Solar PV System Sizing for Electricians

Step 1: Annual consumption — from utility bills (typically 10,000-15,000 kWh/year for US homes). Step 2: Required array size (kW-DC) = annual_kWh / (peak_sun_hours × 365 × 0.80). For Phoenix (5.5 sun-hours): 12,000 / (5.5 × 365 × 0.80) = 7.5 kW-DC. For Seattle (3.5 sun-hours): same consumption = 11.7 kW-DC.

Standard residential modules: 400-450W each. A 7.5 kW system = 17 × 440W panels. Commercial modules may exceed 600W. Roof area required: approximately 18 sq ft per 400W panel.

String inverters: sized at 100-120% of array DC capacity. NEC 690.8(A) limits inverter output current to the rating marked on the inverter. A 7.5 kW-DC array typically uses a 7.6 kW AC inverter.

DC/AC ratio (inverter loading ratio): 1.0-1.25 is typical. Higher ratios (up to 1.3) save inverter costs but clip power during peak production. Most design software optimizes this ratio for site-specific conditions.

Microinverters: one per panel, eliminate string sizing constraints and mismatch losses. Module-level power electronics (MLPE) — either microinverters or DC optimizers — are required by NEC 690.12 for rapid shutdown compliance on most residential installations.

Conductor sizing rule: NEC 690.8(B) requires PV circuit conductors sized at 125% of maximum circuit current (Isc) for continuous conditions. Since PV is a continuous source: effectively 1.25 × 1.25 = 1.5625× Isc. Example: 10A Isc string needs conductors rated for 15.6A minimum.

Overcurrent protection: NEC 690.9 requires OCPD rated at ≤ 2 × Isc if two or more strings are paralleled. Each string typically protected with a 15A or 20A fuse in the combiner box.

Grounding: NEC 690.41-690.47 allows both grounded and ungrounded (floating) PV arrays. Most modern string inverters use transformerless (ungrounded) topology with ground fault detection. Equipment grounding conductors follow NEC 250.

NEC 690.12 requires rapid shutdown capability: upon initiation, conductors more than 3 feet from the array or 1 foot inside the building must reduce to ≤ 30V within 30 seconds. Array-level: modules must reduce to ≤ 80V within 30 seconds and ≤ 1V within 30 seconds per module (module-level shutdown).

Compliance methods: microinverters (inherently compliant — AC output stops when grid drops), DC optimizers with rapid shutdown (SolarEdge, Tigo), or listed rapid shutdown boxes at each module (for string inverter systems).

The rapid shutdown requirement is driven by firefighter safety — ensuring that rooftop conductors are de-energized during fire emergencies. Signage requirements: NEC 690.56 requires placards at the main disconnect indicating rapid shutdown type and operation.

NEC 705.12 governs interconnection of PV inverters to the utility grid. Two methods: (1) Supply-side connection (NEC 705.12(A)) — connects between meter and main breaker, no bus ampacity limit. (2) Load-side connection (NEC 705.12(B)(2)) — connects to a breaker in the main panel, limited by the 120% rule.

120% Rule: The sum of all supply breakers (main + PV backfeed) cannot exceed 120% of the bus rating. A 200A panel: 200 × 1.2 = 240A, minus 200A main = 40A maximum PV backfeed breaker. At 240V: 40A × 240V = 9,600W maximum inverter output.

If the PV system exceeds the 120% rule capacity, options include: supply-side tap, main panel upgrade, or center-fed panel with PV breaker at opposite end from main breaker.