What cable size for a home battery?

Depends on inverter power rating, not battery capacity. A typical 5kW inverter needs 4mm² or 6mm² cable on a 25-32A circuit. Smaller 3.6kW systems can use 2.5mm² on 16-20A. Calculate from inverter maximum AC output current - manufacturers provide this figure.

Can I add battery to existing solar PV?

Yes - AC-coupled batteries are designed for retrofit. They have their own inverter and connect to your consumer unit independently from the solar. You need a spare way in the consumer unit and appropriate cable run to the battery location. No changes to existing solar wiring needed.

Does battery storage need its own circuit?

Yes, battery inverters need dedicated circuits - they can both import and export significant power. Don't share with other loads. The circuit breaker size matches the inverter rating. Some installations use RCBOs for combined protection.

What about DC cables for battery systems?

DC-coupled systems have DC cables between battery and hybrid inverter - these are typically specified by the inverter manufacturer and included with the system. AC-coupled systems don't need DC cables externally. DC cable sizing is part of the system design, not a separate calculation.

Do I need G98/G99 for battery storage?

Yes, grid-connected batteries need DNO notification. G98 (simple notification) covers systems up to 16A per phase - most domestic batteries qualify. Larger systems need G99 application and may require DNO approval before connection. Your installer handles this.

Can I install battery without solar?

Yes - standalone batteries charge from the grid at cheap rate times (overnight) and discharge during peak rates. Same electrical requirements as solar-paired systems. Makes financial sense with time-of-use tariffs like Octopus Go, Intelligent Octopus, or Economy 7.

Where should the battery be located?

Most home batteries are wall-mounted in garages, utility rooms, or externally. Consider: cable route to consumer unit, ventilation requirements, temperature range, and accessibility. Shorter cable runs reduce cost and losses. Some batteries are indoor-only, others rated for outdoor.

What protection does battery circuit need?

MCB or RCBO sized to inverter rating (typically 20-40A Type B). RCD protection required as for any circuit. Some batteries have internal protection and isolation. Consumer unit must have capacity for additional way - may need upgrade if full.

Can battery provide backup power in outages?

Some systems offer backup capability, but many don't - the inverter shuts down when grid fails (anti-islanding). Backup-capable systems need additional wiring to protected circuits. This is a system feature, not just electrical installation - specify at purchase if required.

How do I calculate battery inverter current?

Inverter AC current = Power (W) ÷ Voltage (V). A 5000W inverter = 5000 ÷ 230 = 21.7A. Add 10% margin = 24A, so 25A or 32A circuit. Three-phase inverters divide power across phases. Manufacturer datasheets give exact maximum current figures.

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Battery Storage Cable Sizing Calculator

Calculate the correct cable size for home battery storage systems and retrofit installations

Battery Storage Cable Quick Facts

Typical home batteries 3.6-13.5kWh capacity
Inverter rating determines cable size (3-5kW typical)
AC-coupled systems connect to existing consumer unit
DC-coupled integrates with solar inverter
G98/G99 notification required for grid-connected systems

Cable Size Calculator

BS 7671:2018+A2:2022 compliant

1. Load Current

1kW ≈ 4.35A at 230V

2. Cable Length (meters)

3. Installation Method

Lighting circuit (3% voltage drop limit)

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Common Battery Storage Installations

Small battery system (3.6kW inverter)

16A circuit, 2.5mm² cable sufficient for short runs

Medium battery system (5kW inverter)

25-32A circuit, 4mm² or 6mm² cable typical

Large battery system (8kW+ inverter)

40A+ circuit, 6mm² or 10mm² cable depending on length

Retrofit to existing solar PV

AC-coupled system, dedicated circuit from consumer unit

Standalone battery (no solar)

Grid charging only, standard AC circuit to battery inverter

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Battery storage pairs with solar PV systems for maximum benefit. Consider adding an EV charger to use stored energy for transport. For whole-home energy efficiency, see our heat pump calculator.

Battery Storage Cable Sizing

Battery storage cable sizing calculates conductor sizes for home battery inverter connections. The inverter rating (kW) determines current draw, not battery capacity (kWh). A 5kW inverter draws about 22A, requiring appropriately sized cable and circuit protection. AC-coupled systems need dedicated circuits; DC-coupled share solar inverter connection.

Battery Storage Installation Costs (2024)

Typical UK costs for home battery storage systems including installation.

Installation Scenario	Materials	Labour	Total
Battery electrical connection only	£150-300	£200-400	£350-700
Consumer unit modification	£100-250	£150-300	£250-550
Battery system 5kWh (supply + install)	£2500-4000	£500-1000	£3000-5000
Battery system 10kWh (supply + install)	£4500-7000	£600-1200	£5100-8200
Battery retrofit to existing solar	£3000-5500	£500-1000	£3500-6500

Prices as of 2024. Battery prices continue to fall - check current market rates.

AC-Coupled vs DC-Coupled Systems

AC-coupled batteries have their own inverter and connect to the AC consumer unit - ideal for retrofit to existing solar. DC-coupled batteries connect on the DC side of the solar inverter - more efficient but requires compatible inverter. AC-coupled needs separate electrical circuit; DC-coupled shares solar circuit but may need DC cable upgrades.

Battery Capacity vs Inverter Power

Battery capacity (kWh) indicates energy storage - how long it can supply power. Inverter power (kW) indicates maximum charge/discharge rate. A 10kWh battery with 5kW inverter can discharge for 2 hours at full power, or longer at lower power. Cable sizing uses inverter kW, not battery kWh.

Grid Connection Requirements

All grid-connected battery systems must comply with G98/G99 engineering recommendations. This ensures safe disconnection during grid faults (anti-islanding) and limits export. DNO must be notified before connection. Systems over 16A per phase need application rather than simple notification. Non-compliance is illegal and dangerous.

Frequently Asked Questions

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