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Read the nameplate: amps, RLA and LRA for generator sizing

Every sizing chart wants your appliances in watts. Your appliances have never heard of watts; their labels speak amps. This page translates: where the numbers hide, what running amps, RLA and LRA each mean, how to turn them into the watts a generator has to carry, and what to do about the one figure almost no label prints (the starting surge). Every worked example below comes from real plates read in a real house.

Where the plate hides

• Central AC: a metal plate on the side of the outdoor condenser. This is the richest label in the house: it prints running and starting figures both (RLA and LRA, below).
• Fridge / freezer: inside the door wall on many fridges; on the back panel for most freezers. Expect amps and volts, usually nothing about starting.
• Sump, well and septic pumps: a small plate or sticker on the motor, or, when the motor sits in a pit, the manual. Pump makers also publish amp draws in their spec tables.
• Dryer / range: on the back panel, or inside the door frame. Ranges print a total kilowatt rating (all elements at once) rather than amps.

If you can read the label, take a photo of it. The model number alone is often enough to pull the same figures from the manufacturer's manual online.

The numbers, translated

One warning: nameplate running amps are maximum ratings. A fridge stamped 8.5 A does not draw 8.5 A around the clock; it cycles, and the plate covers the worst case (defrost heater, ice maker, hot day). Sizing on plates is deliberately conservative; you are buying headroom, not error.

Worked examples: real plates, converted

Two lessons in that table. First, the ACs: tonnage barely matters to the surge: the 1.5-ton's locked-rotor draw is nearly the 2-ton's, because LRA tracks compressor design more than cooling capacity. Second, notice which rows say "(from LRA)" and which say "(estimated)". That split is the whole problem, next.

The number that isn't printed

Starting amps, the surge, appear on AC plates as LRA, and occasionally on freezers. On almost everything else (fridges, pumps, washers) the label gives you running amps and silence. There are three honest ways across that gap:

• Find the manual by model number. Pump and motor makers often publish locked-rotor or start amps in spec tables even when the plate omits them.
• Estimate from the appliance type. Motor loads surge in fairly predictable multiples of their running draw: fridges and freezers roughly 3×, sump pumps around 1.6–2×, well pumps around 2×. Our sizing calculator does exactly this when you enter amps off a plate without LRA, and marks the result (est.).
• Measure it. A clamp meter with an inrush (peak) mode on the circuit reads the real surge in one start. Electricians do this; it settles every argument.

What not to do: size a generator on running watts alone. The running total tells you what the machine must carry all day; the largest single surge tells you whether it survives the next motor start. Generators fail outages on the second number.

Nameplate math vs. a real outage

Plate math is worst-case math, and a real outage is kinder than the arithmetic in three specific ways:

• Voltage sag works in your favour. When a big motor starts on a generator, voltage dips for a moment; you see it as lights dimming. A locked rotor at sagged voltage pulls less than its full-voltage LRA, so the real surge lands under the plate figure.
• Running amps are ceilings, not averages. The loads you summed from plates rarely all draw their maximum in the same minute.
• The surge lasts about a second. Generators, inverter models especially, ride brief transients that a strict reading of the "starting watts" rating says they shouldn't.

That is why a real house can run, on an 11,000 W-starting inverter generator, an AC whose plate surge computes to 14 kW: lights dim, the engine hunts for a second, and it holds. The case nameplate math still warns about, correctly, is two motors starting in the same second. Add the two AC plates above together, 58.3 + 56.3 A, roughly 27 kW for an instant, and sag stops rescuing you. Independent thermostats make that coincidence a matter of luck over a long outage. Two things remove the luck: a soft starter on the compressor (planning figure: starting current around 0.4 × LRA, per Hyper / Eltwin application guidance), or load management that staggers motor starts.

Do it with your own plates

The generator sizing calculator accepts the numbers exactly as your labels print them:

1. Tick the appliances you need through an outage.
2. On any row, tap custom, switch to Amps (nameplate), and enter the running amps and volts from your plate. Add LRA if it is printed, and the starting figure then comes straight from your equipment, not a typical.
3. Read the recommendation, and try the soft-starter scenario if an AC is in your list.

Or see it done: open the calculator with a real house loaded: two ACs, twin sump systems, a septic pump and four cold boxes, every figure from plates like the ones in the table above.

Sources & notes

• Worked examples: nameplates read directly from equipment in one house (fridge, freezer, two central ACs), October 2026; amps-to-watts conversions as shown. Anonymized; readings are maxima as printed, not measured averages.
• Sump pump figure: Glentronics (Basement Watchdog) primary-pump specification (½ HP model, 115 V, 4.2 A), from the manufacturer's instruction manual.
• Typical surge multiples and appliance figures: TallyGauge appliance wattage dataset, compiled from manufacturer data.
• Soft-starter planning figure (reduced start current ≈ 0.4 × rated LRA for conventional single-phase scroll compressors): Hyper Engineering / Eltwin SureStart application guidance and datasheet, as used in the calculator's soft-start scenario. An estimate, not a guarantee of any specific start.
• This is sizing guidance built on nameplate figures, not electrical advice. Confirm transfer equipment, circuits and load management with a licensed electrician.