The inverter is the brain of an on-grid system. It finds the best operating point of the array every second, turns DC into grid-quality AC, and disconnects instantly if anything is unsafe. It is also the component most likely to need replacement during the system's life, so choosing and matching it well matters.
What happens inside
- DC switch isolates the array for service.
- MPPT stage (a DC-DC converter) adjusts the operating voltage of each input to keep the array at its maximum power point as sunlight and temperature change.
- DC link stores energy between the stages.
- Full bridge of fast switches (IGBTs or MOSFETs) chops DC into a sine-shaped AC waveform locked to the grid's voltage and frequency.
- Filter smooths the output so current harmonics stay low.
- Relays connect to the grid only when every check passes, and open in milliseconds when something is wrong.
Reading the datasheet
Values for a typical 5 kW single-phase string inverter (illustrative):
| Item | Typical value | Why it matters |
|---|---|---|
| Max DC input voltage | 600 V | String Voc on the coldest morning must stay below this |
| MPPT voltage range | 90–550 V | String Vmp must stay inside, hot and cold |
| Start-up voltage | 100–120 V | Below this the inverter will not wake |
| Number of MPPTs / strings per MPPT | 2 / 1 | Separate MPPTs allow different roof faces or string lengths |
| Max input current per MPPT | 16 A | Must exceed the string's Imp; modern modules give 13–18 A |
| Max short-circuit current per MPPT | 20 A | Must exceed string Isc |
| Rated AC output | 5,000 W, 230 V | Matches the sanctioned load and phase |
| Max / European efficiency | 98.0% / 97.3% | Weighted efficiency reflects real operation |
| Protection rating | IP65 | Fine outdoors, but still keep it shaded |
| Operating temperature | −25 to +60 °C, derating above about 45 °C | Hot west walls cost output |
| Current THD | under 3% | Grid quality |
Three-phase inverters for 5–100 kW usually accept up to 1,000–1,100 V DC and have higher MPPT ranges, so strings are longer.
Matching the string to the inverter
Every string must satisfy three checks: cold Voc, hot Vmp, and current.
Worked example: Pune, 550 Wp modules, the 5 kW inverter above
Module: Voc 49.9 V, Vmp 41.96 V, Imp 13.11 A, Isc 14.0 A; Voc coefficient −0.27 %/°C, Pmax coefficient −0.35 %/°C (used for Vmp, because the datasheet gives no Vmp coefficient). Pune design minimum 5 °C; cell maximum 70 °C.
- Cold Voc per module: 49.9 × [1 + 0.0027 × 20] = 52.6 V
- Hot Vmp per module: 41.96 × [1 − 0.0035 × 45] = 35.4 V
| Modules in one string | Cold Voc (limit 600 V) | Hot Vmp (MPPT 90–550 V) | Verdict |
|---|---|---|---|
| 9 | 473 V | 318 V | OK |
| 10 | 526 V | 354 V | OK, best fit for 5 kW |
| 11 | 579 V | 389 V | Passes, but only 3.5% margin |
| 12 | 631 V | 424 V | Fails: exceeds 600 V |
Current: Imp 13.11 A < 16 A and Isc 14.0 A < 20 A, so one string per MPPT is fine.
The DC/AC ratio
DC/AC ratio = array kWp ÷ inverter rated AC kW
- 10 × 550 Wp = 5.5 kWp on a 5 kW inverter gives 1.10.
- Indian modules rarely deliver their full STC rating because of heat, so a ratio of 1.1–1.2 is common for homes, and up to about 1.3 for commercial sites.
- Above that, the inverter clips (caps output at its rating) around noon on clear, cool days. A little clipping is fine; a lot wastes energy. Model ratios above 1.3 in design software.
- Below about 0.9 the inverter is oversized: more cost, slightly lower efficiency at part load.
Also check what the DISCOM compares with the sanctioned load: some use the inverter's AC rating, some the array kWp.
Efficiency
- Maximum efficiency is measured at the best load point.
- European (weighted) efficiency averages efficiencies at several load levels, weighted for typical sunshine. It is the better number for comparison.
- A 1% difference in weighted efficiency on a 5 kWp Jaipur system is about 1% × 7,700 = 77 units a year.
String inverter, microinverter or optimiser?
| Option | Strength | Trade-off |
|---|---|---|
| String inverter | Lowest cost, simple, the standard for Indian homes | Shade on one module affects the string |
| Microinverter (one per one to four modules) | Module-level MPPT, safe low-voltage DC, good for shaded or multi-face roofs | Higher cost; electronics on a hot roof |
| String inverter with optimisers | Module-level MPPT and monitoring | Added cost and parts |
Key points
- An on-grid inverter tracks maximum power, converts DC to AC, synchronises with the grid and disconnects when unsafe.
- Check each string for cold Voc below the maximum DC voltage, hot Vmp inside the MPPT window, and current within the input limits.
- A DC/AC ratio of 1.1–1.2 suits most Indian homes; higher ratios cause clipping.
- Compare weighted (European) efficiency, not just maximum efficiency.
- Use BIS-registered inverters meeting IS 16221 and IS 16169, and keep them shaded and ventilated.
Check yourself
- A string of 10 modules has a cold Voc of 52.6 V per module. Can it go on a 600 V inverter?
- What DC/AC ratio does 6.6 kWp on a 6 kW inverter give?
- Why do new high-current modules sometimes need a different inverter from older ones?
Answers
- Yes: 10 × 52.6 = 526 V, below 600 V.
- 6.6 ÷ 6 = 1.10.
- Their Imp and Isc (14–18 A) can exceed the maximum input current per MPPT of older inverters (often 11–13 A), causing current limiting or a mismatch with the input rating.