Run time rating (at 25% / 50% load)

What the load fraction means in watts

A claim of “12 hours at 50% load” means half of the rated running watts, not half of the starting watts, on a full tank of whichever fuel is being quoted. On a 7,500 W running unit that is 3,750 W; on a 3,500 W inverter it is 1,750 W. The same percentage therefore describes completely different households, which is why comparing headline runtimes across differently sized generators tells you almost nothing.

A realistic essentials load looks like this: a refrigerator averaging 100–200 W as it cycles, a furnace blower at 400–800 W when it is calling for heat, LED lighting at 50–150 W, a modem, router, and chargers at 50–100 W, and a sump pump at around 800 W intermittently. That totals roughly 1,000–1,800 W most of the time. On a 3,500 W inverter that is 30–50% load, and the half-load runtime figure is a fair planning number. On a 9,500 W open-frame unit the same load is 10–20%, well below the quarter-load figure on the spec sheet.

Which is an argument against oversizing that has nothing to do with price. An engine held at 3,600 RPM under 12% load is burning a large share of the fuel it would burn at 40%, all night, to make very little electricity. Matching the generator to the load you will actually run is a fuel-economy decision as much as a purchase decision.

Why the fuel curve bends

An engine burns fuel to overcome its own friction, its pumping losses, and the magnetising load of the alternator before it delivers a single watt to an outlet. At a fixed 3,600 RPM that overhead is roughly constant regardless of electrical output. Consumption therefore looks like a fixed amount plus a proportional amount, which is why quarter-load burn typically lands at 55–70% of half-load burn rather than at 50%, and why no-load burn on a mid-size open-frame unit is often 0.2–0.35 gal/hr.

Spec sheets that publish both figures show this directly. A unit quoting twelve hours at 25% and eight hours at 50% from the same tank is telling you that quarter-load consumption is two-thirds of half-load consumption. Where only the 25% figure is published — and it usually is, because it is the larger number — you can estimate the 50% runtime at roughly 60–70% of it and be close enough to plan with.

Inverter generators bend the curve favourably. Because the engine can drop toward 2,000 RPM at light load, the fixed overhead falls with the speed rather than staying constant, which is why inverter runtimes at 25% look implausible next to conventional units of similar output. It is also why eco mode is worth switching on for overnight refrigerator-and-router duty, and worth switching off before starting a big motor, since the engine needs to already be at speed to supply the surge.

Planning an outage around the number

The question that actually matters is whether the generator gets through the night without somebody going out at 3 a.m. with a jerry can in the rain. Work backwards: pick the unattended hours you need, usually eight to ten; estimate your real running watts; find the consumption figure closest to that load fraction; and check it against the tank. If it does not reach, the three fixes are a smaller load, a bigger tank, or propane from a larger cylinder.

Refuelling is a scheduled shutdown, not a top-up. Gasoline spilled onto a hot muffler ignites, and every manual specifies stopping the engine and letting it cool — five to fifteen minutes is the usual figure. That means the refrigerator and the furnace are off during refuelling too, so it is worth doing at a convenient moment rather than at the moment the engine coughs and dies.

Extended-run fuel tanks — auxiliary tanks that gravity-feed the generator’s own — solve the interval and bring real caveats. They defeat the certified evaporative-emissions fuel system on newer EPA and CARB units, they generally void the warranty, and a poorly sealed one is a gasoline-vapour source sitting beside a running engine. On propane the equivalent is safer and simpler: a 100 lb cylinder, or two 20 lb cylinders on a changeover regulator, which lets you swap one while the other keeps the generator running.

The limits that are not about fuel

Portable generators are not continuous-duty machines. Manufacturers typically specify an oil change after the first 20–30 hours of operation and every 50–100 hours or annually thereafter. A four-day outage at twenty hours a day is eighty hours, which means a mid-outage oil change on most units. Have oil, a filter if the engine takes one, and a spare spark plug in the same cupboard as the cords, because none of them will be purchasable locally during the outage.

Low-oil shutdown is the sensor most likely to end a run early, and on most small engines it triggers on level rather than on pressure. A generator sitting on a slope can therefore shut itself down with perfectly adequate oil in the sump. Level the machine on a flat, dry surface, and check the level daily during a long outage rather than trusting that it has not burned any.