How to Size a Standby Generator From a Site Walk

Why this matters

A standby quote is only as good as the load sheet behind it, and the load sheet is only as good as the walk. Size from the square footage or the panel's main breaker and you either sell a unit too big for the customer's budget or one that drops the house when the air conditioner starts. Size from nameplates read on site and you sell the right unit, explain why, and have a document the inspector and the next tech can follow. This is the walk, step by step. The method behind the totals is 'Generator Sizing for Residential'; starting arithmetic is owned by 'Motor Starting kVA and the NEC Code Letter'.

Before you start

Bring: a camera, a flashlight, a clamp meter, the load sheet (paper or the job form), and the spec sheets for the units you expect to quote.

Check first: what fuel is at the house (natural gas service, an existing propane tank, or neither), what the customer wants the generator to do in their own words, and whether anyone in the house depends on powered medical equipment, which moves those circuits to the top of the list whatever else happens.

What you will not do on a sales walk: remove a panel dead front, open a condenser's electrical compartment, or meter anything energized. Nameplates are read where they can be read with covers on. If a reading truly needs a cover off, that is a visit by a licensed electrician qualified for the exposure, in the PPE the shop's NFPA 70E program sets, not a step in a sales call.

Step 1: Walk the panel first

Open the panel door (not the dead front) and photograph the panel schedule and every breaker label. Note the main breaker size, the service voltage, and any subpanels.

Why first: the panel is the map of everything the house can draw. Walking the house before the panel means finding loads in the order you happen to pass them, and missing the ones in the attic, crawlspace or detached garage.

What breaks if you skip it: the well pump on its own breaker labeled "spare" never makes the sheet, and the generator is quoted without the one load a rural customer cannot live without.

The main breaker is not the load. A 200 A main on a house that peaks at 60 A tells you the service size, not the generator size.

Step 2: Read every motor and heating nameplate

Walk to each significant load and photograph its nameplate. Record volts and amps, or watts. For motors, record running current and either locked-rotor amps or the code letter.

• Condenser or heat pump: the outdoor unit's data plate gives compressor RLA and LRA. Read it off the cabinet; do not open it.
• Well pump: the control box label usually gives horsepower and volts; the code letter is on the motor's own nameplate down the well, so look for it repeated on the control box or in the install record.
• Furnace or air handler blower, sump and ejector pumps, refrigerators and freezers: volts and amps from the label.
• Resistance loads: water heater, dryer, range, baseboard heat, in watts.

Why nameplates and not typical values: two 4-ton condensers can differ by a third in locked-rotor current. A typical-value chart sizes the average house, and you are standing in this one.

What breaks if you skip it: the largest starting load is guessed, and the largest starting load is what sizes most residential generators.

Step 3: Note the gas and electric appliances separately

As you walk, mark which appliances run on gas or propane. Gas appliances barely load the generator electrically (a furnace's blower and igniter, a gas range's controls), but they share the fuel supply with it. Record each one's input rating in BTU per hour from its plate.

Why: the electrical load sheet and the fuel load sheet are two different documents built from one walk. The gas total goes to the fuel side, converted per 'Natural Gas and Propane Heat Content and Generator Consumption', and the gas fitter sizes the supply from it.

Step 4: Decide the scope with the customer

Two scopes, and the choice changes the unit:

• Whole house: every circuit on generator, usually with a service-rated transfer switch. Most comfortable, largest unit, or a smaller unit with load management.
• Essential loads: a chosen list moved to a subpanel fed by the transfer switch. Smaller unit, but the customer lives without whatever is off the list.

Put the decision in the customer's words on the sheet. "Heat, water, fridge, freezer, sump, some lights, the AC" is a scope. "Essentials" is not.

Step 5: Fill the sheet and flag the largest motor

Here is the sheet from a ranch house on natural gas with a 200 A panel, the same condenser and well pump worked in 'Motor Starting kVA and the NEC Code Letter'. Running kW for each load is volts times amps; starting kVA comes from that card.

SITE-WALK LOAD SHEET (example) ranch, 200 A
Item             Plate          Run kW  Start kVA
Condenser 4-ton  RLA 18.6 A      4.46   25.0 BIG
Well pump 1.5hp  9.6 A, ltr K    2.30   13.5
Furnace blower   7.0 A 120 V     0.84
Refrigerator     6.0 A 120 V     0.72
Chest freezer    4.0 A 120 V     0.48
Sump pump        7.0 A 120 V     0.84
Lights, recepts  estimate        1.50
Microwave        1,500 W         1.50
ESSENTIAL RUNNING TOTAL         12.64
Water heater     4,500 W         4.50
Dryer            5,600 W         5.60
WHOLE-HOUSE RUNNING TOTAL       22.74
GAS: furnace 100,000 BTU/h, range 65,000 BTU/h

Reading it. Each motor's running kW is its volt-amps written as kW: 18.6 A times 240 V is 4,464 VA, entered as 4.46. That overstates real kW slightly, because a running motor's power factor is below 1.0, and overstating is the right direction on a sizing sheet. The condenser is flagged BIG because its 25.0 kVA start is the largest on the site, nearly twice the pump's 13.5.

