Methodology: every formula, every table, every source

This page is the calculator's open book. The tables below are imported from the same data files the calculator executes — they cannot drift from what the tool actually computes. Values marked assumption are ours and are disclosed in the tool's output; everything else traces to the cited source on the date shown.

Educational estimate using a simplified NEC 220.83 method — final sizing must be confirmed by a licensed electrician.

Whole-house mode. Connected load = general lighting and receptacles at 3 VA per sq ft (NEC 220.12) + the running watts of every selected appliance + HVAC. Running load then applies an NEC 220.83(A)-style demand factor: the first 8 kW at 100%, the remainder at 40%. Two disclosed simplifications: we factor the whole connected load including HVAC (the code's 220.83(B) variant takes HVAC at 100% when adding heating/cooling — our approach can size slightly lower for heavy-HVAC homes, which the 80% loading rule cushions), and we fold small-appliance circuits into the 3 VA/sq-ft allowance. Citation basis: EC&M "One-Family Dwelling Unit Load Calculations" (accessed 2026-08-02); the 2026 NEC renumbers 220.83 to 120.83 with a unified table — we cite the method, not the code text.

Essentials mode. No demand factoring. Loads that draw continuously (refrigeration, pumps, fans, lighting, AV) are summed straight. Short-cycle loads (microwave, dishwasher, washer, dryer, range) are excluded from the continuous sum and instead feed the peak check and the duty-cycled energy math. This mirrors how generator wattage worksheets ask "what runs at the same time."

EV charging counts at amps × 240 V (Tesla Wall Connector spec: configurable 12-48 A, up to 11.5 kW). Paused-during-outage EVs are excluded from running load and flagged as requiring a load-management setting.

Surge = running load + the largest single motor-start delta (or the largest short-cycle load, whichever is bigger). Air-conditioner starting demand is derived from manufacturer locked-rotor amps using the 30% voltage-dip method: starting VA ≈ LRA × 0.7 × 240 V × 0.7 ≈ 0.49 × LRA × 240. Basis: Goodman GSXN4/GSXN3 condenser spec sheets for RLA/LRA; the voltage-dip method from L. Rozenblat's generator motor-starting reference (generators.smps.us), both accessed 2026-08-02.

A soft starter reduces the modeled start draw by 65% (midpoint of Micro-Air's published "reduces the peak current by 50 to 70%"), floored at the running load. Well and septic pumps without published LRA use the Cummins/Kohler "2-3× running" guidance at 3×; Franklin Electric's AIM manual (Table 5) is stricter for generator sizing of submersible pumps — its minimum generator sizes (2 kW for 1/2 hp, 4 kW for 1 hp, 7.5 kW for 2 hp) are worth checking if a well pump is your largest motor.

The recommendation is the smallest air-cooled class where 0.8 × rating ≥ running load and rating ≥ surge. The 80% figure follows Kohler's spec-sheet rating note (standby rating applicable to variable loads averaging 80% of rated power) and standard installer guidance. If no class passes but the running load fits under 26 kW, we recommend 26 kW with a load-managing transfer switch; above that is liquid-cooled, licensed-installer territory and the tool says so. Sizing math uses nameplate LP ratings; the natural-gas derate is displayed, never silently applied. Service check: if generator output amps (kW × 1000 ÷ 240) exceed the service rating, we flag a load-managed transfer switch or service evaluation.

For each system we solve two constraints: enough inverters that continuous output covers the running load (and published surge covers the surge), and enough battery modules that usable energy covers duty-cycled demand × outage hours. Usable energy counts 85% of nameplate for the plug-in class (vendors publish nameplate only — disclosed assumption) and 100% for Powerwall 3 (its 13.5 kWh is AC-side on the datasheet). Where a stack cannot honestly cover the load — multi-day electric heat is the classic case — the tool says so instead of quoting a heroic unit count.

Natural gas: indefinite (utility-supplied); we show the interpolated burn rate. LP: usable gallons = tank × 80% (standard fill/reserve practice); hours = usable ÷ burn rate, interpolating linearly between the published ½-load and full-load figures. Battery: usable kWh ÷ duty-cycled average draw. Portable: anchored to the Westinghouse WGen9500DF (example unit) — manufacturer claim of 8 h at 50% load on the 6.6-gal tank (≈0.82 gal/hr), scaled linearly with load. Linear interpolation is a disclosed approximation; real burn curves are not linear.

Every range requires at least three independent sources (manufacturer pricing + retail listings + installer cost guides) and is presented as a range — never a point estimate. Standby installed: 26 kW $13,000-19,500; 10-14 kW $7,000-11,000; 18-24 kW $9,000-16,000. Portable path: unit $700-1,700 + interlock installed $400-850 (to ~$1,500 with permits in high-labor markets). Maintenance: contracts $250-600/yr, service visits $150-450, DIY kit $25-40. Line-item breakdown: install labor (electrical, placement, commissioning) $1,500-6,000; gas line run $500-2,000 (NG); $800-2,500 (LP with tank); automatic transfer switch hardware (often bundled) $500-1,500; concrete/composite pad + site prep $300-1,200; permit + inspection $100-500.

Full source lists render next to every price in the calculator output. The federal 25D residential clean-energy credit expired 2025-12-31; all battery prices shown are gross.

Three headline rules fire first: natural gas + day-plus outages + set-and-forget → standby; under-12-hour outages + no gas + quiet constraint → battery; minimize-cost + manual operation accepted → portable. Everything else falls to a transparent score (fuel, outage length and frequency, budget posture, quiet, electric-heat loads, and whether a battery would need an installer-quote-scale stack). When the top two paths land within the tie band, the tool shows a "close call" instead of fake confidence. Portable is excluded outright by the quiet toggle or a set-and-forget budget.