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What is the difference between a string inverter and a micro-inverter?

A string inverter is a single, centrally located device that converts DC power from multiple panels (connected in series, forming a 'string') to AC. If one panel in the string underperforms (due to shading or soiling), it degrades the output of the entire string. A micro-inverter is a small inverter installed on each individual panel, converting that panel's DC output to AC independently. Micro-inverters eliminate the single-underperforming-panel problem, provide module-level monitoring, enable flexible system design across multiple roof orientations, and carry 25-year warranties. Their disadvantage is higher upfront cost (Rs 3,000-5,000 per panel premium vs string inverter).

What BIS certification is required for solar inverters in India?

BIS IS 16221 (Solar Photovoltaic Energy Systems — Inverters) certification is mandatory for all grid-connected solar inverters sold and installed in India. The BIS licence is issued to manufacturers (not importers) — only products manufactured under a valid BIS licence are legally compliant. From 2022, DISCOM net metering applications require BIS IS 16221-certified inverters. The BIS licence number can be verified at bis.gov.in. Additionally, IEC 62109-1 and IEC 62109-2 (safety for power converters in PV systems) are specified by most quality EPC contractors and larger project developers.

What is anti-islanding in solar inverters and why is it critical?

Anti-islanding is a safety protection function in grid-connected solar inverters that automatically shuts down the inverter when the grid power supply fails or is disconnected by the DISCOM. Without anti-islanding protection, a solar inverter continues generating and exporting power into the local distribution network even when DISCOM has isolated the line for maintenance or fault repair — creating a dangerous energised 'island' that can electrocute DISCOM linemen. IEEE 1547 (adopted in India as IS 16169) mandates anti-islanding protection for all grid-connected inverters. Testing to this standard is a mandatory component of BIS IS 16221 certification.

What is the efficiency of a solar inverter and how does it affect system performance?

Solar inverter efficiency is the ratio of AC output power to DC input power — typically 97-98.6% for modern string inverters. This means 1.4-3% of all solar energy captured by the panels is lost in the inverter as heat. Efficiency is not constant — it varies with load level, and most inverters are most efficient at 50-75% of rated load. Two efficiency metrics are used: peak efficiency (at optimal conditions) and Euro or CEC efficiency (a weighted average across realistic operating conditions). CEC efficiency is the more relevant metric for Indian conditions. A 1% improvement in CEC efficiency on a 100 kWp system generating 1,40,000 kWh/year delivers 1,400 kWh additional generation — worth approximately Rs 8,400-14,000/year at commercial electricity tariffs.

What is the correct MPPT voltage window selection for a solar string inverter?

The MPPT (Maximum Power Point Tracking) voltage window of an inverter specifies the DC input voltage range within which the MPPT algorithm operates. The solar string's Maximum Power Point voltage (Vmpp_string = Vmpp_panel x panels in series) must fall within the inverter's MPPT window at all operating temperatures. Vmpp increases at low temperatures (morning, winter) and decreases at high temperatures (afternoon, summer). Verify: at maximum expected string Voc (cold winter morning) does not exceed inverter maximum input voltage; at minimum expected string Vmpp (hot summer afternoon) remains above inverter minimum MPPT voltage. Typical margin: 10% above and 10% below the MPPT window boundaries.