What Size Generator Do I Need? Sizing Calculator and Room-by-Room Guide
Generator sizing is where buyers lose the most money — in both directions. Undersize, and the generator stalls the moment your air conditioner tries to start, or trips repeatedly under load. Oversize, and you hand over $1,500–$3,000 extra upfront, then burn more fuel every hour it runs, forever.
Here’s what makes it worse: most «sizing help» comes from people selling generators, and the friction-free answer — «just get the 24 kW, you’ll never worry» — happens to be the one that maximizes their ticket.
This guide gives you the same method a good installer uses, in four steps you can do at your kitchen table in 20 minutes. You’ll leave knowing your number — and knowing whether the size you’re quoted is right or padded.
The 30-Second Answer (Then Do the Math)
| Your situation | Size class that usually fits |
|---|---|
| Essentials only: fridge, lights, sump pump, Wi-Fi, furnace fan | 7.5 – 10 kW |
| Small home (<2,000 sq ft), gas heat, one small AC, managed loads | 10 – 14 kW |
| Typical home (2,000–3,000 sq ft), one central AC, whole-home coverage | 18 – 22 kW |
| Larger home (3,000–4,000 sq ft), one big or two small AC units | 22 – 26 kW |
| Large home, two AC units, well pump, electric water heater | 26 – 32+ kW |
These rows are starting points, not answers — two houses with identical square footage can need sizes two classes apart depending on what runs on electricity. The math below is what actually decides it. (And before signing anything, a professional load calculation should confirm it — more on that at the end.)
First, Understand the Only Two Numbers That Matter
Running watts (rated watts): what an appliance consumes while operating normally.
Starting watts (surge watts): what motor-driven appliances demand for the first second or two of startup — typically 2 to 3 times their running watts. Air conditioners, well pumps, sump pumps, refrigerators, and freezers all surge.
Generators are rated on both. Sizing failures almost always come from ignoring starting watts: a 3-ton AC that runs at 3,500W can demand 8,000W+ at the instant of startup. If your generator can’t cover the surge on top of everything else already running, it trips — usually mid-outage, usually at 2 a.m.
Step 1: Inventory Your Loads (Room-by-Room Wattage Chart)
Walk through your home and check what applies. Use nameplate data where you can find it (a sticker on the appliance); otherwise these typical values are reliable for planning:
Kitchen
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator/freezer | 200 – 700 | 1,200 – 2,200 |
| Chest freezer | 100 – 400 | 900 – 1,500 |
| Microwave | 1,000 – 1,500 | — |
| Electric range (per burner) | 1,500 – 2,500 | — |
| Electric oven | 2,000 – 3,600 | — |
| Dishwasher | 1,200 – 1,500 | 1,500 – 3,000 |
| Coffee maker | 900 – 1,200 | — |
Climate (the budget-maker)
| Appliance | Running watts | Starting watts |
|---|---|---|
| Central AC, 2-ton | 2,500 – 3,000 | 5,000 – 7,500 |
| Central AC, 3-ton | 3,000 – 4,000 | 7,000 – 9,500 |
| Central AC, 4-ton | 4,500 – 5,500 | 9,500 – 12,000 |
| Heat pump (heating mode) | 3,000 – 5,000 | 6,500 – 11,000 |
| Gas furnace (blower fan only) | 500 – 800 | 1,500 – 2,500 |
| Window AC unit | 500 – 1,200 | 1,200 – 2,800 |
| Space heater | 1,500 | — |
Water & mechanical
| Appliance | Running watts | Starting watts |
|---|---|---|
| Sump pump, 1/3 HP | 800 | 1,500 – 2,500 |
| Sump pump, 1/2 HP | 1,000 | 2,000 – 3,000 |
| Well pump, 1/2 HP | 1,000 | 2,000 – 3,500 |
| Well pump, 1 HP | 2,000 | 4,000 – 6,000 |
| Electric water heater | 4,500 | — |
| Tankless electric water heater | 10,000 – 28,000* | — |
| Septic pump | 800 – 1,500 | 1,800 – 3,500 |
*Whole-home electric tankless heaters are generator-killers — they alone can exceed a mid-size generator. Homes with them either exclude them from backup or size dramatically up.
