Monday, April 10, 2023

KSTAR Single Phase String Inverters


KSTAR New Energy makes single phase grid-tied inverters ranging from 1 kW to 10 kW.  I tested a 3000S, a 5000D, and a 6000D that were produced in KSTAR's factory outside of Shenzhen.  Their single phase inverters are marketed for locations with a 230 V line to neutral (L-N) grid.  They also work with the split phase 240 V line to line grid that is typical in the US and Canada.  They do not have UL 1741 certification, so they would require special engineering approval to be used for permanent installations with most US and Canada power utilities.

Residential inverters used in the US and Canada usually have an attached junction box with terminal connections for DC and AC wiring.  In the rest of the world, inverters usually have MC4 connectors for the DC string input, and a watertight three-pin plug connection for the AC output.  It is much more convenient having the plug connections when testing inverters and PV panels.  It also avoids potential electrical code concerns when DC wiring up to 600 V and 240 Vac are in the same junction box.


The KSTAR inverters all included MC4 crimp connectors for terminating the DC strings.  The AC connector will accept SOOW or SJOW cable with a outside diameter of up to 16 mm.  I used 3-wire 12 AWG SOOW cable that is rated for up to 25 Amps.

The 3000S has a single string input, and a "nominal" output power of 3 kW.  It is a light inverter, with a stated weight of 8 kg.  Out of the box, the measured weight was 7.3 kg.  The light weight makes it very easy for a single person to install.  When hooked up to a test string of 10 72-cell panels, the efficiency was 85-86%. This is much lower than the spec efficiency of 97% or the 96% efficiency at nominal 380 V listed on the inspection and test sheet that was included with the inverter.  With input power of 3070 W and input voltage of 367.7 V, the output power was 2620 W, for an efficiency of 85.3%.  KSTAR sales and engineering were unable to explain the low efficiency.

The 5000D and 6000D have the same external dimensions and connections on the bottom.  The weight of the 5000D is 11.74 kg, while the 6000D weighs 12.48 kg.   This suggests the 6000D has different internal circuitry, likely larger inductors and capacitors, to support the higher power rating.

The efficiency of the 5000D and 6000D inverters ranged between 89 and 91%.  The screenshot of monitor data below shows a total input power of 6240 W with AC output power of 5570 W, for an efficiency of 89.3%.  This test was done with a large difference between the PV1 and PV2 voltages to represent typical residential PV installations which are not optimized for the inverter's 380 nominal string voltage.

The KStar inverters are reasonably priced and easy to install, but the low efficiency makes them unattractive compared to Growatt and Ginlong Solis inverters.