How to Size a Solar Inverter
The simple relationship is inverter AC kW = array DC kW ÷ DC/AC ratio. A well-chosen DC/AC ratio keeps the inverter operating near its peak efficiency most of the time. PV panels rarely reach their STC (Standard Test Condition) nameplate output in real weather — they are rated at 25 °C cell temperature, which is much cooler than a 40 °C hot summer afternoon. Oversizing the DC array relative to the inverter squeezes more usable energy out of a given inverter size with minimal clipping.
Typical DC/AC Ratios by Region
| Region | DC/AC ratio | Why |
|---|---|---|
| Low DNI (Pacific NW, UK, Germany) | 1.10-1.20 | Lower peak production → less clipping benefit |
| Moderate DNI (Northeast US, Midwest) | 1.20-1.30 | Standard sweet spot |
| High DNI (Southwest, Southern CA) | 1.15-1.25 | Would clip more, so slightly less oversizing |
| East/West split arrays | 1.30-1.45 | Never both peak at once → more DC safe |
| Battery-coupled systems | 1.00-1.10 | Want to harvest every DC watt into the battery |
Common System Sizes
- Small residential: 12 × 400 W = 4.8 kW DC → 4 kW inverter (DC/AC 1.20)
- Typical residential: 24 × 400 W = 9.6 kW DC → 7.6-8 kW inverter (DC/AC 1.20-1.26)
- Large residential: 36 × 415 W = 14.94 kW DC → 11.4-12 kW inverter (DC/AC 1.25-1.31)
- Small commercial: 80 × 540 W = 43.2 kW DC → 36-38 kW inverter (DC/AC 1.14-1.20)
NEC 690.8 Breaker Sizing
The NEC requires the AC output circuit breaker to be sized at 1.25 × the inverter's maximum continuous output current. For an 8 kW 240 V inverter that is 33.3 A × 1.25 = 41.7 A, which rounds up to a 50 A breaker on a 6 AWG THWN circuit. The 125% rule applies to all continuous-duty circuits per NEC 210.19 and is separate from the interconnection "120% rule" used for backfeed panel sizing.
Frequently Asked Questions
What is clipping?
When the array produces more DC power than the inverter can convert, excess energy is dissipated as heat in the PV modules. On a 1.25 DC/AC ratio, clipping occurs only during peak sun and typically costs 1-3% of annual production.
Should I use a microinverter system?
Yes if your roof has shading, multiple orientations, or if you plan to expand in phases. Microinverters (Enphase) give panel-level MPPT and flexible string sizing.
What size wire from inverter to panel?
Size for 125% of max AC current (matching the breaker rule) per NEC. For a 40 A breaker, that's 8 AWG copper THWN minimum.
Do I need a rapid shutdown?
NEC 690.12 requires rapid shutdown within 1 ft of the array boundary within 30 seconds of activation. Microinverters handle this at the panel level; string systems need module-level rapid shutdown devices.
How do I calculate actual kWh production?
Use NREL's PVWatts — it accounts for your local solar resource, panel tilt, azimuth, and system losses. A typical residential system produces 1,200-1,600 kWh per installed kW per year in the US.
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