“But my AC nameplate says 34 amps – so a 26 kW gen is enough, right?”

That homeowner question is the single most common sizing trap. A 34 A × 240 V = 8.16 kW running load, but the locked-rotor (starting) surge can hit 65–85 A for a typical 3‑ton AC. Nameplate only tells you running amps. You need to size by real starting watts, and the two brands treat that surge very differently. Below are three concrete cases – each one walks you through the numbers, the mechanical reason, what it means for your install, and when the story flips.

1 The locked-rotor surge: a 34 A AC that needs ~20 kW to start

The number. A typical 3‑ton air conditioner draws 34 A running (8.16 kW). Its locked-rotor amps (LRA) are around 72 A – that’s 72 × 240 × 0.9 (pf) ≈ 15.6 kW starting surge, peaking for 100–200 ms. Both the Generac Guardian 26 kW (rated 26 kW LP / 24 kW NG) and the Briggs & Stratton generator PowerProtect 26 kW (26 kW LP / 24 kW NG) can nameplate handle it. But the mechanism differs: the Generac generator uses a G‑Force engine with a heavy flywheel that stores rotational inertia to bridge the first half‑cycle of the start without voltage dipping below 30 % of nominal. The Briggs PowerProtect uses the commercial‑grade Vanguard V‑twin, which has a stiffer governor response but less flywheel mass – it relies on the engine’s torque rise to pull through the surge.

Worked consequence. In a real install with a 3‑ton AC plus a ½‑hp well pump (≈5.6 kW running, ~11 kW start), the total surge can hit 26+ kW. A Generac 26 kW unit with Smart Management Module (SMM) can shed the well pump during the AC start and bring it back after ~30 seconds, keeping the house online. The same load on a Briggs 26 kW without load‑shedding will either trip the generator’s main breaker or stall the Vanguard engine – not because the engine is weaker, but because the flywheel inertia is lower and the voltage dip during the surge can drop below the ATS pickup threshold (typically 60 % of nominal), causing the transfer switch to disconnect the house.

2 Dual‑fuel derate: why NG robs 8–15 % of your “26 kW”

The number. The Generac Guardian 24 kW (model 7210) is rated 24 kW on LP and 21 kW on natural gas – a 12.5 % derate. The Briggs PowerProtect 26 kW is rated 26 kW LP / 24 kW NG, a 7.7 % derate. That difference comes down to fuel energy content and engine compression: natural gas has roughly 10 % lower BTU per cubic foot than propane, and the engine’s static compression ratio determines how efficiently it can burn that leaner fuel. The Generac G‑Force engine runs a 9.5:1 compression; the Briggs Vanguard V‑twin runs 10.2:1 (per commercial spec), extracting more of the NG’s energy per cycle.

Worked consequence. If you’re on NG (most suburban homes), a “26 kW” Briggs delivers 24 kW usable; a “26 kW” Generac delivers 22–23 kW (depending on model). For a house with a 4‑ton AC (LRA ~85 A, ≈18.3 kW start) plus a 4.5 kW oven, fridge, and lights – total surge ~24.5 kW – the Generac on NG is already at its ceiling with no margin; the Briggs on NG has about 1.5 kW headroom. That headroom is the difference between the generator starting the AC on a 95 °F afternoon and the voltage collapsing before the compressor engages. The mechanism is volumetric efficiency: at the same displacement, a 10.2:1 compression engine makes more power on low‑BTU fuel than a 9.5:1 engine because the flame speed is higher and combustion is more complete.

3 Noise vs real‑world placement: 58 dB vs 69 dB changes where you can put the generator

The number. The Generac Guardian 24–26 kW in Quiet‑Test mode is rated at ~58 dBA. The Briggs PowerProtect 26 kW is rated at about 68–69 dBA normal operating sound. That 10–11 dB difference is not small – acoustically, a 10 dB reduction is perceived as half as loud. The mechanism is enclosure design: Generac uses a multi‑chamber aluminum housing with tuned intake silencer and a low‑speed fan (the Quiet‑Test mode runs the engine at reduced RPM for self‑test only). Briggs uses a standard steel enclosure with less acoustic treatment, and the Vanguard engine at 3600 RPM is inherently louder because of its larger piston displacement and higher exhaust velocity.

Worked consequence. Many local noise ordinances limit standby generators to 65 dBA at the property line. At 69 dBA, the Briggs may force you to locate the unit >25 ft from the property line or install a sound‑attenuating enclosure (another $800–1,500). The Generac at 58 dBA can sit within 10–15 ft of a property line without violating typical 60‑dBA setbacks. For a homeowner with a small lot (50 ft wide), that placement freedom can be the deciding factor – you can keep the gen near the AC disconnect rather than running 60 ft of feeder cable, which saves $400–800 in copper and trenching. The mechanism here is not a spec trick: the lower noise allows a shorter electrical path, which reduces voltage drop and improves starting surge delivery, especially for the AC circuit.

Real‑watts comparison: three decisive specs

Topology/standards per the cited standards; all product ratings are manufacturer-stated values from the cited datasheets, current to 2026-06; derived/illustrative figures are labelled as such. This is not an independent head-to-head test. Generac is a brand affiliated with this site; competitor names are used for identification only.