Sources and methods — Panel Headroom

Electrical load and panel headroom

National Electrical Code (NFPA 70) article numbers are cited at a high level only. The code is copyrighted; this site does not reproduce its tables. Licensed electricians apply the edition adopted by the local authority having jurisdiction (2023 Article 220, 2026 Article 120, or another).
Existing-loads method (measured demand at 125% plus new load): commonly described for NEC 2023 220.87 / 2026 120.87. Public paraphrase used as a check: City of Orlando, EV Charging Station Permit Checklist; also discussed in trade commentary (Electrical Contractor Magazine).
Optional dwelling-unit method structure (first 10 kVA at 100% and remainder at 40% under 2023 220.82; publicly described 2026 changes include 2 VA/ft², an 8 kVA first tier, and EVSE at 100% outside the tier). Heating/cooling selections are simplified on this site.
EV charging as a continuous load, with Article 625 as the usual home for installation rules: U.S. DOE Alternative Fuels Data Center, “Charging Electric Vehicles at Home,” retrieved 2026-11-11. Branch-circuit 125% sizing (32 A on a 40 A circuit, 40 A on 50 A, 48 A on 60 A) is the usual continuous-load arithmetic; confirm against the adopted code and the EVSE nameplate.

Honda Power Equipment, “What wattage generator do I need?” wattage estimation guide, Home Applications table, retrieved 2026-11-11. Starting wattage in that table is the approximate draw at start, not an increment already added on top of running watts.
Portable generator carbon monoxide: U.S. Consumer Product Safety Commission guidance to use generators outdoors and at least 20 feet from the house, with exhaust pointed away (CPSC portable-generator safety materials and 2026 winter-weather safety tips).

Electricity prices: U.S. EIA, Electric Power Annual Table 2.10, residential 2025 (final), Form EIA-861, epa_02_10.html, retrieved 2026-11-11.
Residential natural gas prices: EIA, average residential price in dollars per thousand cubic feet, annual series, file ng_pri_sum_a_epg0_prs_dmcf_a.xls, release 2026-10-30. Six states without a 2025 annual figure in that file (Connecticut, Maine, Maryland, North Carolina, Rhode Island, South Carolina) use 2024 and are flagged in the data file.
Heat content: EIA, heat content of natural gas delivered to consumers, Btu per cubic foot, 2025, same release date. U.S. average in that series is 1,036 Btu/cf. EIA’s FAQ on converting $/Mcf to $/therm is at FAQ 45 (it rounds the U.S. 2025 average to about 1,037 Btu/cf).
HSPF2, SEER2, COP, and cold-climate criteria (COP at 5°F ≥ 1.75; 70% capacity; HSPF2 minima): ENERGY STAR, Heat Pump Equipment Key Product Criteria, retrieved 2026-11-11, based on 10 CFR 430 Appendix M1.
Conversion 1 kWh = 3,412 Btu: usual EIA conversion. 1 therm = 100,000 Btu.

Angi, “How Much Does It Cost to Upgrade an Electrical Panel? [2026 Data],” updated 2026-10-29: 200-amp panel plus installation $1,300–$2,000; national average upgrade cost $1,600; most homeowners $1,200–$2,000.
Fixr, electrical panel upgrade guide, updated 2025-01-31: upgrade to 200 amps $1,800–$4,500; typical 100-to-200 amp without relocation about $3,000; national average range $1,500–$4,500.
These sources do not agree. The articles treat the discrepancy as a scoping and dating problem, not as a number to average away. Local quotes, meter-base work, and utility fees can fall outside both ranges.

Machine-readable copies of the EIA table and the Honda wattage table used by the calculators live in /data/ on this site.