Portable Generator Size Calculator

Multiply the total watts you want to power by a duty factor — 1.25 for light loads or 1.5 for heavy motor and A/C loads — and round up to the nearest portable inverter-generator size. If you need 2,500 W for an RV with lighter loads, 2,500 × 1.25 = 3,125 W of surge headroom, which rounds up to a 3,500-watt portable inverter generator. For a heavier 2,000 W campsite with an air conditioner, 2,000 × 1.5 = 3,000 W → a 3,000 W unit.

Portable generators live and die by their surge capacity. RV air conditioners, small pumps, and power tools all draw a startup spike far above their running watts, and a portable unit has less thermal mass than a big standby generator, so leaving surge headroom matters even more. The duty factor bakes that surge into the recommendation, and rounding up to a real product size (2,000, 2,200, 3,000, 3,500, or 4,000 W) means the number maps to a generator you can actually buy.

The duty factor is a portable-friendly surge multiplier. Instead of hunting the exact starting watts of every device, you classify the whole load:

• Light (1.25×) — electronics, lights, phone and laptop charging, a small 12V fridge or cooler, a fan, a coffee maker. These loads are mostly resistive or have tiny motors, so a 25% cushion covers their modest surge and the 80% continuous practice.
• Heavy (1.5×) — an RV rooftop air conditioner, a microwave, power tools, a water pump, or anything with a real compressor or induction motor. These surge 2–3× at startup, so a 50% cushion keeps the generator from bogging or tripping when the compressor kicks on.

When in doubt, pick heavy. A slightly oversized portable simply runs quieter and more fuel-efficiently at part load — modern inverter generators throttle the engine to the load. Under-sizing, by contrast, means the A/C won't start or the generator overloads and shuts down. To break down individual device watts first, use the power consumption calculator.

The single biggest driver of RV generator size is the rooftop air conditioner. A common 13,500 BTU RV A/C runs at about 1,300–1,500 W but surges to 2,800–3,000 W at compressor startup. A 15,000 BTU unit runs near 1,800 W and can surge past 3,500 W. That surge is why a 2,000 W portable — despite being popular and quiet — usually cannot start an RV A/C without help.

Two solutions bring RV A/C within reach of a small portable:

• A soft-start module (e.g., a capacitor-based soft starter) cuts the compressor inrush by 60–70%, letting a single 2,000–2,200 W inverter generator start and run a 13,500 BTU A/C.
• A 3,000–3,500 W generator starts the A/C directly and still has room for a microwave, fridge, and charging. This is the sweet spot for most RVers who want to run A/C plus essentials.

If your rig has a 30-amp shore connection, the theoretical ceiling is 30 A × 120 V = 3,600 W, so a 3,000–3,500 W generator matches the coach's own limit well. Fifty-amp rigs (two 120 V legs) often pair two 2,000 W inverter generators in parallel or step up to a 4,000 W+ unit.

Example 1 — Tent camping, light loads. 12V fridge on an inverter 60 W, LED lights 40 W, fan 50 W, phone charging 30 W, coffee maker 900 W (intermittent). Peak simultaneous ≈ 1,080 W. At 1.25× that's 1,350 W → round up to a 2,000 W inverter generator. Quiet, light, and plenty.

Example 2 — RV with A/C. Total simultaneous 2,000 W (A/C running 1,400 W + converter 300 W + charging 300 W), heavy duty. 2,000 × 1.5 = 3,000 W → a 3,000 W generator, which also covers the A/C's 2,800 W startup surge.

Example 3 — Job site. Circular saw 1,500 W (surges to ~2,300 W), work lights 200 W, phone charger 20 W. Total 1,720 W, heavy duty. 1,720 × 1.5 = 2,580 W → a 3,000 W unit handles the saw's inrush comfortably.

Example 4 — Default 2,500 W light. 2,500 × 1.25 = 3,125 W → 3,500 W, the calculator's default answer. The jump from 3,000 to 3,500 comes from rounding up to a real product size that fully clears the surge.

Portable inverter generators shine at part load. Because the engine speed tracks the load rather than running at a fixed RPM, a lightly loaded inverter unit sips fuel and stays remarkably quiet. Typical figures:

• A 2,200 W inverter generator runs 4–8 hours on a ~1 gallon tank at 25–50% load; many extend to 10+ hours in eco mode.
• A 3,500 W unit runs 7–11 hours on 1.5–2.5 gallons depending on load.
• Noise ranges from about 48–58 dBA at part load for quality inverter units — quiet enough for most campgrounds.

