IEC TS 62257-350:2025

IEC TS 62257-350 ®
Edition 1.0 2025-12
TECHNICAL
SPECIFICATION
Renewable energy off-grid systems -
Part 350: Recommendations for selection of inverters
ICS 27.160  ISBN 978-2-8327-0854-5

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CONTENTS
FOREWORD . 3
INTRODUCTION . 5
1 Scope . 6
2 Normative references . 6
3 Terms, definitions and abbreviations . 6
3.1 Terms and definitions . 6
3.2 Abbreviations . 7
4 Overview . 7
5 System architecture and inverter selection . 7
6 General considerations . 8
6.1 Overview . 8
6.2 IP rating. 8
6.3 Ambient temperature . 8
6.4 Altitude . 8
6.5 Direct sunlight . 9
6.6 Efficiency . 9
6.7 No-load and stand-by power requirements . 9
6.8 Ventilation requirements . 9
6.9 Earthing arrangements . 9
6.10 Waveform quality . 9
6.11 Compatibility with loads that produce significant harmonics . 9
6.12 Electromagnetic interference . 9
6.13 Load growth . 9
6.14 Protection . 10
7 Variable frequency drives and T1I-c systems – REN systems operating with no
storage, DC source to AC application . 10
7.1 General . 10
7.2 Characteristics of the inverter . 11
7.2.1 Frequency control . 11
7.2.2 Maximum power point tracking . 11
7.3 Sizing of the inverter . 11
7.3.1 General. 11
7.3.2 AC voltage . 11
7.3.3 AC current. 11
7.3.4 DC voltage . 11
7.3.5 DC current . 11
7.4 Variable frequency drive standards . 12
7.5 Variations on the architecture . 12
8 Uni-directional grid forming inverters that work with batteries – Type T2I systems
– REN production with energy storage . 12
8.1 General . 12
8.2 Characteristics of the inverter . 13
8.2.1 Grid forming . 13
8.3 Sizing of the inverter . 14
8.3.1 Input DC voltage . 14
8.3.2 Input DC current . 14
8.3.3 AC current and power . 14
8.3.4 AC voltage . 14
8.3.5 Power factor handling range. 14
8.4 Variations on the architecture . 15
9 Uni-directional inverters that synchronize to the grid – Type T3I systems – REN
and diesel production without energy storage . 15
9.1 General . 15
9.2 Characteristics of the inverter . 16
9.2.1 General. 16
9.2.2 Grid following . 16
9.2.3 Maximum power point tracking . 16
9.2.4 Active power control . 17
9.2.5 Grid support . 17
9.3 Sizing of the inverter . 17
9.3.1 Input DC voltage . 17
9.3.2 Input DC current . 17
9.3.3 Output AC current and power . 17
9.4 Variations on the architecture . 18
10 Bi-directional grid forming inverters that work with batteries – Type T4I systems,
RE and diesel with energy storage . 18
10.1 General . 18
10.2 Characteristics of the inverter . 19
10.2.1 Grid forming . 19
10.2.2 Battery management . 19
10.2.3 Frequency modulation . 19
10.2.4 Master-slave. 20
10.2.5 Reactive power control . 20
10.3 Sizing the inverter . 20
10.3.1 General. 20
10.3.2 Input DC voltage . 20
10.3.3 Input DC current . 20
10.3.4 Output AC voltage . 20
10.3.5 Output AC current . 21
10.4 Variations on the architecture . 21

Figure 1 – Type T1I-c system – DC to AC with no batteries . 10
Figure 2 – Type T2I system – DC to AC with batteries . 13
Figure 3 – Type T3I system – DC to AC with diesel but no batteries . 16
Figure 4 – Type T4I system – DC to AC with diesel and batteries . 19

Table 1 – Types of inverter and system architecture it is applicable to . 8

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
Renewable energy off-grid systems -
Part 350: Recommendations for selection of inverters