Everything else
| Appliance | Running watts | Starting watts |
|---|---|---|
| Lights (LED, whole home) | 200 – 500 | — |
| TV + streaming + Wi-Fi/modem | 150 – 400 | — |
| Desktop computer setup | 300 – 800 | — |
| Washing machine | 500 – 1,000 | 1,500 – 2,300 |
| Electric dryer | 5,000 – 6,000 | 6,000 – 7,000 |
| Garage door opener | 500 – 750 | 1,200 – 1,500 |
| EV charger, Level 2 | 7,200 – 11,500 | — |
| Medical: CPAP | 30 – 60 | — |
| Medical: oxygen concentrator | 300 – 600 | 600 – 1,200 |
Step 2: Do the Two-Line Calculation
- Add the running watts of everything you want available at the same time during an outage. Be honest — nobody runs the oven, dryer, and both ACs simultaneously during a hurricane, and pretending you will is how you get upsold.
- Add the single largest starting surge on top. Only one motor starts «first» — the correct method is: total running watts + the biggest individual (starting − running) difference among your motor loads.
Then add a 20–25% headroom margin. Generators shouldn’t live at 100% load: continuous max-load running shortens engine life, worsens fuel burn, and leaves nothing for the load you forgot. Headroom is not waste — it’s engineering.
Step 3: See It Done — Three Worked Examples
Example 1: «Keep the essentials alive» — a 1,600 sq ft home with gas heat
Wants powered: fridge (700W run), sump pump 1/3 HP (800W), furnace fan (700W), lights (300W), Wi-Fi + TV (300W), microwave used occasionally (1,200W).
- Running total: 4,000W
- Largest surge: sump pump (+1,700W above running)
- Subtotal: 5,700W → +25% headroom → ~7,100W
- Verdict: a 7.5–10 kW unit. A small standby — or even a quality portable with an interlock — covers this comfortably [→ /portable-generators/best-portable-generators-home-backup/].
Example 2: «Whole home, one AC» — a 2,600 sq ft home
Wants powered: 3-ton AC (3,500W run), fridge (700W), electric water heater (4,500W), sump pump 1/2 HP (1,000W), lights + electronics (800W), kitchen headroom (2,000W).
- Running total: 12,500W
- Largest surge: AC (+5,000W above running, assuming ~8,500W start)
- Subtotal: 17,500W → +20% headroom → ~21,000W
- Verdict: the classic 22 kW home. But note two levers: a soft-start kit on the AC (cuts that surge roughly in half) plus a load-management module on the water heater could bring an 18 kW — even a 16 kW — into play, saving $1,000–$2,000 [→ /standby-generators/22kw-vs-24kw-vs-26kw-generac/].
Example 3: «Everything, rural» — a 3,800 sq ft home, well water, two ACs
Wants powered: 4-ton AC (5,000W) + 2-ton AC (2,800W), well pump 1 HP (2,000W), fridge + freezer (1,000W), electric water heater (4,500W), lights/electronics (1,200W), kitchen (2,500W).
- Running total: 19,000W
- Largest surge: 4-ton AC (+5,500W)
- Subtotal: 24,500W → +20% headroom → ~29,500W
- Verdict: 26–32 kW class with load management — or, smarter: soft-starts on both ACs and sequencing modules can pull this into a 26 kW unit. At this scale, a professional load calc isn’t optional; it’s where thousands of dollars get decided.
Step 4: Know the Three Sizing Levers Before You Buy
These three tools change what size you need — and salespeople don’t always volunteer them, because each one shrinks the invoice:
- AC soft-start kits ($300–$500 installed). Cut compressor starting surge by 40–70%. The single highest-ROI accessory in backup power: one kit can move you down a full size class.
- Load-management modules ($150–$400 each). Smart relays that briefly shed low-priority loads (water heater, dryer, second AC) when the generator approaches capacity, then restore them. They let a 14–18 kW unit deliver a «whole home experience» in a house that naively calculates to 24 kW.
