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AC or DC? System Topologies for Solar Systems

Introduction: How is the Battery Connected?

The previous articles covered much about energy storage systems. One important question remained unanswered: How is this technology optimally integrated into the home?

When integrating battery storage, there are two fundamental concepts:

  • AC-coupled systems (alternating current)
  • DC-coupled systems (direct current)

This chapter explains both topologies with their advantages and disadvantages.

AC-Coupled Solar Systems

In alternating current-coupled systems (AC systems), the inverter is connected directly after the solar modules and supplies the household grid directly with alternating current.

Structure

Solar modules (DC)
    ↓
Inverter (DC→AC)
    ↓
Household grid (AC) ←→ Battery + charge controller
    ↓
Public grid

Operating Principle

  1. The inverter converts module direct current immediately to alternating current
  2. The battery with charging electronics is connected after the inverter
  3. The battery is supplied with alternating current
  4. For charging, AC must be converted back to DC
  5. During discharge, DC is converted back to AC

Energy exchange between solar modules and battery occurs via alternating current.

Advantages of AC Coupling

AC coupling offers decisive advantages particularly for existing systems:

Advantage Explanation
Easy retrofitting Battery can be installed later
Manufacturer-independent Different components can be combined
Flexible placement Battery can be located away from inverter
Proven technology Established components
Scalability Easy expansion possible

Disadvantages of AC Coupling

The flexibility comes at a price – primarily regarding efficiency:

Disadvantage Explanation
Multiple conversion DC→AC→DC→AC = losses
Lower efficiency Typically 85–90% roundtrip
More components Separate battery inverter required
Higher costs More hardware needed
More complex installation More cabling

Typical Efficiency

With AC coupling, losses occur at each conversion:

  • Solar modules → Inverter: ~97%
  • Inverter → Battery charger: ~97%
  • Battery charging/discharging: ~95%
  • Battery → Inverter: ~97%

Total roundtrip: ~85–90%

DC-Coupled Solar Systems

In direct current-coupled systems (DC systems), inverter and battery are connected in parallel directly after the solar modules.

Structure

Solar modules (DC)
    ↓
DC-DC converter
    ├── Battery (DC)
    └── Inverter (DC→AC)
            ↓
        Household grid (AC)
            ↓
    Public grid

Operating Principle

  1. Both components are supplied directly with direct current
  2. The battery can use DC directly for charging
  3. Direct current is converted to alternating current only at the end
  4. Fewer conversion steps = higher efficiency

Energy exchange between solar system and battery occurs via direct current.

Advantages of DC Coupling

The direct connection via direct current brings clear advantages, particularly in efficiency:

Advantage Explanation
Higher efficiency Fewer conversion losses
Fewer components Only one central inverter
Lower costs More economical long-term
Better efficiency Typically 92–95% roundtrip
Faster charging Direct DC path to battery

Disadvantages of DC Coupling

The higher efficiency is bought at the cost of some restrictions:

Disadvantage Explanation
Manufacturer dependency Components often from the same manufacturer
No easy retrofitting System must be planned as a whole
More complex planning DC cabling more demanding
Limited cable lengths Keep DC cables short
Less flexibility Harder to expand

Typical Efficiency

With DC coupling, fewer conversion steps:

  • Solar modules → DC-DC converter: ~98%
  • Battery charging/discharging: ~95%
  • DC → Inverter → AC: ~97%

Total roundtrip: ~92–95%

The Direct Comparison

To facilitate the decision between AC and DC coupling, we compare both concepts directly:

Criterion AC Coupling DC Coupling
Efficiency 85–90% 92–95%
Retrofitting Easy Difficult
Flexibility High Limited
Initial costs Higher Lower
Long-term costs Higher (losses) Lower
Complexity More components Fewer components
Manufacturer choice Free Often restricted

The Hybrid Inverter: The Best of Both Worlds

Modern hybrid inverters dissolve the strict separation between AC and DC.

Concept

In hybrid inverter systems, all components converge in one central device:

  • Integrated MPPT for solar modules
  • DC-DC converter for the battery
  • Inverter for the household grid
  • Intelligent energy management

Advantages of the Hybrid Concept

The integration of all functions in one device offers the best characteristics of both worlds:

Advantage Explanation
Optimal efficiency Intelligent route selection DC or AC
Compact One device instead of several
Simple installation Less cabling
Optimally coordinated All components matched to each other

Energy Management: The Brain of the System

Regardless of topology, the Energy Management System (EMS) is decisive. It functions as the brain of the system.

The Four Core Tasks

1. Load Management

The EMS recognises:

  • Current electricity demand of consumers
  • Available capacity and power of the solar system
  • Decides when which consumers are supplied

Example: Power-intensive devices such as dishwashers or EV charging stations should run during the day when the solar system produces surplus.

2. Grid Feed-in

With a full battery storage and covered self-consumption:

  • Surplus is fed into the public grid
  • Feed-in tariff possible
  • Environmentally friendly: green electricity in the grid

3. Battery Management

Decides when the battery:

  • Is charged (surplus available)
  • Is discharged (more demand than generation)
  • Is conserved (grid electricity cheaper)

Primary goal: Always maintain an energy buffer.

4. Smart Home Integration

A good EMS:

  • Integrates into the smart home network
  • Recognises consumption of all devices
  • Can control devices optimally
  • Continuously optimises the interplay

Advantages of an EMS

A good energy management system brings measurable benefits for system operation:

Advantage Explanation
Higher self-consumption Use more solar electricity yourself
Lower electricity costs Less grid consumption
Longer battery lifespan Optimised charge/discharge cycles
More comfort Automated control
Transparency All data at a glance

Which Topology for Whom?

AC Coupling Recommended for:

  • Existing systems without storage (retrofitting)
  • Maximum flexibility desired
  • Different manufacturers already present
  • Battery far away from inverter

DC Coupling Recommended for:

  • New systems with storage from the outset
  • Maximum efficiency important
  • Everything from one supplier preferred
  • Short DC paths possible

Hybrid Inverter Recommended for:

  • New systems of any size
  • Simple installation desired
  • Best efficiency sought
  • Future-proof solution needed

Conclusion

Summary: The choice of system topology affects:

  • Efficiency (5–10% difference possible)
  • Flexibility for later changes
  • Costs short and long-term
  • Installation effort For most new systems, a hybrid inverter is today the best choice – it combines the advantages of both concepts. For retrofitting existing systems, there is often no alternative to AC coupling.

The Complete Article Series "Energy Storage for Solar Systems"

  1. From Frog Legs to Batteries: How Does an Energy Storage System Work? – Fundamentals
  2. Lithium vs. Lead: Which Battery for the Solar System? – Technology comparison
  3. Power Electronics: Inverters and DC-DC Converters – Power conversion
  4. The All-Rounder: Hybrid Inverters – Everything in one device
  5. AC or DC? System Topologies for Solar Systems – You are here

Related Article Series

How Does a Solar System Work?

Heat Pump Series:

Battery Storage and Powerstations:

Sources