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

  • AC-coupled battery — connects to the AC side of the solar PV system; has its own built-in inverter/charger; charges from the AC supply (solar or grid); simplest retrofit to existing systems
  • DC-coupled battery — connects to the DC side of the solar array; charged via the solar inverter's DC bus; more efficient (one DC-to-AC conversion instead of two); specified on new-build systems with a hybrid inverter
  • Hybrid inverter — a single inverter that handles both solar PV and battery; replaces the standard string inverter; required for DC-coupled systems
  • Round-trip efficiency — energy recovered from storage as a percentage of energy put in; AC-coupled: ~88–92%; DC-coupled: ~92–96%; the difference is meaningful at scale
  • G98/G99 re-notification — adding a battery to an existing solar PV system changes the connection characteristics; a new G98 notification (or G99 application if combined export exceeds threshold) is required
  • Export capacity — a battery that can export to the grid (when fully charged and solar is also generating) adds to the property's total export capacity; this is what determines G98 vs G99 eligibility for the combined system
  • DNO export limiting — some DNOs require export limiting on battery + solar combined systems to keep total export within the G98 threshold; the battery inverter must be export-limited accordingly
  • MCS MIS 3012 — the MCS installation standard for battery energy storage systems; holding this scope is required to MCS-certify battery-only or battery + solar installations
  • Notifiable (Part P) — battery installation involves new electrical circuits in the dwelling; notifiable under Part P; Competent Persons Scheme self-certification applies (see building regs part p solar)
  • Battery safety — lithium-ion batteries carry fire risk; installation must comply with manufacturer guidelines for clearances, ventilation, and proximity to flammable materials; BS EN IEC 62619:2022 covers safety requirements for stationary battery systems
  • BESS capacity — Battery Energy Storage System; typical domestic capacity range: 5kWh–30kWh; most common: 10kWh (Tesla Powerwall 2: 13.5kWh; GivEnergy: 9.5kWh; Solis: various)
  • Virtual Power Plant (VPP) / Demand Flexibility Service — aggregators (Octopus Energy, EDF, Kaluza) can remotely dispatch a home battery for grid balancing; the homeowner receives payments; requires DNO-approved export capability

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:

  • Dedicated AC circuit from the consumer unit to the battery inverter (typically 32A or 40A single-phase; size per manufacturer guidance)
  • Battery mounted on an internal wall (indoor) or in a suitable external-rated enclosure (outdoor)
  • Minimum clearances from combustible materials (check manufacturer spec: typically 300–600mm)
  • Communication cable between battery inverter and existing solar inverter (for some systems that coordinate DC/AC generation signals; not always required)

Common products:

  • Tesla Powerwall 2 — 13.5kWh; AC-coupled; gateway unit controls grid/solar/battery flows; proven track record; single unit; premium price
  • GivEnergy Hybrid Battery — modular (up to 30kWh); AC-coupled via GivEnergy inverter; popular with UK installers; good monitoring via Givenergy cloud
  • Solis RHI Hybrid — multiple options; AC or DC-coupled versions

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:

  • AC output power (e.g., 5kW, 10kW)
  • Battery DC input voltage range (must match the selected battery bank specification)
  • EPS (Emergency Power Supply) capability: can supply defined loads during grid outage; not all hybrid inverters offer true EPS
  • Backup power rating: the kW available during grid outage
  • G98/G99 compliance for the combined system

Common UK hybrid inverter products:

  • SolarEdge Home Hub — DC-coupled; SolarEdge Backup Interface required for EPS; strong monitoring
  • GoodWe ET Plus — popular UK hybrid inverter; competitive price; EPS capability
  • Solax X-Hybrid — modular; strong UK market share; competitive
  • SolaX Power, GivEnergy All-in-One, Sungrow SH — growing UK presence

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:

  • The existing solar PV system details
  • The battery system added (make, model, capacity in kWh, inverter/charger power in kW)
  • The new total export capacity (solar inverter export + battery inverter export capacity)

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:

  • Battery system commissioning procedure
  • Electrical safety: DC battery circuit protection, fusing, isolation
  • Fire safety: installation clearances, ventilation, emergency procedures
  • Documentation: battery installation certificate (separate from the solar PV MCS certificate)

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