Generator Load Calculation Worksheet

An installer tells you that you need a 24 kW generator. Is he right, upselling you, or — just as costly — undersizing you to win the bid? A generator load calculation worksheet is how you answer that question yourself: a circuit-by-circuit inventory of running watts, starting watts, and the diversity that separates real peaks from paper ones. This is the quote-checker’s tool, and it takes about an hour with your breaker panel and your appliance nameplates.

Work through the four steps below — inventory, starting watts, diversity, headroom — using the reference tables, then run the worked example and compare your result against any installer’s recommendation. This worksheet sanity-checks a pro’s sizing; it does not replace the licensed electrician’s formal load calculation, permits, or inspections.

Generator load calculation worksheet, step 1: the running-watts inventory

Open your breaker panel and list every circuit you want on generator power during an outage. For each, find the running watts: the nameplate on the appliance (volts × amps = watts), the owner’s manual, or the reference ranges below. Nameplate data beats internet charts — your 4-ton AC’s actual draw is what matters, not the average 4-ton AC’s.

Reference ranges for the usual residential loads (approximate; verify against your equipment):

Load	Typical running watts	Typical starting watts
Central AC / heat pump, 3-ton	3,000–3,800	9,000–17,000
Central AC / heat pump, 4–5 ton	4,500–6,000	14,000–25,000
Gas furnace blower	600–900	1,500–2,500
Electric water heater	4,000–5,500	— (resistive)
Electric range / oven	5,000–12,000	— (resistive)
Electric dryer	4,500–5,500	— (resistive)
Refrigerator	150–400	800–1,200
Freezer (chest)	150–300	600–1,000
Well pump, 1/2 HP	~1,050	2,150–4,100
Sump pump, 1/3 HP	~800	1,300–2,900
Microwave	1,000–1,500	—
Lights + outlets + electronics (whole house)	1,000–2,000	—
Garage door opener	500–750	1,000–1,500
Pool pump, 1.5 HP	~1,500	3,000–4,500
EV charger (Level 2)	7,200–11,500	—

Two notes before you add anything up. First, resistive loads (water heaters, ranges, dryers, anything that makes heat with electricity) have no starting surge — what you see is what you get, which is why electric heat and hot water dominate generator sizing so brutally. Second, write down your numbers in a third column; the ranges above are for orientation, and your worksheet runs on nameplates.

Step 2: the starting-watts adder (motors only)

Motors demand a starting surge — typically 2 to 3 times running watts for common residential motors, and far more for large HVAC compressors, where locked-rotor amps (LRA) on the nameplate times voltage gives the surge in volt-amps. The generator must ride through the largest single starting event while carrying everything else already running.

The worksheet method the pros use: total running watts of everything on simultaneously, plus the starting surge of the single largest motor (or more precisely, the largest motor’s additional starting watts above its running draw — but adding the full surge is the conservative version most DIY worksheets use, and conservative is fine here). You do not add every motor’s surge together, because motors essentially never start in the same second — with one exception noted in Step 3.

So if your running total is 14,000 watts and your largest motor is a 4-ton AC with a 14,000-watt starting surge, your starting requirement is roughly 14,000 + 14,000 = 28,000 watts momentarily. That number looks alarming, and it is supposed to: it is the number that separates a generator that starts your AC from one that stalls trying. Standby generators handle motor starting better than their nameplate kW suggests — this is what motor-starting kVA ratings and load-shedding modules are for — but the worksheet must show the surge honestly before the installer’s experience discounts it.

Step 3: diversity — the part amateurs skip

Diversity is the difference between a paper total and a real peak. Your worksheet’s raw sum assumes the dryer, the oven, the water heater, the AC, and the well pump all demand maximum power in the same minute. They don’t. Thermostatically controlled loads cycle; cooking and laundry are intermittent; the water heater runs perhaps an hour or two a day.

Apply diversity judgment circuit by circuit. The honest approach: include at 100 percent everything that runs continuously or simultaneously by design (HVAC while it’s calling, the refrigerator, the well pump during water use, lights and electronics), include intermittent large loads at a realistic coincidence (the dryer or the range, not both at full tilt — unless your household genuinely runs both), and exclude or defer loads you will shed during an outage (pool heater, EV charger, second water heater) if your transfer switch has load management.

The exception to the one-surge-at-a-time rule: correlated starts. When utility power returns after an outage — or when a generator first picks up the house — every thermostat in the house is calling at once, and multiple compressors can attempt to start within seconds of each other. Good ATS setups stagger this with time delays; your worksheet should note it as a scenario and confirm with the installer how the transfer switch sequences loads on pickup. If the installer has no answer for staggered startup, that tells you something about the installer.

“A load calculation without diversity is a fantasy novel. The art is knowing which loads genuinely coincide — and which ones you’ll shed without noticing.”

