Generator shopping lists running watts and starting watts as if one number could describe a whole home—it cannot when compressors, well pumps, and microwaves time their peaks together.
This master guide merges every what size generator for… scenario plus conceptual posts on surge, spec sheets, usable capacity, runtime, and inverter vs conventional fuel use.
Each scenario below is a case study with illustrative bands—your nameplates and transfer switch limits win.
Case studies synthesize EN cluster generator articles and NEC-informed practice. Never backfeed—use listed transfer equipment. CO safety and fuel storage are mandatory but outside watt math.
| Essentials stack | 800–2,500 | Fridge start | 5–15 | Fridge + lights + comms |
| Room AC + essentials | 2,000–4,500 | AC LRA | 15–40 | Stagger loads |
| Whole-house 3 ton | 4,000–8,000+ | High | 40–100+ | 10–22 kW class |
Typical ranges for planning — confirm with nameplate labels and your use pattern.
For whole-home off-grid design, see the Off-Grid Solar System Guide 2026. Build a defensible load list with How to Build a Load List for Off-Grid Solar Sizing, then model concurrent peaks in the WattSizing Calculator.
Case study: 1.5 ton AC + refrigerator
Central 1.5 ton ~2,000–2,800 W running; LRA surge drives class. Fridge +180 W running, +1,200 W start. Conservative portable: 5,500–7,500 W with overlap; soft-start may drop one class. 240 V feed required for standard central condenser.
Worked stack: 1.5 ton system ~2,400 W running + fridge 180 W + lights 100 W = ~2,680 W steady. LRA event on condenser ~5,000–6,500 W equivalent planning with blower—5.5–7.5 kW portable class conservative.
Mini-split caveat: 1.5 ton ductless may run 2,000 W max with gentler start—do not size like central LRA.
Fuel planning: 5.5 kW gen at 50% load may use 0.5 gal/h gasoline—12 h outage needs 6 gal plus reserve, not 2 gal.
Case study: 2 ton AC
Running ~2,200–3,700 W system; plan 7.5–10 kW for AC + fridge + essentials without heroic staging. Mini-split 2 ton often smaller generator—do not use central LRA tables.
Transfer switch: 240 V condenser requires interlocked feed—never backfeed the meter. Many 7.5 kW portables provide 30 A 240 V twist-lock—match LRA not brochure running watts.
Mini-split alternative: If you install a 2 ton ductless for the same space, revisit generator class—often 3.5–5 kW instead of 7.5–10 kW for comparable comfort.
Evening outage table:
| Load | W |
|---|---|
| 2 ton central (running) | 2,700 |
| Fridge | 180 |
| Lights/router | 150 |
| Subtotal running | 3,030 |
Add surge margin for compressor restart while fridge cycles—7.5–10 kW class for central; 3.5–5 kW possible for inverter mini-split with staging.
Evening outage table:
| Load | W |
|---|---|
| 2 ton central (running) | 2,700 |
| Fridge | 180 |
| Lights/router | 150 |
| Subtotal running | 3,030 |
Case study: 3 ton AC
Running ~3,000–4,500 W; surge stacks higher. 10–12 kW class common for whole-comfort backup. Stagger well pump and kitchen.
Feeder reality: 3 ton on 240 V often 30–50 A service to condenser—generator 240 V/30 A twist-lock common on 10–12 kW portables.
Stagger: Run well pump only when AC compressor is idle if borderline on class.
Whole-home interlock: 200 A service with 12 kW gen is common upgrade path—still cannot run two central units simultaneously without load shed.
Case study: 2,000 sq ft house essentials
Not square footage alone—insulation, climate, and fuel. Essentials-only (fridge, lights, network, one room AC) often 3,500–7,500 W; whole-house comfort 10–22 kW depending on all-electric vs gas heat.
Climate split: 2,000 sq ft in Minnesota with electric heat ≠same in Texas with gas heat + 3 ton AC. Benchmark your utility 12-month kWh, divide summer peak month.
Essentials-only panel: Fridge + furnace blower + lights + one sump often 3–5 kW running—far below whole-house comfort.
Case study: apartment backup
Fridge + lights + network + phone often 1,800–2,500 W continuous. Window AC adds 900–1,500 W running—3,000–3,500 W class practical. Landlord rules and venting limit fuel types.
Landlord constraints: No balcony → gasoline forbidden; battery stations cap at 1,800 W often—limits window AC size.
Window AC sizing: 8k BTU ~800 W running fits 2,000 W station with fridge stagger.
Balcony distance: Generator 20 ft from window per CO guidance—cord length affects voltage to window AC.
Battery station: 2 kWh stations restart fridge but rarely run 8k BTU AC—read continuous not surge on spec.
Case study: camping trailer
1,000–2,200 W inverter generators for lights, charging, small microwave. No roof AC on smallest units; 13.5k BTU RV AC needs 2,500–3,500 W+ with surge headroom.
30 A RV service: 3,600 W at 120 V—theoretical max; practice 3,000 W continuous planning for AC + microwave overlap.
Microwave + AC: 1,500 + 1,500 W needs 3,000 W inverter gen minimum.
