Domain 3: System Design and Production Estimation
Domain 3 is technical design and production estimation: turning qualified load and site data into an array, inverter, and wiring that meet the yield target and the code. Items test string sizing at temperature extremes, DC/AC ratio choices, and the loss factors behind a production estimate.
Sizing logic
- Annual target (kWh) / specific yield (kWh per kW-year) = array size in kW. Typical fixed-tilt U.S. specific yield runs about 1,200–1,600 kWh/kW-yr; use local data.
- A DC/AC ratio near 1.1–1.3 is normal; clipping losses stay modest at moderate ratios, and the extra DC harvests low-light and shoulder-season hours.
String sizing
- Maximum string voltage uses module Voc corrected to the record low ambient temperature at the site.
- Minimum string voltage uses Vmp corrected to the record high cell temperature; the inverter MPPT window must still be satisfied.
- NEC 690.7 limits PV system voltage to 600 V for one- and two-family dwellings, with higher limits for other occupancies.
- Keep voltage drop modest - commonly 2% or less on DC circuits and 3% or less overall.
Production estimate
- kWh = kW × peak sun hours × 365 × derate. A well-built system derate is often 0.75–0.85, covering soiling, mismatch, wiring, inverter, and availability.
- Module degradation is about 0.5% per year; report year-1 and 25-year values and label the estimate P50 (expected) versus P90 (conservative, exceeded in 9 of 10 years).
Worked example
An 8 kW array at a site with 4.8 peak sun hours and a 0.80 derate: 8 × 4.8 = 38.4 kWh/day; × 365 = 14,016 kWh ideal; × 0.80 = 11,213 kWh/yr, or 1,402 kWh/kW. If the customer needs 12,000 kWh, the array is about 7% short - add modules, accept a partial offset, or reduce load first.
Traps
Using STC Voc for cold-temperature string math; ignoring inverter minimum voltage on hot days; applying a generic derate from a different climate; presenting one production number with no stated assumptions.