What Size Generator Do I Need? Complete Sizing Guide & Calculator (2025)

So you're staring at generators online or in the store, completely overwhelmed by the numbers. 5,000 watts? 10,000? 22,000? Feels like they're speaking another language, doesn't it? Been there. Last year when that ice storm knocked out power for three days, I grabbed what I thought was a beefy generator only to discover it couldn't handle my fridge AND furnace kicking on simultaneously. Total rookie move. That frustration is exactly why figuring out what size generator you truly need matters so much.

It's not just about lights on. Picking the wrong size means wasted money upfront, wasted fuel later, or worse - fried appliances. Too small and you're playing musical plugs during an outage. Too big and you're burning cash for power you never use. Let's cut through the confusion and nail this down step-by-step. No engineering degree required, promise.

Why Generator Size Matters More Than You Think

Think of your generator like shoes. You wouldn't wear clown shoes to run a marathon or ballet slippers to hike a mountain. Wrong size equals discomfort or disaster. With generators, undersizing causes constant tripping and potential equipment damage from "brownouts" (low voltage). Oversizing guzzles fuel inefficiently and costs way more upfront.

Here's the kicker most folks miss: Appliances need a huge surge of power when they first switch on (starting watts), often 2-3 times their normal running wattage. Your refrigerator might run on 700 watts but needs 2,200 watts just to start its compressor. If your generator can't handle that startup surge, the fridge won't run at all. That crucial difference between starting watts and running watts is where most sizing mistakes happen.

The Core Question: What Exactly Are You Powering?

This is where the rubber meets the road. Are you prepping for:

Essential Items Only: Fridge, phone charging, a few lights, maybe a fan? (Think short outages, camping)
Partial Home Backup: Add furnace blower, sump pump, well pump, microwave? (Multi-day outages, colder climates)
Whole House Coverage: Central AC, electric stove, water heater? (Major storms, frequent/long outages)

Your answer dramatically shifts the what size generator do I need equation. Let's break down each scenario.

Calculating Your Power Needs: No Math Ph.D. Required

Forget complex formulas. Grab a notepad and flashlight (ironic, I know). We'll build your power list room by room. Need help finding wattage? Look for:

The yellow EnergyGuide label on appliances
A metal plate or sticker on the item itself (often near the power cord)
The owner's manual (usually online if you lost yours)

Don't guess. Seriously. Guessing leads to cold showers or fried circuit boards.

Appliance/Device	Typical Running Watts	Typical Starting Watts	Notes
Refrigerator (18 cu ft)	700	2,200	Starting surge is critical!
Freezer (15 cu ft)	600	2,000	Similar surge issue
Furnace Blower (Gas Furnace)	800	2,300	Essential for winter outages
Sump Pump (1/2 HP)	800	2,300	Prevents basement flooding
Well Pump (1/2 HP)	1,000	2,500	No power = No water
Portable Space Heater	1,500	1,500	No surge, but high constant draw
Central AC (2.5 Ton)	3,500	10,500	Massive surge - needs big gen!
Microwave	1,000	1,000	No significant surge
LED Light Bulb	10	10	Negligible
Laptop	50-100	50-100	Low draw
TV (55" LED)	100	100	Low draw

Your Step-by-Step Power Audit

List Essentials: Write down everything absolutely critical during an outage (fridge, furnace, minimal lights, phone charger).
Add Comfort Items: What would make life bearable? (Coffee maker? TV? Specific outlets?)
Find the Watts: Write running AND starting watts for each item (use the table above as a cheat sheet).
The Magic Formula: Add up all RUNNING watts you'd use simultaneously. Write this number down.
Find the Biggest Surge: Identify the single HIGHEST starting wattage on your list.
Calculate Minimum Size: Take your total running watts and add the largest starting surge. Example: Total running watts = 3,500 + Largest surge (Fridge @ 2,200) = 5,700 watts minimum generator size needed.

Sounds simple, right? Here's where people stumble: They add ALL starting surges together. Don't do that! Your fridge compressor kicks on, then the furnace blower starts 10 seconds later – they don't surge at the exact same millisecond. Only the largest surge matters.

Real-Life Power Scenarios & Generator Recommendations

Let's translate those numbers into actual generator sizes. Here are common setups based on what I've seen work (and fail) in real outages.

Scenario 1: Keeping the Basics Alive (3,000 - 5,500 Watts)

Goal: Prevent food spoilage, charge devices, run a few lights, keep furnace circulating.

