A whole house generator keeps everything running when the power goes out — but buying the wrong size wastes money. Too small and it can’t handle your loads. Too large and you overpaid for capacity you’ll never use.
Here’s how to figure out what you actually need.
Understanding Generator Sizing (kW)
Generators are rated in kilowatts (kW). Your home’s electrical loads determine what size you need. Here’s what common appliances draw:
| Appliance | Running Watts | Starting Watts |
|---|---|---|
| Central AC (3-ton) | 3,500 | 7,000 |
| Electric furnace blower | 500 | 1,000 |
| Refrigerator | 150 | 400 |
| Well pump (1/2 HP) | 1,000 | 2,000 |
| Sump pump (1/3 HP) | 800 | 1,300 |
| Electric water heater | 4,500 | 4,500 |
| Electric dryer | 5,000 | 6,000 |
| Lighting (whole house) | 1,000 | 1,000 |
| Home office (computer, monitor, router) | 500 | 500 |
Starting watts matter. Motors (AC, pumps, compressors) draw 2-3x their running watts when they kick on. Your generator needs to handle these surges.
Quick Sizing Guide
Small Home (under 1,500 sq ft)
10-14 kW — Covers essentials: heating/cooling, refrigerator, lights, well pump, a few outlets. Won’t run everything simultaneously but keeps the house livable.
Medium Home (1,500-3,000 sq ft)
16-22 kW — Most popular size. Covers HVAC, all kitchen appliances, lights, sump pump, and most daily-use circuits. The 20kW Generac Guardian is the most commonly installed unit in this range.
Large Home (3,000+ sq ft)
22-36 kW — Whole-house coverage including electric water heater, dryer, EV charger, pool equipment, and multiple HVAC zones. Some large homes with all-electric heating may need 36kW+.
Natural Gas vs. Propane
Standby generators run on either natural gas or propane (LP):
Natural Gas
- Unlimited fuel supply (utility line)
- No tank to refill
- Slightly lower output per unit vs. propane
- Best choice if you have a gas line
Propane (LP)
- Works anywhere (no gas line needed)
- Higher energy density = more power per gallon
- Requires a tank (250-500 gallon typical)
- Need to monitor fuel level and schedule refills
- Best for rural areas without natural gas
Massachusetts-Specific Considerations
- Nor’easters and ice storms can cause multi-day outages. Size your generator to run continuously for 3-5 days minimum.
- Permits required — Massachusetts requires electrical and gas permits for generator installation
- Setback requirements — generators must be placed specific distances from windows, doors, and property lines (varies by town)
- Transfer switch — required by code; automatically switches your home to generator power during outages
- Sound ordinances — some North Shore towns have noise restrictions; modern generators (60-70 dB) generally comply
Top Brands We Install
- Generac — market leader, widest range of sizes, excellent parts availability
- Kohler — premium build quality, quieter operation, commercial-grade engineering
- Briggs & Stratton — solid mid-range option, good value
Get Properly Sized
The best way to size a generator is a professional load calculation. Spencer’s generator installation service includes a free on-site assessment where we calculate your exact electrical loads and recommend the right unit.
Free estimates. Licensed electricians serving Peabody, Salem, Danvers, Beverly, and 30+ North Shore MA communities. Call (978) 293-5770 or book online.