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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IEC TS 62257-350 has been prepared by IEC technical committee 82: Solar photovoltaic energy
systems. It is a Technical Specification.
The text of this Technical Specification is based on the following documents:
Draft Report on voting
82/2464/DTS 82/2529/RVDTS
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this Technical Specification is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
This first edition cancels and replaces IEC TS 62257-9-7:2019. This edition constitutes a
technical revision.
A list of all parts in the IEC 62257 series, published under the general title Renewable energy
off-grid systems, can be found on the IEC website.
Future standards in this series will carry the new general title as cited above. Titles of existing
standards in this series will be updated at the time of the next edition.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
– reconfirmed,
– withdrawn, or
– revised.
INTRODUCTION
The IEC 62257 series provides technical standardization to different stakeholders (including
but not limited to project developers, financing agencies, testing agencies, installers, etc.)
involved in electrification projects for access to electricity for those not solely connected to the
regional grid, through the setting up of off-grid renewable energy and hybrid systems (including
micro-grids) with a voltage less than or equal to 1 000 V for AC (alternating current) or a voltage
less than or equal to 1 500 V for DC (direct current).
Access to electricity is one of the predominant policy actions designed to increase the well-
being of populations, together with access to clean water, improved healthcare, education,
personal advancement, and economic development. Increasing access to electricity through
utilization of renewable off-grid electricity also directly or indirectly supports various United
Nations Sustainable Development Goals (https://sdgs.un.org/goals), depending on the
application.
The IEC 62257 technical specifications focus on enabling access to electricity by concentrating
on, but not being specific to, developing countries. This series should not be considered as all-
inclusive for access to electricity. That means that the technical specifications could be used
for rural electrification, also for electrification of remote sites in developed countries, or any
requirement for electricity access that cannot be met by attaching solely to the national utility
grid.
One of the main objectives of this series is to provide the minimum sufficient recommendations,
including items for safety, sustainability of systems, and at the lowest life cycle cost, relevant
to the renewable energy and hybrid off-grid systems field of application.

1 Scope
This part of IEC 62257, which is a technical specification, specifies the criteria for selecting and
sizing inverters suitable for different off-grid applications integrating solar as an energy source.
NOTE As well as off-grid systems, this document can also apply to inverters where a utility grid connection is
available as a backup for charging batteries, but it is not intended to cover applications in which inverters synchronize
and inject energy back into a utility grid, even though this capability can incidentally be a part of the functionality of
the inverters.
Single and multi-phase applications are included.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60529, Degrees of protection provided by enclosures (IP Code)
IEC 61683, Photovoltaic systems - Power conditioners - Procedure for measuring efficiency
IEC 61800, (all parts), Adjustable speed electrical power drive systems
IEC TS 61836, Solar photovoltaic energy systems - Terms, definitions and symbols
IEC 62109, (all parts), Safety of power converters for use in photovoltaic power systems
IEC TS 62257-2, Recommendations for renewable energy and hybrid systems for rural
electrification - Part 2: From requirements to a range of electrification systems
IEC TS 62257-7-1:2010, Recommendations for small renewable energy and hybrid systems for
rural electrification - Part 7-1: Generators - Photovoltaic generators
IEC 62548, Photovoltaic (PV) arrays - Design requirements
IEC 62548-1, Photovoltaic (PV) arrays - Part 1: Design requirements
3 Terms, definitions and abbreviations
For the purposes of this document, the terms and definitions given in IEC TS 61836 and the
following apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
– IEC Electropedia: available at https://www.electropedia.org/
– ISO Online browsing platform: available at https://www.iso.org/obp
3.1 Terms and definitions
3.1.1
renewable energy
REN
energy from a source that is replenished at a higher rate than it is consumed
3.1.2
hybrid system
multi-source system with at least two kinds of power generation technology
3.1.3
storage
equipment in which energy produced by one of the generators of the system can be stored,
such as in a battery
3.1.4
DC bus
node of the electrical system through which DC power flows
3.1.5
AC bus
node of the electrical system through which AC power flows
3.1.6
black start
process of starting an electrical power supply without relying on any other external generating
source
3.2 Abbreviations
PCE power conditioning equipment
STC standard test conditions (1 000 W/m , AM 1,5 spectrum. 25 °C)
4 Overview
Inverters are used to convert DC power into AC power. For off-grid renewable applications,
there are different types of inverters that can be used depending upon the architecture of the
system.
Some systems work with batteries, and the inverters use their energy to create a local AC micro-
grid (grid-forming inverters) and some inverters synchronize to an existing AC grid.
In addition, some inverters can be bi-directional and not only convert the DC battery voltage
into AC but can also reverse the process and charge the battery from the AC grid.
It is necessary to correctly select inverters used in these applications based on the features
they possess and to size them depending on the loads they feed or the energy sources they
are fed by.
5 System architecture and inverter selection
Different types of system architectures of isolated electrification systems are given in
IEC TS 62257-2. Not all of them require inverters, but several do. Depending on the situation
encountered, the inverters will require certain features.
Table 1 shows the system architectures from IEC TS 62257-2 that require inverters and gives
the general correspondence of which inverter is suitable for which application, with some
exceptions noted.
Table 1 – Types of inverter and system architecture it is applicable to
Type of inverter Principal system Description Example Variations on
architecture applications architecture
Variable frequency T1I-c REN systems Solar water pumping Some drives also use
drives operating with no diesel or grid
storage and no diesel
generators or grid
backup
Grid forming inverters T2I REN systems with Solar home systems Some systems allow
that work with storage and no diesel generators or grid to
batteries generators or grid connect via an
backup additional battery
charger
REN and battery are
coupled on the DC
bus
Grid-tie inverters that T3I REN systems Reduction of diesel Some s
...