What's Powered: Fridge, freezer, furnace blower (gas), LED lights, phone chargers, TV, laptop.
Critical Surge: Fridge or Furnace Blower (approx. 2,200-2,300W)
Typical Running Load: 1,500 - 2,500W
Recommended Size: 4,000 - 5,500 Watt Generator

Top Picks:

Honda EU7000is (5,500W Running / 7,000W Surge): Super quiet, incredibly reliable, but pricey ($4,000+). My neighbor swears by his during hurricane season.
Champion 4500-Watt Dual Fuel (4,500W Running on Gas): Great value ($700-$900), runs on gas or propane, inverter tech keeps power clean for electronics. A solid workhorse.
Generac GP3500iO (3,500W Running): Lightweight inverter ($1,000-$1,200), quieter than most. Good for essentials if your furnace surge is low.

Warning: Don't cheap out below 4,000W for this setup unless you have very low-surge appliances. That fridge surge will trip smaller units.

Scenario 2: Comfortable Resilience (6,000 - 10,000 Watts)

Goal: Add major appliances like well pumps, sump pumps, microwaves, or multiple large loads.

What's Powered: Essentials + Well Pump/Sump Pump + Microwave + Coffee Maker + More Lights/Outlets.
Critical Surge: Well Pump (approx. 2,500-3,000W) or Sump Pump.
Typical Running Load: 3,000 - 6,000W
Recommended Size: 8,000 - 10,000 Watt Generator

Top Picks:

DuroMax XP13000HX (10,500W Running): Dual fuel powerhouse ($2,000-$2,500), handles heavy loads with ease. Big and loud though.
Westinghouse WGen9500 (9,500W Running): Reliable, electric start ($1,000-$1,300). Good balance of power and price. Their customer service is hit or miss, FYI.
Generac GP8000E (8,000W Running): Trusted brand ($1,000-$1,200), solid performance. Not an inverter, so avoid sensitive electronics.

Scenario 3: Whole Home Power (10,000+ Watts)

Goal: Seamless backup. Powers central AC, electric water heaters, electric stoves/ovens, multiple large circuits.

What's Powered: Almost everything except maybe massive electric dryers/stoves simultaneously.
Critical Surge: Central AC Unit (HUGE - often 8,000W to 15,000W+)
Typical Running Load: 7,000W - 15,000W+
Recommended Size: 18,000 - 24,000 Watts (or higher)

Pick This Route If: You experience frequent/long outages, have medical equipment, or want true uninterrupted power (usually requires a professionally installed standby generator).

Top Brands (Standby Units):

Generac Guardian (22kW - 26kW models common): Industry leader ($5,000-$12,000 installed). Automatic transfer switch kicks in within seconds of outage. Installation is key – get a certified pro.
Kohler (20kW - 26kW models): Premium build quality ($6,000-$14,000+ installed), often quieter than Generac. Their service network isn't as vast everywhere.
Briggs & Stratton (20kW+): Solid alternative ($5,000-$11,000 installed).

Standby units solve the what size generator do I need question comprehensively but require significant investment and professional installation. Portable generators in this range (like the DuroMax XP13000HX) can technically power many whole-home circuits if wired through a transfer switch, but starting a large AC unit is usually beyond them.

Critical Factors Beyond Wattage

Choosing generator size isn't just math.

Fuel Type: Gas, Propane, or Dual Fuel?

Gasoline: Most portable generators. Pros: Highest power output per dollar, readily available. Cons: Short shelf life (degrades), harder to store safely in large quantities, messy refills during storms.
Propane (LPG): Standby & some portables. Pros: Stores indefinitely, burns cleaner (less engine carbon buildup), safer storage. Cons: Less power output than gas (approx. 10-15% less on same gen), tanks take space. My dual fuel Champion saved me during a gas shortage.
Dual Fuel: Best of both worlds (like Champion models). Flexibility is king during extended outages.

Inverter vs. Conventional Generators

Conventional: Cheaper per watt. Good for power tools, resistive loads (heaters, lights). Cons: Loud, power output isn't perfectly smooth ("dirty power"), can damage sensitive electronics (laptops, phones, TVs, modern furnace boards!).
Inverter: More expensive. Pros: Very quiet, ultra-clean power safe for all electronics, much more fuel-efficient especially at partial loads (which is most of the time). If you're running modern gadgets, inverter tech is worth the premium.

