Solar inverter is the heart of a solar system as it converts DC electricity into AC electricity. Typically apart from PV module, the largest hardware related expenses are done in solar inverters and accordingly it becomes important to select a right inverter for your rooftop solar application. There are number of options available for inverter selection which include the Micro inverters, String inverters and Central inverters.

In case of rooftop projects, we only focus on the Micro inverters as well as String inverters. However, following 5 considerations should be taken for evaluating the different options for your solar inverter.

CRITERIA 1: What should be the size of my inverter?

The string inverter are ranging from 5 kW-200 kW. It is important that you select the right inverter which meets the requirement of your solar panels. The optimal size of the inverter depends upon:

• Roof area
• Multiple roofs/Single roof
• Location of the inverter
• Anticipated DC/AC Ratio

CRITERIA 2: What is my application?

Solar inverters in rooftop projects are used as table top mounted or wire wall mounted or roof mounted. Sometimes, it becomes very important how the solar inverters will be installed, whether the inverter will be intalled in open area or covered area, what will be the operating temperatures.

It is also important to decide the inverter considering redundancy in your project capacity. If it is a large project, then you can consider multiple inverter rather than a single inverter in order to get redundancy so that if one inverter is under fault, other inverters are generating and you get adequate generation.

Sometimes, space constrains also matter while selecting a solar inverter. The noise level of inverter and internal heat generation are also few factors which should be considered while selecting the inverter.

CRITERIA 3: How to evaluate the cost of ownership in inverter?

String inverters are available with multiple options in terms of their efficiencies, the warranty lifetime, their capability to handle larger DC to AC Ratio. However, inverter selection becomes critical if we consider all these factors. For e.g. Some companies are giving inverters with 20 years life warranty while some other companies are providing 5, 7 or 10 years warranty of the inverter.

Hence, the cost of replacement during 25 years lifetime becomes important. It is also important to consider the cost of other accessories which are required along with inverters. For e.g. Some inverters are having in-built features for monitoring the generation. However in some inverters you have to install external monitoring system for monitoring the generation and other data and it can add significant costs over and above the inverters basic costs.

Number of MPPTs are also to be taken into considerations while inverter selection. If your inverter is having more number of MPPTs, it will be producing optimized results while operating under shadow conditions.

CRITERIA 4: How do I do my inverter selection?

The technical criteria for inverter selection include the MPPT voltage range, no. of MPPTs, highest and lowest operating temperatures, efficiencies of inverters, DC side and AC side protections and communication system for data monitoring.

It is equally important to check if your inverter is meeting the regulatory guidelines. For e.g.- In Indian context, the inverter should be IEC certified and should meet the grid support parameters as per CEA guidelines. The typical inverter parameters are listed below for your reference:

COMPARISON OF TBEA, HUAWEI AND SUNGROW INVERTERS
Input (DC)	TBEA	HUAWEI	SUNGROW
Model No	TBEA TS208KTL‐HV	HUAWEI SUN2000‐185KTL‐INH0	SG110CX
Max. PV input voltage	1500 V	1,500 V	1100 V
Start‐up input voltage	600 V	550 V	200 V / 250 V
Nominal PV input voltage			585 V
MPP voltage range	550‐1500V	500 V ~ 1,500 V	200 – 1000 V
MPP voltage range for nominal power			550V – 850 V
No. of independent MPP inputs	12	9	9
Max. number of PV strings per MPPT	3	2 or 3	2
Max. PV input current	26 A	26 A	26 A * 9
Max. current for input connector	30 A		30 A
Max. DC short‐circuit current	33 A	40 A	40 A * 9
Output [AC]
Max. AC output current	173A	134.9 A	158.8 A
Nominal AC voltage	3 / PE, 800 V	800 V, 3W + PE	3 / N / PE, 400 V
AC voltage range	640‐920Vac	640‐960 Vac	320 – 460 V
Nominal grid frequency / Grid frequency range	50 Hz / 45 – 55 Hz	50 Hz/60Hz	50 Hz / 45 – 55 Hz, 60 Hz / 55 – 65 Hz
THD	< 3 %	< 3 %	< 3 % (at nominal power)
Efficiency
Max Efficiency/ Euro Efficiency	99 % / 98.7 %	99.03% / 98.66%	98.7 % / 98.5 %
Protection
DC reverse connection protection	Yes	Yes	Yes
AC short circuit protection	Yes	Yes	Yes
Leakage current protection	Yes	Yes	Yes
Grid monitoring	Yes	Yes	Yes
Ground fault monitoring	Yes	Yes	Yes / No
DC switch/ AC switch	DC switch available	DC switch available	Yes
PV String current monitoring	Yes	Yes	Yes
Q at night function	Yes	Yes
Anti PID and PID recovery function	PV array Anti PID Function	PV array Anti PID Function	Optional
Overvoltage protection	DC Type II/ AC Type I	DC Type II/ AC Type II
Isolation method	IT, Transformerless	Transformerless	Transformerless
Ingress protection rating	IP65	IP66	IP66
Night power consumption	< 3 W	< 3W	< 2W
Operating ambient temperature range	(‐25 to 60 ℃)	(‐25°C ~ 60°C)	-30 to 60 ℃ (> 50 ℃ derating)
Allowable relative humidity range (non‐condensing)	0 – 100 %	0 – 100 %	0 – 100 %
Cooling method	Smart forced air cooling	Smart forced air cooling	Smart forced air cooling
Max. operating altitude	4000 m	4000 m	4000 m (> 3000 m derating)
Display	LED Indicators, WiFI + APP	LED Indicators, Bluetooth	LED, Bluetooth + APP
		+ APP
Communication	Rs 485/PLC (optional) /GPRS (optional) /WIFI (optional)		RS485 / Optional: Wi-Fi, Ethernet
		USB, RS485, MBUS

 

CRITERIA 5: What should be the brand of inverter that can be used for my rooftop solar system?