What Size Generator Do I Need?
Quick answer
Essentials-only backup (fridge, furnace fan, lights, well pump): 7,500–9,500 running watts covers most homes. Whole-home with central A/C: typically an 18–26kW standby unit. The surge draw of motors, A/C, well pump, is what actually sets the size.Running watts vs starting watts
Every motor in your home draws a surge at startup, often 2–3× its running load. A well pump that runs at 1,000W may demand 3,000W for the second it starts; central A/C is the biggest ask of all. Generators must be sized to survive the worst-case simultaneous start, not the average hum, which is why honest sizing lists your motor loads first.
Quick sizing tiers
- Portable + interlock, essentials: 7,500–9,500 running watts, fridge, furnace, well, lights, internet
- Small standby (10–14kW): essentials plus comfort, careful with A/C
- Whole-home standby (18–26kW): everything including central air
- Large homes / multiple A/C units: 26kW+ or load-managed switching
The alternative worth pricing
For outages measured in hours rather than weeks, a battery (Powerwall) now competes head-on: silent, instant, fuel-free, and eligible for state and utility storage programs that generators don't get. Many homes land on a hybrid, battery for the frequent short outages, portable generator as the long-outage insurance. We'll price both honestly.
Build your number: a worked sizing
Walk the math once and the ranges make sense. Essentials for a typical home: refrigerator (700W running, 2,200W starting), furnace blower (800W/2,300W), well pump (1,000W/3,000W), lights and electronics (600W), sump pump (800W/2,000W). Running total: ~3,900W. The sizing constraint is the worst start: if the well pump kicks on while everything runs, you need ~3,100W running + 3,000W surge headroom, a 7,500W/9,500W-surge portable covers it with margin.
Add central A/C (3,500W running, 8,000W+ starting) and the equation changes class: now you're specifying an 18kW+ standby or a soft-starter to tame the surge. This is why 'what size' has no answer without your appliance list, and why we do the audit before quoting, not after.
The fuel-supply check people skip
A natural-gas standby is only as good as the gas line feeding it. Generators draw serious fuel, a 22kW unit wants ~280 cubic feet per hour at full load, and a meter and line sized for a furnace and range may not carry the addition. The fix (meter upgrade, line segment) is routine but must be in the plan, not discovered at commissioning.
Propane setups trade that question for tank math: the same 22kW unit burns roughly 3.5 gallons per hour at full load, so a 500-gallon tank buys about five days of heavy runtime. Sizing the tank to your outage expectations is part of honest system design.
Portable, standby, or battery: matching machine to outage profile
Sizing assumes you've picked the right category, so a quick decision frame. Outages at your address rare and short (a few hours, few times yearly): a battery covers them silently and adds daily rate-arbitrage value, the generator may be over-solution. Outages seasonal and long (storm country, days at a time): standby generator or generator-plus-battery hybrid earns its keep. Budget-constrained but resilience-minded: the portable-plus-interlock setup at $500–$1,500 delivers 80% of the protection for 15% of the standby price.
Your utility's reliability data (ask us, we know the local grids) plus your own outage memory sets the profile honestly. The wrong-category purchase is the most expensive sizing error of all.
Frequently asked questions
Not everything at once, but with an interlock and circuit discipline (skip A/C and dryer), it runs the essentials comfortably. That's the classic storm setup for most homes.
Plan for the A/C's starting surge: most 3–4 ton units want an 18–22kW standby (or a soft-start kit that lets a smaller unit cope). The nameplate LRA number decides it, bring it to your assessment.
No, an oversized generator costs more, burns more fuel, and runs inefficiently at low load. Sized-to-need with load management beats brute force.
Often yes, gas furnace blowers draw modestly (800–1,200W) and a 2,000W inverter generator with a proper transfer connection can keep heat alive. It's the minimalist winter setup, and vastly better than nothing in an ice storm.
It bogs, voltage sags (hard on motors and electronics), and breakers or protection shut it down. Chronic overloading shortens engine life significantly. The fix is management, not muscle: stagger heavy loads instead of buying the next size up.
Get it handled
Generator Installation
Standby generator installation keeps your home powered automatically during outages. Installed cost typically runs $5,000–$15,000 including the transfer switch.
View service →Generator Sizing Calculator
Find the right standby generator size for the circuits you want to back up.
Open calculator →Related questions
How Many Powerwalls Do I Need?
One Powerwall backs up essentials (fridge, lights, internet, a few outlets). Two covers most homes including short A/C use. Three or more delivers true whole-home backup with air conditioning and EV charging. Your consumption data gives the exact answer.
Read answer →Can a Powerwall Run My Air Conditioner?
Yes, the Powerwall 3's 11.5kW output starts and runs most central air conditioners, something older batteries struggled with. The real question is runtime: A/C draws 3–5kW, so plan capacity (or solar) for outages longer than a few hours.
Read answer →