Solar Battery Storage Installation: AC-Coupled vs DC-Coupled, DNO Notifications and G99 Implications

Quick Answer: Solar battery storage systems connect to an existing or new solar PV installation to store excess generation for use when the sun isn't shining. AC-coupled batteries (Tesla Powerwall, GivEnergy, Solis Hybrid) are the easiest retrofit to an existing system — they connect on the AC side without modifying the existing solar inverter. DC-coupled systems are more efficient and typically specified on new installations (combined hybrid inverter). Battery storage triggers a new G98/G99 DNO notification and may require building regulations notification. MCS certification scope must include battery storage.

Summary

Battery storage is the fastest-growing add-on to UK domestic solar PV. A 10kWh battery can store a summer day's excess solar generation and supply the evening and overnight household load — potentially reducing grid import to near zero in summer. In winter, the battery is charged from off-peak grid electricity (Octopus Go, Economy 7) and discharged during peak tariff periods.

For solar PV installers expanding into battery storage, understanding the coupling architecture, DNO implications, and MCS scope requirements is essential. Batteries add significant value to the customer (and to the job value) but also add complexity and liability.

Key Facts

Quick Reference Table: AC-Coupled vs DC-Coupled Comparison

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Aspect AC-Coupled DC-Coupled (Hybrid Inverter)
Best for Retrofitting to existing solar New installations; greenfield builds
System complexity Lower (no existing inverter changes) Higher (hybrid inverter required)
Round-trip efficiency ~88–92% ~92–96%
Common products Tesla Powerwall, GivEnergy Hybrid, Solis SolarEdge Home, GoodWe ES, Solax Hybrid
Grid charging Yes (all AC-coupled batteries) Yes (hybrid inverters charge from grid too)
Relative installed cost Higher total cost (battery + existing inverter remain) Lower for new install; higher for retrofit (replace inverter)
Monitoring Two separate monitoring platforms Single platform (battery + solar)

Detailed Guidance

AC-Coupled Battery: Retrofit to Existing Solar

For a customer with an existing solar PV system, an AC-coupled battery is by far the most practical option. The existing solar inverter is unchanged; the battery connects to the household AC supply.

How it works:

  1. The battery inverter/charger monitors the household AC bus
  2. When solar generation exceeds household consumption, excess flows onto the AC bus; the battery inverter detects this and charges the battery
  3. When solar generation falls below consumption (evening, overnight), the battery inverter discharges, supplying AC power to the household
  4. The battery can also be charged from the grid on a schedule (e.g., overnight on cheap tariff)

Installation requirements:

Common products:

DC-Coupled Battery: New Installations with Hybrid Inverter

For new installations, a DC-coupled system (hybrid inverter) is the architecturally cleanest and most efficient solution.

How it works:

  1. The solar array connects to the hybrid inverter's DC input (MPPT input), as with a normal string inverter
  2. The battery connects to the hybrid inverter's battery DC bus
  3. The inverter manages energy flows between solar array, battery, household AC, and grid in a single unit
  4. Solar charges the battery directly on the DC bus — no DC→AC→DC conversion; lower conversion losses

Hybrid inverter selection: Key specifications:

Common UK hybrid inverter products:

DNO Notifications for Battery Storage

New battery (retrofit to existing G98 solar): Adding a battery changes the connection characteristics. A new G98 notification must be submitted, stating:

If the combined export capacity (solar + battery) exceeds 3.68kW, a G99 application is required before connection.

Export limiting for battery systems: Many AC-coupled battery systems can export battery charge to the grid (battery discharge + excess solar = combined export). If this combined export could exceed 3.68kW, the battery inverter must be export-limited to keep total export within the G98 threshold, or a G99 application must be made.

In practice, many DNOs accept G99 applications for battery + solar systems up to 5–10kW relatively quickly, as battery storage is increasingly recognized as beneficial to the grid (through demand flexibility services). G99 is not the barrier it might appear; it is a notification and approval process, not a refusal.

Demand flexibility and grid export: Some VPP (Virtual Power Plant) arrangements require the battery to be able to export to the grid on demand from the aggregator. This typically requires a G99 connection approval that specifically permits grid export from the battery. Confirm the DNO approval covers the intended operating mode before signing the customer up to a VPP scheme.

MCS MIS 3012: Battery Storage Certification

To MCS-certify battery storage installations (required for the installation to meet the MCS standard and for any associated grant eligibility), the installer must hold MCS scope that includes battery storage under MIS 3012.

Most certification bodies offer this as an add-on to the solar PV MCS 012 scope. The additional assessment covers:

Frequently Asked Questions

Should I always specify a hybrid inverter when also doing new solar PV?

If the customer has any interest in battery storage now or in the future: yes. A hybrid inverter costs approximately £200–£500 more than a standard string inverter of equivalent rating. Adding a standard inverter now and then replacing it with a hybrid inverter for battery retrofit adds £500–£1,000 in unnecessary cost. Specify hybrid from the start if battery storage is a realistic future upgrade.

Is a Tesla Powerwall better than a GivEnergy battery?

Both are well-regarded products in the UK market. Tesla Powerwall 2 is a premium, proven product with good reliability data; it is more expensive and requires Tesla-approved installers. GivEnergy offers more modularity (capacity can be expanded by adding modules) and is generally more installer-friendly in terms of commissioning. The right choice depends on the customer's capacity needs and budget.

Can I install a battery without solar PV?

Yes. A standalone battery (no solar) charged from the grid during off-peak periods and discharged during peak tariff periods can provide significant savings on Octopus Agile or other time-of-use tariffs. This is a growing use case. The installation is simpler (no DC wiring; just the AC-coupled battery inverter) but G98/G99 notification is still required if the battery can export to the grid.

Regulations & Standards