Generac vs Onan onboard RV generator

Both brands publish real test data. The difference is not marketing slogans; it is 68 dB vs 72 dB at 7 meters, 5.5 kW continuous vs 4 kW, and how many hours the engine will run before the first valve job.

Continuous vs surge rating — the number that actually matters

Onan (Cummins) Quiet Diesel 4.0 kW and 5.5 kW models are rated for continuous output at 104°F ambient. Generac 5.5 kW and 7.0 kW RV units publish similar continuous ratings with slightly higher surge headroom on the gasoline/LP versions. The '7,000 watt' marketing number on a Generac is usually the starting surge; continuous is closer to 5,500-6,000 W depending on fuel type and altitude.

Real load math: two 15k BTU roof ACs on a 40 ft coach can pull 3,200-3,800 W running once both compressors are up. Add microwave (900-1,200 W), induction cooktop (1,800 W), and water heater element (1,400 W) and you are at or over the continuous rating of a 4-5.5 kW unit. The diesel Onan 5.5-8 kW units (common on diesel pushers) give more headroom because the engine does not bog as easily under sustained load.

Per Cummins Onan and Generac published derating tables, every 1,000 ft above 500 ft costs ~3-4% power on gasoline units. LP units derate less. Diesel units hold power better with altitude. If you boondock at 8,000 ft in Colorado with two ACs, a 5.5 kW gas unit may only deliver ~4.2 kW continuous — enough to run one AC + essentials, not both.

Noise — the spec that determines whether neighbors hate you

Onan Quiet Diesel 5.5 kW is rated ~67-69 dB(A) at 7 meters (23 ft) under full load in Cummins literature. Generac 5.5-7 kW gas/LP units are typically 70-73 dB at the same distance. 3-5 dB is perceptible; 6 dB is roughly twice as loud to the human ear. In a tight campground with 30 ft spacing, the difference is the difference between 'background hum' and 'that generator is annoying'.

Installation location and enclosure design matter as much as the raw number. A generator mounted under a slide-out or behind a bedroom wall will sound louder inside the coach than the 7-meter spec suggests. Diesel units with water cooling and heavier enclosures are quieter inside the rig than air-cooled gas units of the same wattage.

State and federal park quiet hours are real (see the dedicated quiet-hours article in this batch). Many CA, WA, and CO state parks enforce 10 pm-8 am generator curfews with dB limits at the site boundary. A 72 dB unit that is legal on paper can still get you a warning if the enclosure leaks sound toward the next site.

Runtime, fuel type, and maintenance reality 2026

Gasoline Onan and Generac units in RV duty are typically rated for 1,000-2,000 hours before major service (valves, rings). Diesel Onan units on pushers routinely reach 4,000-6,000+ hours with documented oil analysis and scheduled injector work. LP versions of either brand sit in between.

Fuel consumption at 50% load (realistic for one AC + lights + fridge): a 5.5 kW gas unit burns ~0.6-0.8 gal/hr; the equivalent diesel ~0.35-0.45 gal/hr. Over a 10-day boondock with 4 hrs gen per day, that is 24-32 gal gas vs 14-18 gal diesel. The diesel advantage grows if you are running the gen 6-8 hrs/day for work or hot weather.

Per EPA and CARB certification data, newer 2025-26 units from both brands meet Tier 4 / Phase 3 emissions when new. Real-world emissions after 1,000 hours depend on maintenance (air filter, oil, fuel quality). Dirty fuel in a gas unit produces more carbon and valve deposits than clean diesel with good filtration.

Frequently asked questions

Is a diesel generator worth the extra $4,000-7,000 on a new coach?›

Only if you run the generator more than ~300 hours per year or boondock at altitude with multiple ACs. For weekenders who run the gen 40-80 hours/year, a quality gas unit is adequate and cheaper to service.

Can I add a soft start to a 4 kW generator to run two ACs?›

Micro-Air EasyStart and similar devices reduce inrush by 60-70%. On a 4 kW unit you can often start one 15k BTU AC, then the second, but sustained dual running may still exceed continuous rating once both are at speed. Test before you rely on it.

How loud is 'too loud' for most campgrounds?›

Anything over 70 dB at 20-25 ft is noticeable to neighbors in still air. 65-68 dB is the sweet spot for modern diesel or enclosed gas units. Many parks post 60 dB at site boundary as the de-facto limit even if not printed.

Do I need to exercise a generator if I mostly use shore power?›

Yes. Both brands recommend 2-4 hours under load every 4-6 weeks to keep seals lubricated, charge the starting battery, and prevent varnish in carb/injectors. A 2-hour no-load run is better than nothing but not as effective as real load.

Which brand has better parts availability in remote areas?›

Cummins/Onan has a larger established RV dealer network for diesel units. Generac has grown fast in the portable and home standby space; RV-specific support is good but not yet as dense as Onan in every state.

Continue your research on PickRV

Sources

1.Cummins Onan RV generator specifications and dB ratingsCummins Onancompany or industry source
2.Generac RV generator continuous/surge and noise dataGeneraccompany or industry source
3.EPA nonroad spark-ignition engine emission standardsU.S. EPAgovernment or university source
4.RV Industry Association generator installation guidelinesRVIAcompany or industry source
5.CARB generator certification lookupCalifornia Air Resources Boardgovernment or university source

Sources are a mix of government and university publications and the company or industry documentation for the specific equipment, store policy or standard discussed — each row above says which one it is, so you can weigh it yourself. Last verified 2026-06-05. Report broken links to support@pickrv.com.

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