Parallel capability lets you link two identical smaller inverter generators to add their outputs — two 2,000 W units make roughly 3,600–4,000 W usable. This is popular with RVers because a single 2,000 W generator is light and quiet for boondocking, while the pair starts the A/C when needed. If you expect to grow into an air conditioner later, buying a parallel-capable model is smart future-proofing. For fixed installations and larger loads, compare the sizing math in the what size generator do I need calculator.

Campers and RVers often operate at elevation, exactly where portable generators lose the most output. The same derating rules apply: roughly 3–3.5% less power per 1,000 feet above about 1,000 feet, plus about 1% per 10°F above 77°F. A 3,000 W portable at a 7,000 ft mountain campsite delivers only about 3,000 × (1 − 0.035 × 6) ≈ 2,370 W — potentially not enough to start an RV air conditioner that needs 2,800 W of surge. If you camp high, size up one step or fit a soft-start module on the A/C. Many manufacturers offer high-altitude jet kits for their portable engines; check the manual before a mountain trip.

Portable generators split into two designs, and for RV and camping use the choice matters as much as the wattage. Understanding the trade-off keeps you from buying a unit that is loud, thirsty, or unsafe for your electronics.

Inverter portables generate AC, convert it to DC, and electronically rebuild a clean sine wave. Because the engine speed tracks the load instead of holding a fixed 3,600 RPM, they are dramatically quieter (48–60 dBA — think normal conversation), sip fuel at part load, and produce under 3% total harmonic distortion (THD) that is safe for laptops, phones, and sensitive RV control boards. They are also lighter and often parallel-capable. The downsides are a higher price per watt and a lower ceiling — most stop around 4,000 W in a single unit. For campgrounds with noise limits and rigs full of electronics, inverter units are the clear pick.

Conventional open-frame portables run the engine at a constant speed to hold 60 Hz. They deliver more watts per dollar and reach higher outputs, but they are louder (65–75 dBA), heavier, less fuel-efficient at low load, and their higher THD can trouble delicate electronics. They shine on job sites and for high-wattage backup where noise and power purity matter less than raw output and cost.

The parallel option. Many RVers split the difference by buying two small parallel-capable inverter generators: one 2,000 W unit is featherlight and near-silent for boondocking, and linking the pair yields ~3,600–4,000 W to start the air conditioner when needed. It costs more than a single mid-size unit and means hauling two boxes, but it maximizes flexibility and redundancy — if one fails, you still have power. Note that parallel kits only combine two identical units, so buy the pair together and keep both on the same maintenance schedule.

1. Add up the watts you'll run at once. Include your air conditioner, fridge, charging, and any tools that overlap. Enter the total.
2. Pick light or heavy duty. Light for electronics and small loads; heavy for A/C, motors, and power tools.
3. Read the recommended portable size. The tool shows continuous need, surge need, and a real inverter-generator rating.
4. Check A/C startup. If you run an RV air conditioner, confirm the recommended size (or a soft-start module) covers its 2,800–3,500 W surge.
5. Derate for altitude. Above ~3,000 ft, size up one step or add a soft starter.

• Buying a 2,000 W unit for an RV A/C. Without a soft-start kit it usually can't start a 13,500 BTU air conditioner.
• Ignoring the duty factor. Sizing on running watts alone leaves nothing for the compressor surge.
• Forgetting altitude. Mountain campsites cut output 15–25%, exactly where you least expect it.
• Skipping parallel-capability. If you might add an A/C later, a parallel-ready model saves buying twice.
• Running open-frame generators near quiet campsites. Many parks cap noise; inverter units are far quieter.

For a fixed home or larger loads, use the generator size calculator, see real appliance fit at what will a 5,000-watt generator run, and total a whole coach or home with the electrical load calculator for a house.

For portable use above all, weight and noise are part of the sizing decision, not afterthoughts. A generator you can lift into the truck and run without annoying the next campsite is one you will actually bring along, so resist over-buying watts you rarely use. Size for your real simultaneous load plus the duty margin, favor a quiet parallel-capable inverter unit for anything with electronics or an air conditioner, and add a soft starter before you jump a whole size class for the A/C alone.