Portability & Installation

Portable: You store it, wheel it out, start it manually, run extension cords or connect to a manual transfer switch. Requires fueling every 6-12 hours typically.
Standby: Permanently installed outdoors (like an AC unit). Automatic transfer switch monitors utility power. Kicks on within seconds automatically. Runs on natural gas (NG) or large propane (LP) tanks. Hands-off convenience at a high cost. The ultimate answer to "what size generator do i need" for whole-home comfort.

Runtime & Fuel Efficiency

A giant generator running a small load is inefficient. Check the specs for fuel consumption at 25%, 50%, and 75% load. Inverter generators shine here, sipping fuel at partial capacity. A 6,000W conventional gen running only 1,500 watts might burn fuel faster than a 3,500W inverter gen doing the same job. Factor fuel storage and availability into your size decision.

Common Pitfalls to Avoid

Ignoring Surge Watts: This is the #1 mistake. That starting surge is non-negotiable.
Forgetting Hidden Loads: Garage door opener? Security system? Tankless water heater igniter? Attic fan? Add them in.
Assuming "Whole House" Label Means Everything: Even large portable "whole house generators" (12kW-15kW) often can't start massive AC units or electric dryers simultaneously. Know your peak surge!
Overloading Circuits: Even with enough generator capacity, plugging too many items into one outlet circuit trips breakers. Spread the load.
Neglecting Transfer Switches: Backfeeding power through a household outlet (a "suicide cord") is EXTREMELY DANGEROUS and illegal. It kills utility workers. Use a proper transfer switch.
Underestimating Noise: Big conventional generators are loud. Consider neighbors and your sanity. Check decibel ratings (dB). Inverters are significantly quieter.

Frequently Asked Questions (FAQs)

What size generator do I need for a 2000 sq ft house?

This is impossible to answer without knowing your essential circuits. A small 2000 sq ft home with gas furnace/water heater might only need 5000W for essentials. A similar sized all-electric home with central AC easily needs 18,000W+. Do your power audit! Forget square footage – focus on appliance lists.

Can I run my central air conditioner with a portable generator?

It depends entirely on your AC unit's starting surge. Smaller window units? Often yes (check the surge!). Modern 3-ton+ central AC? Usually requires a large standby generator (18kW+). Check your outdoor AC unit's data plate for LRA (Locked Rotor Amps) and convert to watts (LRA x Volts = Watts). That starting wattage is brutal.

How much generator do I need for a refrigerator and freezer?

You'll need at least 3,500-4,000 running watts and enough surge to handle whichever has the bigger startup surge (fridge is usually worse). A quality 5000W inverter generator handles this safely.

What size generator for a well pump?

A typical 1/2 HP well pump needs about 1,000 running watts but a 2,500-3,000 watt surge. You'll need a generator capable of at least 4,000 watts running to safely start it alongside other basics.

Is a 7500 watt generator enough for a house?

Potentially! For essentials plus a well pump, sump pump, furnace, fridge/freezer, and some lights/outlets in a typical home, 7500W is often sufficient. But verify your specific appliance surges (especially if you have AC).

Can I run sensitive electronics on any generator?

No. Only inverter generators produce "clean" sine wave power safe for laptops, TVs, and smartphones. Conventional generators can fry circuit boards. Check the spec sheet for "THD (Total Harmonic Distortion)". Look for less than 5% THD, ideally less than 3% for electronics safety.

What size propane tank do I need for a generator?

For portable dual fuel: A standard 20 lb BBQ tank runs a 5000W load for 6-12 hours. For standby generators: A 500-gallon tank is common for multi-day outages running larger homes.

Making Your Final Decision: Key Takeaways

Figuring out what size generator do I need boils down to:

Your Non-Negotiables: What absolutely MUST run? (Food, heat/cooling, water, medical devices).
The Surge Monster: Identify the single highest starting wattage on your list.
Add Running Loads: Total the watts of items you'll run simultaneously.
Size = Total Running Watts + Largest Surge Wattage.
Consider Future Needs & Fuel: Might you need more later? How will you store fuel?
Safety First: Use transfer switches. NEVER backfeed. Prioritize CO safety (keep generators FAR from windows/doors).

Still feel stuck? Add up your running watts and your biggest surge watt using the steps above. Drop those two numbers in a generator forum or ask at a reputable dealer. Knowing those two numbers cuts through 90% of the confusion. The right generator size brings peace of mind when the lights go out – worth every bit of planning.