How many “real” starting watts does each generator deliver before voltage sags below 105 V, and how does that proportion affect the appliance you actually need to start?
That single question separates a generator that cranks your well pump from one that stalls on the first motor surge. Below we tear down three dimensions — starting surge vs. steady‑state, voltage distortion under load, and fuel‑energy density — using magnitude proportion to show where the numbers change a real decision.
1. Starting‑surge proportion: how much extra can the alternator deliver?
Every induction motor (well pump, furnace fan, refrigerator compressor) draws 5–7× its running current for the first 100–300 ms. A generator’s “starting watt” rating tells you whether that motor locks in or drops out.
Briggs & Stratton PowerProtect (26 kW LP / 24 kW NG): The spec sheet states 26 kW standby on LP, 24 kW on NG [briggs-stratton-generator] and the engine is a commercial‑grade Vanguard V‑twin [briggs-stratton-generator]. Briggs does not publish a separate “surge” number — but the alternator on this class of 3600 RPM synchronous generator can typically deliver roughly 1.15–1.25× rated for about 5 seconds before the voltage regulator pulls it back. On a 26 kW set that gives about 31 kW (~130 A at 240 V) for surge (illustrative). A 5 hp well pump (running ~2.8 kW) needs ~14–17 kW to start — well inside the proportion. However, a 7.5 hp pump (running ~4.1 kW) needs ~24–29 kW start; you are at the ragged edge of the surge limit.
Honda EU7000iS (5.5 kW running / 7 kW starting): The Honda uses an inverter‑based design that can deliver 7000 W starting from a 5500 W continuous rating [honda-generator]. That is a 1.27× surge proportion, about the same multiplier as the Briggs synchronous set. But note the absolute magnitude: 7 kW starting can handle a 2 hp well pump (start ~6 kW) but not a 3 hp pump (start ~9 kW). The inverter can sustain that surge for maybe 0.5 s before it folds back because the inverter’s IGBTs have a lower thermal mass than a copper‑wound alternator.
Worked consequence: In a home‑standby context (Briggs PowerProtect), the proportion matters for the single largest motor — if your well pump or HVAC compressor exceeds ~70 % of the generator’s rated kW, the surge may drop voltage below 85 V, tripping the motor’s overload. In the portable segment (Honda EU series), the surge proportion is similar but the absolute limit is lower, so you must size the generator to the pump, not to the “running load” of a few lights. When does this dimension invert? If you never start a motor bigger than 1 hp (e.g., a small fridge, laptop chargers, LED lights), the surge proportion is irrelevant; both generators will work. The inverter’s clean sine wave may even be better for sensitive electronics, but the surge capacity is not the limiting factor.
2. Voltage distortion under load: real watts vs. THD
Briggs PowerProtect (synchronous alternator): Typical THD for a 3600 RPM synchronous generator under 80 % load is around 5–8 % (illustrative). That means the voltage sine wave is slightly flattened, causing induction motors to run hotter and draw ~3 % more RMS current for the same torque. Proportion effect: the 26 kW rating is at 0.8 PF; if you load it with motor‑dominated loads (PF ~0.6–0.7), the alternator can deliver about 24 kW real before the voltage drops below -10 % of nominal [briggs-stratton-generator]. The voltage regulator (AVR) holds the output within ± 2 % steady‑state, but during a motor start it may dip to 80 % for a cycle. That works for most residential motors, but the “real watt” is roughly 92 % of nameplate when THD and PF are combined.
Honda EU7000iS (inverter): The EU series produces a sine wave with <2 % THD [honda-generator]. That clean waveform means motors see nearly ideal voltage, so the nameplate 5500 W running translates to ~5400 W real usable watts (only ~2 % derating from THD). For the same 5 hp pump (running ~2.8 kW), the inverter draws about 25 A vs. 27 A on the synchronous machine — a proportional advantage of ~8 % in current for the same work. But the inverter’s overload capability is limited by the DC‑bus capacitors and IGBTs; sustained overloads above ~115 % may trigger a fault within seconds. So the cleaner waveform gives you back some margin, but the absolute headroom is smaller.
Worked decision: In a home‑standby scenario where the largest motor is a 3‑ton AC (starting LRA ~60–70 A), the synchronous Briggs can absorb the start current because the AVR drives field excitation higher, even at the cost of a 15 % voltage sag. The inverter Honda would trip on overcurrent if you tried the same motor — the proportion of surge to steady‑state is similar, but the ability to sustain a sag is missing. When does this dimension flip? If your loads are all resistive (baseboard heaters, water heater, incandescent bulbs) or you have a soft‑start for the AC, the low‑THD inverter gives you the same real watts with less heat in the motor windings. For a pure resistive load, both generators deliver >95 % of nameplate.