Elevation: 5,000 ft camping loses 15% gen power—13.5k BTU AC may not start on 2,200 W unit that worked at sea level.
Parallel forbidden: Unless manufacturer approves, do not parallel two different generator models for RV AC.
Case study: construction site trailer
Tools + heat vary; plan 3,500–7,500 W if table saws and compressors cycle. GFCI and bonding per local code—beyond watt math.
Table saw + compressor: 1,800 W saw + 2,400 W compressor running—not simultaneous surge if staggered; 5,500 W class site standard.
GFCI nuisance: Long cords on generators trip GFCI—bonding and neutral switching per manufacturer.
Compressor duty: A 2 HP air compressor may draw 2,000 W running with 3,500+ W start—stagger with table saw. GFCI on generator receptacles is mandatory for wet sites.
Temp derate: Open-frame generators lose output in 100°F sun—size 15–20% above calculated W.
Case study: food truck
Hood, refrigeration, grills, AC can exceed 10 kW running combined. Commercial nameplates summed with diversity only if you have data—otherwise sum honestly. Propane cooking reduces electrical peak.
Load list example: Walk-in 1,200 W, hood 800 W, fryer 3,500 W, AC 2,000 W—honest sum 7.5 kW without diversity unless you have logged data.
Propane cooking: Removes fryer electric peak—generator may drop one size class.
Menu-driven loads: Fryer 3,500 W, griddle 2,000 W, heat lamp 500 W—menu decides peak, not truck length. Propane for cooking drops electrical need to hood + fridge + POS.
Permits: Sound ordinances may force inverter gens at night—electrical size unchanged, fuel planning changes.
Case study: essential circuits panel
Critical loads panel approach: fridge, furnace blower, well pump, one lighting circuit—often 5–8 kW if pump and fridge stagger. Transfer switch locks out non-essentials.
Critical load panel: Physically separates well, fridge, furnace blower, one lighting circuit from range and second AC—lets a 5–8 kW unit behave like a larger generator.
Load shedding: Automatic modules drop dryer when well calls—cheaper than oversizing nameplate.
6-circuit transfer switch typical: Fridge, furnace, well, bathroom, kitchen receptacles, lights—sum running W, then worst two-motor surge second.
Automatic shedding: Reliance/load-shed modules drop dryer when well starts—cheaper than 12 kW upgrade.
Furnace blower: PSC blower 300–600 W while gas heat runs—add to winter outage stack even when AC is off.
Case study: medical equipment at home
CPAP, oxygen concentrators, dialysis—list nameplate W × hours. Concentrators 300–600 W continuous are common. Require inverter quality and automatic transfer; add UPS bridge for switchover per clinician guidance.
Oxygen concentrator: Often 350–600 W continuous 24/7—8.4–14.4 kWh/day before other loads.
CPAP: 30–60 W—small but needs pure sine and reliable overnight runtime.
UPS bridge: 10–30 min UPS covers transfer switch gap—size VA for concentrator inrush if present.
Dialysis at home: Some units 1,000–1,500 W for 4 h sessions—add to daily kWh and peak separately from concentrator.
Documentation: Keep clinician letter with minimum runtime hours for utility medical baseline programs where available.
Case study: RV air conditioner
13.5k BTU roof units 1,500–2,000 W running; start 2,500–3,500 W. 3,000–3,500 W inverter generator popular; soft-start kits common in RV community.
13.5k BTU roof: 1,600–2,000 W running; Micro-Air/soft-start popular to run on 2,200 W inverter generators.
Elevation: Generator derates 3–4%/1,000 ft—mountain camping needs upsize.
Soft-start: Micro-Air style devices let 2,200 W gen start 13.5k BTU roof unit that otherwise needs 3,500 W—verify compatibility list.
Case study: tiny house
2,000–4,000 W often covers mini-split + fridge + lights if heat is wood/gas. All-electric heat in cold climate pushes 7.5 kW+.
30 A service @ 120 V only: 3,600 W max—mini-split + fridge + induction one zone is the electrical budget.
Wood heat: Removes 15 kW strip heat from winter generator math entirely.
30 A @ 120 V service: 3,600 W max—budget mini-split 9k BTU, fridge, LED, one 1,200 W cook plate. Wood stove removes winter gen need entirely in many builds.
Tow vehicle: Some tow 7,000 W built-in gen on truck—house cord caps lower than truck capability.
Case study: workshop tools
Table saw 1,500–1,800 W; air compressor 1,500–2,500 W running with high surge. Duty cycle low but peaks matter. 240 V tools need correct generator leg.
Table saw stall: Binding blade spikes 2× running briefly—2,500 W saw wants 3,500 W+ surge class.
240 V tablesaw: Needs generator 240 V leg—cannot run on 120 V-only portables.
Table saw + dust collector: 1,800 W + 1,200 W if both run—4,500 W gen minimum. Welder not covered here—240 V 50 A class separate topic.
Extension cord: 12 AWG max 25 ft for 1,800 W tools—voltage drop causes motor overheat.
