Choosing the right size standby generator comes down to one core question: how much electrical load do you want to run during an outage, and how much extra capacity do you need for appliances that draw a surge of power when they start? Getting this calculation right prevents two expensive mistakes — buying an undersized unit that trips or struggles under load, or overspending on capacity you will never use.
Running Watts vs. Starting Watts
Every appliance has two power figures that matter for sizing. Running watts (also called rated watts) is the continuous power an appliance draws once it’s operating normally. Starting watts (or surge watts) is the extra burst of power certain appliances need for a moment when they first switch on, which can be two to three times higher than their running wattage. Motors are the main culprit: air conditioners, well pumps, refrigerators, and sump pumps all have an electric motor that needs a jolt of extra current to overcome initial resistance and start spinning.
A generator has to be sized not just for the sum of your running watts, but for a scenario where multiple appliances’ starting surges might overlap. This is why simply adding up the “typical wattage” numbers printed in appliance manuals is not enough on its own — you need to account for the highest single starting-watt spike layered on top of everything else already running.
Step-by-Step: Calculating Your Generator Size
- List every circuit or appliance you want to power during an outage: this typically includes HVAC, refrigerator, well pump or sump pump, lighting circuits, a home office setup, and any medical equipment.
- Find the running wattage for each item: check the appliance’s nameplate, owner’s manual, or manufacturer specification sheet for its rated wattage or amperage (watts = volts x amps for single-phase loads).
- Identify which items have a motor or compressor: air conditioners, well pumps, sump pumps, refrigerators, and furnace blowers are the most common motor-driven loads in a home.
- Find the starting wattage for those motor-driven items: this is often listed separately from running watts on the nameplate or spec sheet, sometimes labeled “LRA” (locked rotor amps) for HVAC equipment.
- Add up all running watts for everything you want to run at once: this gives you your baseline continuous load.
- Add the single highest starting-watt figure on top of that baseline: generators only need to cover one worst-case simultaneous startup, not every motor starting at the exact same instant, since that scenario is extremely unlikely in normal household use.
- Add a safety margin of roughly 10 to 20 percent: this accounts for future appliance additions, voltage fluctuations, and manufacturer derating in extreme temperatures.
- Compare your total to generator output ratings: standby generators are sold by kilowatt (kW) output; divide your total wattage by 1,000 to get the kW figure you need, then round up to the nearest standard model size.
Whole-House Sizing vs. Essential Circuits Sizing
Homeowners generally choose between two sizing philosophies. A whole-house approach sizes the generator to run literally everything in the home simultaneously, including central air conditioning at full load, which typically requires a larger unit, often in the 20 to 26 kW range or higher for larger homes with multiple HVAC zones. An essential-circuits approach sizes the generator for a curated list of priority loads, such as refrigeration, well pump, some lighting, and a furnace blower, which can often be covered by a smaller unit in the 10 to 16 kW range, especially when paired with a load-management device that prevents overload by staggering high-draw appliances.
Neither approach is inherently right or wrong; it is a tradeoff between equipment cost and how much of your normal routine you want to maintain during an extended outage.
Common Sizing Mistakes to Avoid
A frequent error is sizing based only on square footage rather than actual electrical load, since two homes of the same size can have very different HVAC systems, appliance counts, and electrical habits. Another common mistake is ignoring the well pump’s starting watts entirely, which can be a nasty surprise on properties with private wells, since well pump motors often have a substantial starting surge relative to their small running wattage. Homeowners also sometimes underestimate future needs, sizing exactly for today’s appliances without headroom for an added home office, an electric vehicle charger, or a future addition to the house.
Why a Professional Load Calculation Still Matters
Online wattage calculators and manufacturer sizing charts are useful for a ballpark estimate, but a licensed electrician or generator dealer will typically perform a more precise load calculation during the site survey, sometimes using a clamp meter to measure your home’s actual current draw under real conditions. This professional step catches nuances a simple checklist can miss, such as how your specific HVAC system’s compressor behaves on startup, or whether your electrical panel’s total capacity itself becomes a limiting factor. Treat your own calculation as a way to have an informed conversation with your installer, not as the final word on the model you purchase.
Key Takeaways
- Generator sizing requires adding running watts for everything you want powered, plus the single highest appliance starting-watt spike.
- Motor-driven appliances like well pumps, HVAC compressors, and sump pumps have starting watts far higher than their running watts.
- Whole-house sizing typically requires a larger generator than an essential-circuits approach paired with load management.
- Building in a 10 to 20 percent safety margin accounts for future appliances and temperature-related derating.
- A professional load calculation during your site survey is more precise than any online calculator and should guide your final purchase decision.
Frequently Asked Questions
How many watts does a typical central air conditioner need?
It varies significantly by system size and efficiency, and both running and starting watts should be pulled directly from your unit’s nameplate or manufacturer specifications rather than assumed, since central AC systems vary widely in electrical draw.
Can I use a smaller generator if I promise not to run everything at once?
Yes, this is essentially how essential-circuits sizing works, often combined with a load-management device that automatically prevents non-priority appliances from drawing power when a high-demand appliance is running.
Do LED lights significantly affect my sizing calculation?
Not usually. LED lighting draws very little power compared to HVAC, water heating, or motor-driven appliances, so it rarely has a meaningful impact on overall generator sizing unless you have an unusually large number of high-wattage fixtures.
What happens if I undersize my generator?
An undersized generator can trip its own overload protection, stall under a large appliance startup surge, or run appliances inefficiently and put excess wear on the engine. This is one of the most common and preventable installation mistakes.
Should I size for future additions like an EV charger?
If you are reasonably likely to add a major appliance such as an EV charger within the generator’s expected service life, it is worth discussing that possibility with your installer during sizing, since upsizing later typically means replacing the unit rather than expanding it.