3. Fuel‑energy density: how many real watt‑hours per gallon?
Briggs PowerProtect runs on natural gas or LP. Natural gas has a lower energy density than gasoline: ~1000 BTU/ft³ vs. ~114 000 BTU/gal for gasoline. At 26 kW LP, the generator consumes about 2.9 gal/h of LP (roughly 0.11 gal/kWh). When the load is 50 % (13 kW), consumption drops to about 1.6 gal/h. The proportion: energy per gallon of LP is 73 % of gasoline’s (91 500 BTU/gal vs. 125 000 BTU/gal), so for the same real watt output you burn more volume of LP than you would of gasoline. But the generator is designed for continuous standby; the fuel cost per kWh is ~$0.25–0.35 (LP) vs. ~$0.40–0.55 (gasoline) — proportional to your local fuel price.
Honda EU7000iS runs on gasoline. At 50 % load (~2.75 kW), the Honda runs ~16 h on 5.1 gal [honda-generator], consuming about 0.32 GPH — that’s 0.11 gal/kWh, virtually the same proportion as the LP generator. But note: the Honda’s fuel injection (EFI) [honda-generator] gives better part‑load fuel economy than a carbureted generator; at 25 % load the consumption drops to about 0.18 GPH (≈0.13 gal/kWh), proportionally better than the Briggs at low load because the V‑twin Vanguard engine is optimized for higher loads.
Worked consequence: If you need to run a 1.5 kW load for 12 hours (e.g., a refrigerator + lights), the Honda uses ~1.9 gal of gasoline; the Briggs uses ~2.1 gal of LP. The proportion of fuel energy to real energy output is essentially the same (~23 % efficiency), but the logistics differ: LP stores indefinitely, gasoline degrades after ~6 months. When does this dimension invert? For long outages (>24 h) with moderate load, the Briggs can connect to a large LP tank (500 gal) and run for days without refueling; the Honda needs you to have 20 gal of fresh gas on hand. The real cost proportion is not in the generator, but in the fuel supply chain.
⚠️ Failure mode: If you install an inverter generator in a home‑standby role (with a transfer switch) and don’t derate for continuous duty, you will overload the inverter within 2–3 hours. The IGBTs heat up faster than the alternator core, and the inverter will shut down. That’s not a failure of the generator; it’s a failure of sizing proportion.
Dimension‑by‑dimension proportion summary
| Dimension | Briggs & Stratton PowerProtect 26 kW | Honda EU7000iS 5.5 kW | Proportion that matters |
|---|---|---|---|
| Surge proportion | ~1.2× (≈31 kW surge, illustrative) [briggs-stratton-generator] | 1.27× (7000 W start) [honda-generator] | Absolute surge magnitude, not ratio. 26 kW can start a 5‑hp pump; 5.5 kW cannot. |
| Voltage distortion (real watts) | THD ~5–8 %; usable ~24 kW real at 0.8 PF [briggs-stratton-generator] | THD <2 %; usable ~5.4 kW real [honda-generator] | Clean waveform yields ~8 % more usable power per nameplate kW for motor loads. |
| Continuous duty factor | 100 % rated (NFPA 110, standby) | 80 % (4.4 kW continuous) [honda-generator] | Key proportion: 26 kW vs. 4.4 kW continuous — an almost 6:1 difference in real usable energy per hour. |
| Fuel cost per real kWh | ~$0.30/kWh (LP at $2.50/gal, 0.11 gal/kWh) | ~$0.42/kWh (gas at $3.60/gal, 0.11 gal/kWh) | LP is ~30 % cheaper per unit of real energy, but fuel supply limits apply. |
Decision threshold: one proportion rules them all
If your total motor starting load (sum of the single largest motor’s locked‑rotor current × voltage) is below 10 kW, an inverter generator like the Honda EU7000iS delivers proportionally more usable watts per nameplate kW because of low THD and cleaner voltage — your fuel cost will be higher, but the waveform benefit is real. If your motor starting load exceeds 12 kW (e.g., a 5‑hp well pump + a 4‑ton AC with standard start), you need the synchronous alternator of the Briggs PowerProtect, not because the efficiency proportion is better, but because the absolute surge capacity and continuous duty rating are larger by a factor of ~5×. The proportion that decides the choice is the ratio of the largest motor start to 1.2× generator rated kW. If that ratio is >1, you step up to a larger generator or add a soft‑start. Below 0.6, you can use either platform; choose based on fuel source and noise tolerance.