2012 Grid-Tied Inverter Buyer's Guide: Page 2 of 2

AC Variables

CEC Rated Power. The California Energy Commission’s (CEC) test protocol allows comparing inverter output for different makes and models by using the same test conditions for each. Electronics function better at cooler temperatures, and when ambient air temperature increases, inverter performance is reduced. This output rating is what each inverter can maintain at 40°C (104°F) and helps designers know what each can produce at higher temperatures.

AC Output Current. This specification is the output amperage to be used in overcurrent protection and conductor-sizing calculations. In addition to these NEC requirements, designers use this value in voltage drop calculations, as AC conductors must be sized for a very small voltage drop (for example, 1.5% or less). Just like on the inverter’s DC input, the AC side also has a voltage range it must operate within. Keeping the voltage drop low reduces the likelihood of nuisance tripping when grid voltage is high. Some inverter manufacturers offer online AC voltage drop calculators to help designers determine appropriate AC output conductor size.

Performance Variables

Efficiency (Peak and CEC). An inverter’s efficiency is a ratio of output power to input power. Because inverters lose some electric energy as heat, as does any kind of conversion, efficiency will always be less than 100%. Efficiency varies depending on conditions, such as ambient air temperature, inverter temperature, and array voltage. In particular, that value depends on how much power the inverter is trying to process. Peak efficiency is the highest ratio between power out to power in, given ideal conditions and power input, and isn’t necessarily representative of efficiency during common operating conditions. 

The CEC-weighted efficiency is more akin to real-world conditions, because it considers the amount of time arrays normally spend at various power levels. Each inverter is tested at several power levels, ranging from 10% to 100% of rated power and at low, medium, and high DC input voltages to calculate a single average efficiency value.

The weighted efficiency is independently verified and is used by designers for comparing inverters. It is also used in calculations that determine an overall system DC-to-AC derate value used in system sizing and energy performance estimates.

Ambient Temperature Range. Exceeding the manufacturer’s acceptable ambient temperature range can result in erratic inverter operation, damage, and premature failure. Going over the limit can also violate NEC 110.3(B), which requires that equipment be installed in accordance with manufacturer’s instructions. Some inspectors are abiding this specification closely—in some cases, local temperature determines which inverters you are limited to using, if you do not want to put the inverter in a conditioned space. Because of this, some installations are limited to microinverters or AC modules, which have a wider operating temperature range. Some string inverter manufacturers will provide additional documentation allowing exceptions outside of the limits shown on their specifications sheets, but acceptance is up to the authority having jurisdiction (AHJ).

Mechanical Specifications

Integrated Disconnects & Combiners. Many inverters include integrated DC and AC disconnects, along with fused combiner boxes. This can eliminate installing external disconnects and combiners, reducing costs and installation time. Local AHJs and/or incentive program requirements may have additional requirements beyond the NEC, so external disconnects may still be required. Another consideration is that inverter servicing/removal is much easier and safer if workers are not subjected to exposed wiring coming in from the PV array or utility grid. Thus, a feature worth considering is the ability for the inverter to be detached from the integrated disconnect/wiring box (without dismounting the disconnect/wiring box).

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Justine Sanchez is a technical editor with Home Power and an instructor for Solar Energy International. She is certified by ISPQ as a PV Affiliated Master Trainer.