How to Size a Home Standby Generator
"What size generator do I need?" is the most common question every homeowner asks before buying a Generac, Kohler, or Briggs & Stratton standby unit. Get it wrong and you either overpay for capacity you'll never use, or buy a generator that trips offline the first time the AC and well pump kick on together. The calculator above gives you a fast answer; this guide explains the math behind it so you can sanity-check the result.
Step 1 — Add up your running watts
List every appliance you want powered during an outage and add up their running watts. Lights and outlets average 1,500-3,000W for a typical home. Add a refrigerator (~700W), microwave (~1,500W), and a few small loads. Then add the big ones:
- Central air conditioning: 3,500-5,000W running, with a 2-3× surge at startup.
- Heat pump: 4,000-6,000W running; in cold weather an electric backup heat strip can add another 5,000-10,000W.
- Electric furnace / electric heat: 7,000-15,000W is common.
- Well pump (½-1 HP): ~1,500-2,500W running, 4,000-6,000W at startup.
- Electric range: ~2,000W typical use, up to 8,000W with all burners and oven.
- Electric dryer: ~5,000W.
- Electric water heater: ~4,500W.
Step 2 — Apply a 25% surge factor
Motors (AC compressors, well pumps, refrigerators, furnace blowers) pull 2-3× their running wattage for a fraction of a second when they start. A generator that can't deliver that surge will brown out or shut down. The standard rule is to multiply your running total by 1.25 (25%) and round up to the nearest standard generator size.
Step 3 — Match to a standard generator size
Generac, Kohler, and Briggs & Stratton sell residential standby generators in standard sizes. These are the most common picks for West Virginia homes:
- 10-12 kW: Essential circuits only — lights, fridge, a few outlets. Good for cabins or homes without central AC.
- 14-16 kW: Most popular for small-to-medium homes with central AC and gas appliances.
- 18-20 kW: Whole-home coverage for typical 2,500-3,500 sq ft homes, even with a well pump and central AC.
- 22-24 kW: Whole-home coverage for larger or all-electric homes — runs electric range, dryer, water heater, and AC simultaneously.
- 26 kW+: Large homes (4,000+ sq ft), shops, or homes with multiple HVAC units. May need a load-shedding module.
Step 4 — Decide between whole-home and managed-load
A whole-home transfer switch backs up every circuit in your panel. A managed-load (load-shedding) module backs up everything but automatically sheds non-essential loads — like the dryer or water heater — when the AC kicks on, so you can use a smaller, less expensive generator. For most West Virginia homes a Generac 22kW with a whole-home switch is the simplest, no-compromise choice.
Why have a licensed electrician verify your size
Every calculator (including ours) is an estimate. Real homes vary — a tankless electric water heater pulls more than a tank model, a multi-stage heat pump cycles differently than a single-stage, and older homes may have undersized service. Before installation, BLS Electrical performs a full NEC-style load calculation on-site to confirm the right Generac, Kohler, or Briggs & Stratton size and to verify your existing 200-amp panel is up to the job.
Ready for a free on-site quote? BLS Electrical LLC (WV066166) is a licensed standby generator installer serving all of West Virginia — Charleston, Kanawha County, Fayetteville, Oak Hill, Beckley, Summersville, Mount Hope, Gauley Bridge, Ansted, Montgomery, and the surrounding Fayette, Nicholas, and Raleigh County areas. Most home installs are scheduled within about a week of the quote. Call (304) 640-5835 or visit our homepage to learn more about our generator installation, transfer switch, and panel upgrade services.