Inverter sizing & DC/AC ratio

Your array makes DC; the inverter turns it into AC, capped at its rated output. The relationship between how much DC you install and that AC cap — the DC/AC ratio — is one of the most misunderstood design choices. A little oversizing is good; too much wastes panels.

The DC/AC ratio

Divide your array’s DC watts by the inverter’s AC watts. A 8 kW array on a 6.4 kW inverter is a 1.25 ratio. Ratios above 1.0 (oversizing the array) are normal and usually smart, because…

Why oversizing helps

Panels almost never hit their lab-rated (STC) output — heat, angle, haze, and dirt mean they mostly run well below nameplate. So an inverter sized to the array’s peak would sit half-empty most of the year. Installing more DC than the inverter’s rating fills more of its capacity for more hours, raising total energy — especially valuable in cloudier conditions and shoulder seasons.

The limit: clipping

Push the ratio too high and on the brightest few hours the array briefly produces more than the inverter can pass. The inverter simply caps output and the excess is lost — that’s clipping. A small amount of clipping is fine (those peak hours are rare, and you gained energy everywhere else). A lot of clipping means you overpaid for panels whose peaks you throw away. Typical residential designs land around 1.1–1.3, depending on orientation and goals.

Inverter architecture: string, optimizers, or micros

Beyond sizing, you choose how the panels connect — and it directly decides how much shading and mismatch cost you (Module 3):

• String inverter — panels wired in series to one central inverter. Simplest and cheapest, but the whole string tracks a single operating point, so a shaded or weak panel drags the rest down. Best for clean, unshaded, single-orientation roofs.
• String inverter + power optimizers — a small optimizer behind each panel does per-panel MPPT while still feeding a central inverter, so a shaded panel is throttled on its own instead of limiting the string — plus per-panel monitoring. Brands: SolarEdge (integrated), Tigo (retrofit). Better for partial shade or mixed orientations, at higher cost.
• Microinverters — each panel gets its own tiny inverter (DC→AC right at the module), so every panel is fully independent: same shading and monitoring benefits, no central inverter, and you can expand panel-by-panel. Brand: Enphase (also APsystems, Hoymiles). Highest per-watt cost; great for complex roofs.

Optimizers and micros are together called module-level power electronics (MLPE). Rule of thumb: clean simple roof → string; shade or several orientations → optimizers or micros.

The simulator models this. In the system editor, each panel group has a Module electronics choice — String, Power optimizers, or Microinverters. Picking optimizers or micros removes module mismatch and recovers much of the shading loss in your results (biggest effect on shaded or multi-orientation roofs; small on a clean roof, where optimizers even give back a little to conversion loss). Try it both ways on a shaded array and compare the annual energy.

And: string vs. hybrid (for storage)

Separate from the above — a hybrid inverter also charges and discharges a battery. Pick it (over a plain string inverter) if storage is in your plan now or later (Module 7). Microinverter systems add storage a bit differently, with an AC-coupled battery.

Next: batteries — capacity vs. power, parallel units, and backup sizing.