Generator size refers to its maximum continuous electrical power output, measured in kilowatts (kW), which dictates how many appliances and circuits it can run simultaneously without overloading. When you select a specific kW rating, it directly changes the main breaker size, the automatic transfer switch (ATS) rating, and the feeder wire gauge required between the unit and your main panel. The most common confusion among DIYers and new homeowners is equating physical dimensions with electrical capacity, or mistakenly using 'peak/surge watts' instead of 'continuous running watts' to size their system.
The Core Concept: Electrical Capacity, Not Physical Footprint
When we discuss the sizes of generators, we are strictly talking about electrical output capacity, not the physical length, width, or weight of the enclosure. A physically compact 10kW inverter-generator can easily out-produce a massive, older 5kW open-frame contractor unit.
People commonly confuse surge watts (the 2-to-3 second burst of power needed to start an electric motor) with running watts (the continuous power needed to keep it spinning). Marketing materials for cheap portable units often highlight a massive '10,000 Peak Watt' number in bold text, while hiding the actual continuous running capacity (often just 7,500W) in the fine print. For permanent standby and hybrid inverter systems, you must size based on continuous kW, ensuring the alternator's inherent surge capacity handles the motor starts.
The Math: Running Watts vs. Locked Rotor Amps (LRA)
To understand why sizing matters, let's run a real numeric example based on a standard suburban home. You want to back up a 3-ton central air conditioner, a 1/2 HP well pump, and basic house loads (fridge, LED lights, Wi-Fi router).
- Basic House Loads: 3,000W continuous.
- Well Pump (1/2 HP, 240V): ~960W running. However, it requires a 2x surge to start = 1,920W surge.
- 3-Ton AC (36,000 BTU): The nameplate shows a Running Load Amps (RLA) of 15A. 15A × 240V = 3,600W running. But the Locked Rotor Amps (LRA) is 90A. 90A × 240V = 21,600W (21.6 kW) of instantaneous surge required just to spin the compressor from a dead stop.
Total Continuous Load: 3,000 + 960 + 3,600 = 7,560W (7.56 kW).
Total Surge Load: If the AC kicks on while the pump and house loads are running, you need 7,560W + 21,600W surge = 29,160W (29.16 kW) of surge capacity.
If you bought a portable generator rated for '10,000W peak', the AC compressor would lock out, brown out your lights, and trip the generator's main breaker. However, a modern 22kW liquid-cooled standby generator has a continuous rating of 22,000W, but its heavy-duty alternator can handle a 2-second surge of roughly 30,000W to clear that LRA hurdle without dropping the voltage below 216V.
Where You Meet This in Practice
Sizing the generator is only step one. The electrical infrastructure connecting it to your home must match the continuous ampacity of the unit. According to NEC Article 702 (Optional Standby Systems), the conductors and overcurrent protection must be sized for the full load or the generator's output, whichever is smaller.
Feeder Wire and Breaker Sizing
Let's look at a standard 22kW single-phase generator operating at 240V.
- Continuous Amps: 22,000W / 240V = 91.6A.
- NEC Sizing Rule: Feeders must be sized at 125% of the continuous load. 91.6A × 1.25 = 114.5A.
- Wire Selection: Per NEC Table 310.16 (75°C column), you must use 2 AWG Copper or 1/0 AWG Aluminum (THHN in conduit) to safely carry this load without exceeding temperature ratings.
- Breaker Selection: A 125A or 150A molded case breaker at the generator, stepping down to a 100A breaker at the transfer switch, depending on the specific ATS rating.
Integration with Solar and Hybrid Inverters
If you are integrating a generator into a 48V solar/battery system using a hybrid inverter (like a Sol-Ark 15K or Victron MultiPlus), generator sizing becomes critical. The generator must be sized to handle the inverter's maximum AC passthrough load plus the battery charging current simultaneously. Furthermore, the generator's continuous kW rating should be at least 1.2x to 1.5x the inverter's maximum charger input to prevent the inverter from rapidly cycling the generator contactor on and off due to voltage sag when heavy loads switch on.
Decision Tree: Picking Your Exact Generator Size
Use this decision path to terminate your search and pick a concrete unit. Do not guess; match your home's profile to the scenarios below.
| Home Profile & Load Requirements | Required Continuous kW | Concrete Pick (Model) |
|---|---|---|
| Essential Circuits Only: Fridge, freezer, sump pump, Wi-Fi, LED lights, gas furnace fan. No central AC, no electric water heater. | 10kW - 12kW | Generac 7178 (10kW Air-Cooled) or Kohler 12RESCL |
| Whole House (Gas Appliances): All above plus electric oven, microwave, well pump, and a small (1.5 ton) AC unit. Gas water heater and gas heat. | 16kW - 18kW | Kohler 16RESCL or Generac 7228 (18kW) |
| Whole House (Electric Heavy): All above plus 3-to-5 ton central AC, electric water heater, electric dryer, and EV charger (Level 2 managed). | 22kW - 24kW | Generac 7043 (22kW Liquid-Cooled) |
Common Sizing Mistakes and How to Avoid Them
Mistake 1: Confusing kVA with kW. Many industrial and larger residential generators are rated in kVA (kilovolt-amps). To find the actual usable kW, you must multiply by the power factor (usually 0.8 for single-phase and three-phase standby). A 25 kVA generator only provides 20 kW of real continuous power. Always verify the kW rating on the data plate.
Mistake 2: Ignoring Altitude Derating. Internal combustion engines lose power as air density drops. If you live above 5,000 feet, a naturally aspirated 20kW generator might only produce 16kW of actual power. You must either buy a larger unit, use a turbocharged model, or accept the derated capacity. Check the manufacturer's spec sheet for the exact derating curve (typically a 3% to 5% loss per 1,000 feet above 500 feet).
Mistake 3: Forgetting the 'Make-Up' Air. A 22kW generator consumes massive amounts of air for combustion and cooling. If you install it in an enclosed space or too close to a property line without proper clearance (usually 18 inches from the house, 60 inches from the front, per NFPA 37 and local codes), it will overheat and shut down on high-engine-temp alarms within 20 minutes of a summer outage.
Frequently Asked Questions About Generator Sizing
Can I just buy a massive 48kW generator to be safe?
No. Running a 48kW generator with only a 4kW continuous load causes 'wet stacking' (unburned fuel and carbon buildup in the exhaust) in diesel units, and severe glazing of the cylinder walls in natural gas engines. Generators need to run at a minimum of 30% to 50% of their rated capacity to maintain engine health. If your load is small, buy a smaller generator or implement a scheduled load-bank testing routine.
How do I size a generator for an off-grid inverter/charger system?
Size the generator based on the inverter's maximum AC input current, not just the house loads. If your Victron Quattro 48/10000 has a maximum AC input of 100A at 240V (24kW), your generator must be able to supply at least 24kW continuously to allow the inverter to simultaneously pass through AC power to the loads and use its 70A charger to refill the 48V battery bank. A 20kW generator will constantly trip the inverter's AC input breaker during heavy charging cycles.
Does adding a soft-start device change the generator size I need?
Yes. Installing a soft-start module (like a Micro-Air EasyStart) on your HVAC compressor can reduce the LRA surge requirement by 60% to 70%. This often allows homeowners to drop down from a 22kW generator to a 16kW or 18kW unit while still comfortably running a 4-ton or 5-ton air conditioner, saving thousands on the initial equipment and gas line installation.