Running watts vs starting watts
Running (rated) watts = continuous load capability. Starting/surge/peak = brief motor inrush. Generators trip on instantaneous overload even if average is fine. Overlap two compressors = worst case.
Timeline model: Second 0: AC inrush 5,000 W + fridge 1,200 W + lights 100 W = 6,300 W need—even if second 30 averages 2,000 W.
Resistive loads: Toaster and kettle have no surge—nameplate = planning W.
How to read generator specs
Nameplate lists max running and surge—use running for steady stack, surge for largest motor start plus other running loads at that second. Inverter generators quote electronically limited surge—still verify.
Dual rating example: 5,500 W running / 6,500 W peak means only 1,000 W surge headroom above running—marginal for fridge + AC unless staged.
Inverter gens: Peak often 110–120% of running for <1 s—read manual duty cycle.
Calculating surge for motor loads
Surge ≈ LRA × voltage (single-phase approximation) for compressors; add 25–50% slack for fan/pump coincidence. Clamp meter capture beats guessing. Soft-start reduces multiplier—test.
Worked surge stack: List each motor with LRA or measured inrush, voltage, and whether starts can coincide. Fridge + well + AC aligned is the rural worst case—stagger with manual procedure if automation is absent.
Power factor: Motors draw VA higher than W—generator kVA limits matter on marginal units.
LRA example: LRA 28 A × 240 V ≈ 6,720 W locked-rotor planning—soft-start might reduce observed peak to 4,000 W—test.
Pump + AC: Well LRA aligned with AC restart is why rural homes need 10 kW where urban apartments need 3 kW.
Rated vs usable output
Altitude, fuel, and temperature derate output. Continuous rating may be 80% of max on some industrial units. Harmonic load from cheap switch-mode supplies can heat windings—leave headroom.
Altitude derate: 5,000 ft can lose 15–20% power—10 kW gen behaves like 8 kW.
Temperature: Hot intake air reduces engine output—open-frame units in sun trip earlier than spec sheet.
Runtime from fuel tank
Runtime ≈ (tank gallons × fuel energy density × efficiency) ÷ load kW. At 50% load, many portables last longer than at 100%—consult manufacturer curves. Propane vs gasoline energy content differs.
Gasoline example: 4 gal tank, ~33 kWh/gal thermal, ~20% engine efficiency at 50% load → rough 6–8 h at 2,500 W—use manufacturer chart for quote.
Propane: 91,500 BTU/gal equivalent—slower fuel line freeze in winter.
Inverter vs conventional fuel use
Inverter units throttle engine—better part-load fuel economy for variable homes. Conventional can be efficient at steady high load. THD lower on inverters—better for electronics and some UPS.
Eco-throttle: Inverter at 300 W load may sip fuel vs conventional idling high always—meaningful for overnight fridge-only.
THD: Inverter <5% THD common; conventional 9–15%—electronics and medical gear prefer inverter.
Eco mode caveat: Eco-throttle may hunt with reactive motor loads—disable eco when fridge compressor makes inverter hunt frequency.
Buying checklist: (1) Running W ≥ steady stack. (2) Peak/surge W ≥ worst motor start + other running loads. (3) 240 V availability if central AC, well, or dryer matter. (4) Fuel runtime at your load, not 50% brochure chart. (5) THD if computers, medical, or inverter UPS loads matter. (6) Transfer switch rated for full generator output.
Stagger card example: Minute 0 fridge only; minute 3 microwave ≤5 min; minute 10 window AC if gen has headroom; never start well pump during AC restart window.
Scenario matrix (running W only, illustrative):
| Use case | Typical class |
|---|---|
| Apartment fridge + lights | 2,000–2,500 W |
| + window AC | 3,000–3,500 W |
| 2 ton central + fridge | 7,500–10,000 W |
| Rural well + fridge + window AC | 5,500–7,500 W |
Surge always adds headroom above these running bands—never buy solely on running watts marketing.
Model your stack in the WattSizing Calculator before buying—round up one generator class if any motor lacks measured start data.
Worked example: surge stack
Running: fridge 180 W + window AC 900 W + lights 100 W = 1,180 W.
Surge: AC 1,800 W + fridge start 1,200 W (aligned worst case)—not additive as 3,000 W always; but plan 2,500–3,000 W+ surge capability above running.
Pick: 3,500 W running / 4,500 W peak inverter generator minimum class for conservative overlap.
FAQs
Running or surge—which limits generator size?
Surge for motors; running if all resistive.
Can I use extension cords for whole house?
No—proper transfer switch and conductor sizing.
Inverter generator for sensitive electronics?
Preferred for low THD; still mind surge.
Why does generator trip at 50% load meter?
Motor start or harmonics—not average W.
How to size for well pump?
Use LRA and 240 V requirement—pumps dominate.
Bigger generator always better?
No—fuel, noise, cost; load management helps.
Propane vs gasoline runtime?
BTU/gal and engine map differ—read mfr chart.
Soft-start on AC worth it?
Often drops one generator class—verify listing.
Medical backup?
Continuous W × hours + UPS bridge—clinical guidance first.
Food truck generator size?
Sum honest peaks or 3,500–12 kW+ range wide.
