PV Inverter Loading Ratio (DC:AC) and Clipping Onset

The array-to-inverter power match: the inverter loading ratio (DC nameplate over AC rating, the DC:AC or ILR).

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Checked against published worked examples from: US EIA.

Details, formula, and sources

ScopeWith the cost-optimal band (commonly 1.1 to 1.3) and the clipping onset as a fraction of STC nameplate. An array is deliberately oversized so it fills the inverter's ceiling for more of the day, but push it too far and the inverter clips every clear-day peak. The clipping ceiling is fixed by the inverter; raising the array lowers the fraction of nameplate at which it bites - the diminishing return that puts the sweet spot near 1.2. A sizing sanity check, not a clipping-loss model.

Formula or tableilr = dc_kw / ac_kw; clip_dc_kw = ac_kw / inv_eff; clip_fraction = clip_dc_kw / dc_kw; verdict by band (< 1.1 oversized, 1.1-1.3 optimal, > 1.3 undersized).

Source and editionThe inverter loading ratio (ILR / DC:AC ratio) and NREL inverter-sizing guidance (clipping begins where the array DC output exceeds P_ac / eta_inv), by name; the relations are public.

Free accessThe loading-ratio definition and the clipping-onset relation are public; NREL inverter-sizing guidance is free at nrel.gov.

What governsEstimate. AHJ and licensed professional govern.

API field namesdc_kw, ac_kw, inv_eff, ilr, clip_dc_kw, clip_fraction

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