- Honest simultaneity. The difference between «everything I own, at once» and «everything I’d realistically run during an outage» is typically 4–8 kW. Size for reality; use load management for the edge cases.
The flip side — three reasons to size UP a class:
- You plan to add an EV charger, hot tub, pool pump, or addition within the generator’s 15–25 year life [→ /standby-generators/how-long-do-whole-house-generators-last/]
- Heat pump heating: if your winter heat is electric heat pump (with resistance backup strips), winter load can exceed summer AC load — size for the strips or manage them
- Medical equipment: never let life-critical loads live inside the headroom margin; they belong in the base calculation with room to spare [→ /outage-prep/power-medical-equipment-during-outage/]
Portable Generator Sizing (Quick Version)
Not ready for a standby install? The same math applies, smaller:
- 2,000–2,500W inverter: fridge OR sump pump + phones, lights, Wi-Fi — with careful juggling
- 3,500–4,500W: fridge + sump pump + lights + TV + occasional microwave — the sweet spot for short-outage households
- 6,500–8,500W: adds a window AC or small central furnace fan + more kitchen use — the practical ceiling of «comfortable portable living»
- 10,000–12,500W: partial-home via transfer switch, including a small central AC with soft-start
Full portable wattage guidance, model by model: [→ /portable-generators/generator-wattage-chart/]. And whatever size you buy, connect it through a transfer switch or interlock — never a «suicide cord» backfeed [→ /portable-generators/connect-portable-generator-to-house/].
Why a Professional Load Calculation Still Matters
Your kitchen-table math will land you in the right class — that’s its job, and it’s how you detect an upsell. But before contracts are signed, a reputable installer should perform a formal load calculation (NEC-based), verify your panel and service capacity, and measure whether your gas meter and line can actually feed the generator you chose at full load — an 18 kW → 22 kW upsize is pointless if your gas supply can’t run either [→ /installation-costs/natural-gas-line-sizing-standby-generators/].
Red flag worth repeating: any installer who quotes a size from square footage over the phone, without a load calc or site visit, is guessing with your money.
Frequently Asked Questions
What size generator do I need for a 2,000 sq ft house? Usually 14–18 kW for whole-home coverage with one AC (18–22 kW if loads are heavy or all-electric), or 10 kW with load management. The deciding factors are your AC tonnage and whether heat/water heating are electric — not the square footage itself.
Will a 22 kW generator run my whole house? For most homes of 2,500–3,500 sq ft with a single central AC — yes, comfortably. Homes with two AC units, a large well pump, or heavy electric heat may need 26 kW+ or load management.
What can a 10,000-watt generator run? Essentials plus comfort: fridge, freezer, sump pump, furnace fan, lights, electronics, microwave, and either a window AC or (with soft-start) a small central AC — with load juggling during peaks.
Is it bad to oversize a generator? It wastes money twice: more upfront, and more fuel per hour forever (bigger engines burn more even at partial load). Lightly-loaded running is also less efficient per kWh. Aim for your calculated load + 20–25%, not the biggest unit on the truck.
What size generator will run central air conditioning? Rule of thumb per AC: about 1 kW of generator per 1,000 BTU… is too crude — use tonnage: a 3-ton AC generally needs 7–9.5 kW available at startup (3.5–4.5 kW with a soft-start kit), on top of everything else running.
How do I find my appliance’s exact wattage? Check the nameplate sticker (watts, or amps × volts), the manual, or measure with a $25 plug-in power meter for 120V items. For hardwired loads (AC, water heater), the nameplate or your installer’s meter is the source.
The Bottom Line
Your generator size is decided by four numbers: honest simultaneous running watts, the single biggest starting surge, 20–25% headroom, and the levers (soft-start, load management) that can legally shrink all of the above. Do the 20-minute math before anyone quotes you — buyers who arrive with their own number consistently avoid the one-class upsell, and that discipline is worth $1,000–$3,000.
Next steps: with your size class in hand, see what it costs installed [→ /standby-generators/whole-house-generator-cost/], and how to collect quotes that can actually be compared [→ /installation-costs/generator-installation-quotes/].