Grid-tied mode evaluates the DC:AC ratio (ILR — Inverter Load Ratio) between a PV array's STC DC rating and the inverter's continuous AC rating, a deliberate oversizing practice used to keep the inverter running closer to full output for more of the day at the cost of some solar-noon clipping. Off-grid mode sizes an inverter's continuous and surge (motor-starting) capacity from a connected load.
Standards followed
DC:AC ratio (ILR) design practice — typical range 1.1-1.35 for grid-tied string/central inverters
What it can do
- Grid-tied: recommended inverter AC rating from array DC kW and a target ILR, or evaluate a candidate inverter's actual ILR with a CONSERVATIVE→AGGRESSIVE grading band.
- Off-grid: required continuous kVA (load ÷ output PF) and required surge kVA (continuous × a motor-starting surge multiple).
Example problem
Grid-tied: 12kWdc array, target ILR 1.2, candidate inverter 10kWac.
- Recommended inverter = 12/1.2 = 10kWac
- Candidate inverter's actual ILR = 12/10 = 1.2 — within the 1.15-1.35 TYPICAL band
10kWac recommended, candidate inverter's ILR = 1.2 (TYPICAL).
ILR is a design choice, not a hard limit — a higher ILR trades a small amount of clipped energy at peak sun for a smaller, cheaper inverter and higher output during low-light conditions; the ideal ratio depends on local irradiance profile and electricity pricing. Off-grid surge sizing here is a simplified continuous×multiple estimate — for inductive/motor loads with a specific locked-rotor current, cross-check against the inverter manufacturer's actual surge rating curve.