will consume more fuel, run hotter, and wear out faster than the same generator at 75% load. The sweet spot for generator longevity is 50% to 75% of rated capacity for continuous operation. This is another reason the 20-25% safety margin is not optional.
The financial penalty of undersizing is also real. A generator that cannot handle the starting loads forces the owner into manual load management: turning off the AC before starting the well pump, waiting for the refrigerator to cycle off before running the microwave. Spending an extra $500 to $2,000 for the next size up is almost always cheaper than dealing with the consequences of undersizing.