Why one motor is flagged: the working rule from the motor-starting card is that the generator carries the running total of everything except the largest motor, plus that motor's starting kVA. Only one motor gets the starting figure, and the sheet has to make clear which one.

Step 6: Total it and compare against the derated rating

Essential scope. Running total 12.64 kW. Everything except the condenser is 12.64 minus 4.46, which is 8.18 kW. The momentary requirement during a condenser start is 8.18 kW running plus 25.0 kVA starting, 33.2 when added as plain numbers. Adding a low-power-factor start to a running kW total that way overstates the true combined figure, which again is the safe direction.

The candidate is a unit rated 19.5 kW on natural gas. At this site, 300 ft and a 90 F design day, its derated output works out to 19.2 kW (13 F above a 77 F rating basis at the sheet's 1 percent per 10 F; the method is in 'Reading a Generator Spec Sheet and Derating'). Running margin: 19.2 minus 12.64 is 6.56 kW, about 34 percent of 19.2.

For the start, say the unit's spec sheet states it can start up to 35 kVA at its rated voltage dip; that figure is illustrative, so read the real line off the real sheet. 35 against 33.2 passes by 1.8 kVA. That margin is thin, so the quote notes that the start gets checked at commissioning, and that a soft starter on the condenser is the fallback if it fails.

Whole-house scope. Running total 22.74 kW against 19.2 kW available. Short by 3.54 kW before any start. Two ways to make it work:

• Load management. A load-shed module that holds the water heater and dryer off while the generator runs, or sheds them when the load climbs. Shedding both removes 4.50 plus 5.60, 10.10 kW, which brings the running load back to 12.64, the essential figure, with the comfort of a whole-house switch. NEC 702.4, as adopted by your jurisdiction (numbering inside it has moved between editions), in substance allows load management to limit what an automatically transferred standby must carry.
• A larger unit whose derated output clears 22.74 kW with margin, plus its own start check.

Present both, with the customer's own trade-off in plain words: "the bigger unit lets you dry clothes during an outage; the smaller one with load management doesn't, and costs less to buy and fuel."

Step 7: Write down what the sheet assumed

Every sheet carries its assumptions in one block: the derate basis used, which motor was flagged, whether the thermostat or load management keeps the condenser and pump from starting together at transfer, and the estimate used for lighting and receptacles. If the condenser and pump start together, the momentary demand is 25.0 plus 13.5, 38.5 kVA plus the rest of the running load, and the 35 kVA unit fails. Whether that happens depends on controls; 'Motor Starting kVA and the NEC Code Letter' covers the two-motor case, and the sheet must say which controls are relied on.

Why it matters later: the inspector reviewing capacity, the installer setting up load management, and the tech answering a complaint in two years all need to know what the number meant.

Finish test: does the sheet hold up?

• Every breaker on the panel photo is either on the sheet or marked excluded. Recount: the photo shows the breaker count, the sheet shows the loads. A 240 V breaker with no matching load is a question for the customer, not a blank.
• One motor flagged largest, with its starting figure traceable to a plate.
• Running total recomputed. Add the column again from the bottom up; the essential total here comes back to 12.64.
• Compared against the derated rating, not the brochure figure, with the derate shown.
• A sanity check against real use. Where the utility offers interval data, a year of the house's peak demand should sit below the whole-house running total, because not everything runs at once. A measured peak above the sheet's total means a load was missed.
• The gas total handed to the fuel side with the generator's full-load consumption added.

At commissioning, the start is proven for real: the house on generator, the condenser called, the generator's voltage dip read on a min-hold meter. A sheet that passed on paper and failed that test is the case written up in 'The New Standby That Could Not Start the Heat Pump'.

References

• NFPA 70, National Electrical Code, 702.4 (optional standby capacity and load management), as adopted by your jurisdiction; check the current edition.
• NFPA 70E, current edition, for any reading that requires an energized cover off.
• Generator maker's spec sheet for derated output and motor-starting capability.
• See related: Generator Sizing for Residential; Motor Starting kVA and the NEC Code Letter; Reading a Generator Spec Sheet and Derating; Natural Gas and Propane Heat Content and Generator Consumption.