Methodology & sources

Every formula, constant, and limit of the backup power planner, in the open. Found an error? support@whetstonetools.com.

1 · Generator sizing — the single-largest-surge rule

minimum size = (sum of all running watts) + (the single largest additional startup surge) , then × 1.25 safety margin, rounded up to the next standard generator size.

This is the rule Generac, Champion, and Honda all state. Motors (compressors, pumps, blowers) draw 2–3× their running watts to start — locked-rotor inrush can be 3–7× for 1–3 seconds. Resistive loads (space heaters, electric water heaters, ranges, microwaves, lights) have no meaningful start surge. You budget only the single biggest surge because motors almost never start at the same instant — but you must include the largest one, or the generator stalls when it kicks on. Adding every surge together massively oversizes; ignoring surge dangerously undersizes.

2 · Appliance load table

Running watts and additional startup surge per appliance, from the Generac "Estimating Power Needs" two-column chart, cross-checked against Champion and DOE. Where sources disagreed we took the more conservative (higher-surge) number, because an undersized generator that can't restart a well pump means no water in an outage. Notable safety-driven choices:

• Central AC (3 ton): ~16,000 W surge. A standard compressor's locked-rotor amps put the real startup spike near 15,000–18,000 W — not the ~3,500 W some charts imply. A typical 5,000–7,500 W portable cannot start it without a soft-start kit; the tool says so and points you to home-standby or a soft-start (which cuts startup surge 50–70%).
• Well pump (3/4 HP): 4,500 W surge (high end). Sources range 3,000–4,500 W; we default to the high end for safety. If your pump's nameplate lists locked-rotor amps (LRA), the real surge ≈ volts × LRA.
• Window AC (12,000 BTU): uses the Champion figure (~1,100 run / 2,200 surge), not a 9,750 W outlier that appears in one chart for an apparent through-wall heat-pump unit.

3 · Battery sizing — capacity and inverter surge

daily energy = Σ (running watts × realistic hours-per-day per load). Hours-per-day use real duty cycles, not peak — a fridge nameplated at 700 W only runs about 1/3 of the time (DOE). Battery nameplate size = usable energy ÷ 90% depth-of-discharge ÷ 95% inverter efficiency, × your days of backup.

The honest part most calculators skip: a battery can hold plenty of kWh and still fail to start a motor if the inverter can't deliver the surge. The tool checks both, and flags a hard fail if your largest motor surge exceeds a typical home battery inverter's ~2× (10-second) surge rating — in which case capacity won't save you; you need a bigger inverter, parallel units, or a soft-start.

4 · Fuel & 20-year cost

Portable fuel burn at ~50% load from Generac/EPA-class figures (interpolated by size); standby figures from Generac spec sheets. Manufacturer standby numbers vary by model and year and are not perfectly monotonic — treat all fuel figures as ±20% estimates. The 20-year comparison sums equipment, install, fuel (your outage-hours/year × fuel price), maintenance, and one replacement where applicable, vs a battery's equipment + a projected mid-life replacement.

Tax credit, 2026: the 30% federal Residential Clean Energy Credit (IRS §25D) that covered home batteries ended for installs after December 31, 2025 (One Big Beautiful Bill Act). The tool does NOT apply it. Check separately for state and utility incentives.

5 · What the tool refuses to do

• No electrical / transfer-switch / sub-panel design. That requires a licensed electrician and a permit. Backfeeding a generator into house wiring without a proper transfer switch or interlock can electrocute utility line workers.
• No runtime guarantees. Estimates come from rated 50%-load figures; real runtime varies with load, temperature, fuel, altitude, and age.
• No medical-device reliability claims. For any CPAP, oxygen, or life-sustaining device, consult the device spec and your clinician, keep a manual backup plan, and register with your utility's medical-priority program.
• It can't know your real outage hours or local fuel prices — those are your inputs, with conservative defaults named as assumptions.

Sources

• Appliance watts & the running-plus-largest-surge rule: Generac "Estimating Power Needs" chart and support.generac.com; Champion Power Equipment wattage chart; Honda generator sizing guide.
• Refrigerator duty cycle (1/3 rule): US DOE, energy.gov "Estimating Appliance and Home Electronic Energy Use."
• Carbon-monoxide safety: US CPSC Portable Generator safety center (~85–100 deaths/year; lethal in minutes in an enclosed space); UL 2201 / PGMA G300 CO-shutoff standards.
• Battery sizing, depth-of-discharge, inverter surge: EnergySage storage guides; battery-sizing references.
• Battery cost (~$1,000–1,450/kWh installed) and degradation: EnergySage.
• Tax-credit termination: IRS Residential Clean Energy Credit page; OBBBA §25D termination after 2025-12-31.
• Standby install cost ($7,500–16,000): HomeGuide / Angi 2026.

Disclaimer: planning estimates only — not safety, electrical, medical, or purchasing advice. Generator and transfer-switch installation must be done by a licensed electrician per local code and permit.