Why Generator Sizing Matters
An undersized generator won't keep your home running during an outage. An oversized one wastes thousands of dollars upfront and burns more fuel than necessary. Getting the size right is the single most important decision in the generator buying process — and it's where many Hamilton County homeowners make costly mistakes.
This guide walks you through exactly how to size a standby generator for your home in Carmel, Fishers, Westfield, or anywhere in Hamilton County, Indiana.
Understanding Generator Size: kW Explained
Generator capacity is measured in kilowatts (kW). The higher the kW rating, the more electrical load the generator can handle simultaneously. Home standby generators typically range from 10kW to 26kW.
Here's a quick translation:
- 10kW — Powers essential circuits only (lights, fridge, sump pump, a few outlets)
- 14kW — Powers most of a mid-size home including one HVAC system
- 18–22kW — Whole-home coverage for most Hamilton County houses
- 24–26kW — Large homes with heavy electrical loads (multiple AC units, pool equipment, EV charger)
Step 1: List Every Appliance and System You Need to Power
Start by walking through your home and listing everything that needs electricity during an outage. For each item, note its running wattage and starting wattage (also called surge wattage — the extra power needed when a motor or compressor kicks on).
Here are the most common loads in Hamilton County homes:
- Central air conditioning (3-ton) — 3,500W running / 7,000W starting
- Central air conditioning (5-ton) — 5,000W running / 10,000W starting
- Gas furnace blower — 700W running / 1,400W starting
- Refrigerator — 150W running / 400W starting
- Sump pump (⅓ HP) — 800W running / 1,300W starting
- Well pump (½ HP) — 1,000W running / 2,000W starting (relevant for rural Cicero, north Westfield)
- Electric water heater — 4,500W running / 4,500W starting
- Gas water heater — 400W running / 400W starting
- Electric range/oven — 3,000W running / 3,000W starting
- Microwave — 1,000W running / 1,000W starting
- Washing machine — 500W running / 1,200W starting
- Clothes dryer (electric) — 5,400W running / 5,400W starting
- Clothes dryer (gas) — 700W running / 1,800W starting
- Lights (whole house LED) — 500–1,500W running
- Garage door opener — 550W running / 1,100W starting
- Home office setup — 300–800W running
- EV charger (Level 2) — 7,200W running / 7,200W starting
- Pool pump — 1,500W running / 3,000W starting
Step 2: Calculate Your Total Running Load
Add up the running wattage of everything you want powered simultaneously. Be realistic — you probably don't use the oven, dryer, and EV charger at the same time.
Example for a typical 2,500 sq ft Carmel home:
- Central AC (3.5 ton): 4,000W
- Gas furnace blower: 700W
- Refrigerator: 150W
- Sump pump: 800W
- Gas water heater: 400W
- Lights: 1,000W
- Microwave: 1,000W
- Home office: 500W
- Garage door: 550W
- Washing machine: 500W
- Gas dryer: 700W
Total running load: ~10,300W (10.3 kW)
Step 3: Account for Starting Surge
The biggest surge in most homes comes from the air conditioner compressor. When your AC kicks on, it briefly draws 2–3x its running wattage. Your generator must handle this surge without tripping.
For our example home, the AC surge adds about 4,000W momentarily, pushing peak demand to roughly 14,300W (14.3 kW).
Rule of thumb: Take your total running load and add the single largest surge item. That's your minimum generator size.
Step 4: Match to a Generator Size
Using our example (10.3 kW running, 14.3 kW peak), a 14kW generator would be tight — it would work but leave zero headroom. A 18kW generator provides comfortable margin and allows for adding loads.
Here's our recommended sizing guide for Hamilton County homes:
- Under 1,500 sq ft (apartment, condo, small home): 10–14 kW
- 1,500–2,500 sq ft (typical Fishers/Westfield home): 14–18 kW
- 2,500–3,500 sq ft (larger Carmel home, 2 AC zones): 18–22 kW
- 3,500–5,000 sq ft (Village of WestClay, Bridgewater estates): 22–24 kW
- 5,000+ sq ft (estate homes, Zionsville): 24–26 kW (may need commercial unit)
Special Sizing Considerations for Hamilton County
Indiana-specific factors that affect generator sizing:
- Dual HVAC systems — Many newer Carmel and Fishers homes have separate upstairs/downstairs AC systems. Two 3-ton units running simultaneously need ~7,000W running and ~14,000W at surge. This alone often pushes you to 22kW+.
- Sump pumps are essential — Hamilton County's high water table means most homes have at least one sump pump. Some have two. Never skip the sump pump in your load calculation — a flooded basement during a power outage is a disaster on top of a disaster.
- Electric vs. gas appliances — Homes with gas stoves, gas water heaters, and gas dryers need significantly less generator capacity than all-electric homes. Know what fuel your appliances use.
- Well pumps — Homes in rural areas (Cicero, northern Noblesville, east Hamilton County) on well water need to factor in well pump wattage and surge.
- EV chargers — Level 2 EV chargers draw 7,200W. If you want to charge your EV during an outage, add this to your total. Most families skip EV charging during outages to keep generator size manageable.
Whole-Home vs. Essential-Circuit Coverage
You have two approaches to generator sizing:
Whole-home coverage means the generator powers everything in your house — every outlet, every circuit. You won't notice the power is out (except for the generator running outside). This requires a larger, more expensive generator and uses more fuel.
Essential-circuit coverage means the generator powers only selected circuits through a load-management transfer switch. You choose what's critical (HVAC, fridge, sump pump, some lights and outlets) and the rest stays off. This allows a smaller, cheaper generator.
Most Hamilton County homeowners choosing Generac or Kohler generators go with whole-home coverage because the price difference between a 14kW and 22kW unit is only $1,500–$2,500 — a modest premium for full coverage.
The Professional Load Calculation
While this guide gives you a solid estimate, a professional load calculation is the gold standard. A licensed electrician will:
- Read your electrical panel to identify actual circuit loads
- Measure your AC compressor's locked rotor amps (LRA) for precise surge calculation
- Account for 240V vs 120V circuits
- Factor in future additions (hot tub, EV charger, pool)
- Provide a written recommendation you can compare across installer quotes
Most Hamilton County generator installers include a free load calculation as part of their quote process.
Common Sizing Mistakes to Avoid
- Ignoring AC surge. Your generator must handle the surge — not just the running load. This is the #1 reason undersized generators trip breakers.
- Forgetting the sump pump. Indiana storms mean rain + power outage = flooding risk. Always include sump pump load.
- Sizing for today only. If you're planning a hot tub, home addition, or EV charger in the next 5 years, size up now. It's far cheaper than replacing the generator later.
- Assuming all 22kW generators are the same. Different brands deliver their rated power differently. Generac's True Power technology provides cleaner power than some competitors at the same kW rating.
Get a Free Sizing Consultation
Not sure what size generator your Hamilton County home needs? Our vetted local installers provide free in-home assessments and load calculations. Tell us about your home and we'll connect you with licensed professionals who can size your system correctly.