The 2026 equipment landscape the worksheet feeds into

The worksheet’s output lands in a market with clear 2026 price bands: 14–18 kW equipment at $3,000–$4,500, 20–24 kW at $4,500–$6,500, and 26 kW+ at $6,000–$8,000+ before installation. Knowing these bands before you run the numbers keeps the worksheet honest — if your honest diversified total lands at 19 kW and you’re tempted to round down to a 14 kW unit to hit a budget, the bands tell you exactly what the next step up costs, which is usually less than the regret of an undersized unit. The worksheet sizes the need; the bands price the answer.

Step 4: headroom, fuel derating, and the final number

Take your diversified running total and add 20 to 25 percent headroom. This covers the loads you forgot, the hotter-than-expected week, and the fact that generators are happiest — and most fuel-efficient — running at 50–80 percent of rating rather than pinned at the redline. Then apply the fuel correction: if you burn natural gas, compare against the NG-derated output (~19.5 kW for a 22 kW unit, ~21 kW for 24, ~22.5 kW for 26 — manufacturer-reported, verify current specs), not the propane headline.

Round up to the next standard standby size. Standby generators come in standard increments (14, 18, 20, 22, 24, 26 kW in the air-cooled residential range), so a worksheet result of 19.3 kW on natural gas means a 24 kW unit, not a 22. Do not round down to save money — the step between sizes is typically hundreds of dollars in equipment, while an undersized generator is a five-figure regret that announces itself during the worst outage of the decade.

Common worksheet mistakes that skew the answer

Four errors show up in DIY worksheets again and again. Mistake one: counting the water heater, dryer, and range at 100 percent simultaneously. These are the three largest resistive loads in most homes, and the raw sum routinely adds 12,000+ watts that never coincide. Unless your household genuinely runs all three at once, diversity-discount them — this single correction moves more worksheets down a size step than any other.

Mistake two: forgetting the well or sump pump. They hide in the panel as quiet 20-amp breakers, but a 1/2 HP well pump’s 2,150–4,100-watt starting surge is a top-three starting event in many homes. Rural worksheets that omit the well pump undersize the surge requirement badly.

Mistake three: using running watts for the HVAC and ignoring LRA. The compressor’s starting surge is the whole game — a worksheet that sizes to the AC’s 4,800 running watts instead of its ~14,000-watt surge produces a generator that stalls on the first hot afternoon of the first outage.

Mistake four: skipping the fuel derate. A worksheet result of “19 kW needed” compared against a 22 kW unit’s propane rating looks comfortable — until you remember the 22 delivers ~19.5 kW on natural gas, leaving almost no headroom. Always compare against the output on your fuel.

Worked example: putting it together

A 2,800 sq ft home, gas heat, one 4-ton AC, electric dryer, well water, standard appliances. The inventory:

Running watts: AC 4,800 + furnace blower 800 + dryer 5,000 + refrigerator 300 + well pump 1,050 + lights/electronics 1,500 + microwave 1,200 (intermittent) = ~14,650 raw. Apply diversity: the microwave and dryer rarely coincide at full draw — count the dryer at 5,000, drop the microwave from the peak, keep the rest: ~13,450 diversified running watts.

Starting adder: the 4-ton AC’s ~14,000-watt surge is the largest single starting event. Starting requirement: ~13,450 + ~14,000 = ~27,450 watts momentary. Add 20 percent headroom to the running side: ~16,100 watts continuous needed. On propane, a 22 kW unit (22,000 W) covers the continuous need with margin and handles the AC surge through its motor-starting capability; on natural gas, the 22’s ~19,500 W output still clears 16,100 with headroom. Verdict: 22 kW, and any installer quoting 26 kW for this house should explain exactly which loads justify it.

Now change one variable: swap the gas furnace for a 3-ton heat pump with 10 kW backup strips, and electrify the water heater. Running total climbs past 20,000 watts diversified; the heat pump’s ~17,000-watt starting surge becomes the dominant event; the strips add 10,000 watts of resistive load in cold weather. The worksheet now lands around 24–26 kW on propane — and on natural gas, the 26’s ~22.5 kW derated output needs the load analysis checked line by line. Same house, same square footage, different fuel mix: two full size steps apart. This is why the worksheet starts at the breaker panel, not the tape measure.

Using the worksheet against an installer’s quote

Now the payoff. When the installer recommends a size, ask to see their load calculation — running watts, starting watts, diversity assumptions — and compare it against your worksheet line by line. The discrepancies are where the money hides: an installer who counted the EV charger and pool heater at 100 percent may be sizing you two steps up; one who ignored the AC’s LRA surge may be sizing you down to win on price.

Three questions separate the pros from the order-takers: How did you account for the HVAC starting surge? Which loads are on the load-management/shed schedule, and what’s the shedding order? What’s my diversified peak against the unit’s output on my fuel? The installer who answers crisply with numbers is the one to hire — licensed, insured, pulling permits, with the transfer-switch amperage, gas work, and warranty terms itemized in writing. Your worksheet got you to the table as an informed buyer; their formal calculation, permits, and inspections carry it across the finish line.

Frequently asked questions