Operating modes and priorities
A home battery should be sized for its intended operating mode. The intended operation may include:
- storing spare solar for use after sunset
- charging during a lower-price tariff period and reducing import later
- keeping selected circuits available during a power cut
- limiting export or import at the grid connection
- combining more than one mode with defined control priorities
A battery reserved for backup leaves less energy for daily solar or tariff cycling. A unit large enough to hold a summer solar surplus may spend much of winter waiting for energy that never arrives. A battery designed around a long overnight charge window may not absorb the same energy during a shorter window.
The design states the priorities in order. For backup, it records how much reserve will remain and which loads need support. For tariff operation, it identifies the import periods the battery is intended to avoid. For solar storage, it uses measured or properly modelled generation rather than the array’s kWp alone.
A kWh allocated to backup reserve is not available for daily solar or tariff operation.
Usable capacity and power ratings
- Usable capacity, in kWh, is the energy the system makes available for the stated use.
- Continuous charge power, in kW, affects how much energy can enter during the available solar or tariff window.
- Continuous discharge power, in kW, affects how much household demand the battery can cover at one time.
- Peak power may be available only for a defined duration or operating mode. It is not a substitute for the continuous rating.
A large battery with a modest discharge limit may last for many hours at a low load but still import from the grid when a kettle, oven and heat pump run together. A high-power system with little usable capacity can cover a short peak but empty quickly.
Use usable battery capacity rather than assuming the nameplate figure is available. If a datasheet lists only nominal capacity and maximum depth of discharge, keep both inputs visible. MGD 003 defines usable capacity for the function being assessed and treats a permanent backup reserve as unavailable to solar self-consumption.
Conversion and standby losses also sit between stored DC energy and electricity delivered to AC loads. Capacity and efficiency figures need a stated measurement boundary. A battery-side value and a whole-system value are not the same measurement.
The battery power rating entry covers continuous, peak, charge and discharge figures in more detail.
Household demand and generation data
Annual consumption is useful context, but it cannot show when energy needs to move. Use interval data where it is available, together with solar generation data for an existing array. A representative run of days is more useful than one unusually sunny day or one winter peak.
For solar storage, identify:
- electricity exported while the battery could have been charging
- demand after solar production falls
- seasonal differences in generation
- existing export payments that storage would displace
- the power available from the solar and inverter arrangement
For tariff operation, identify:
- the energy normally used during the higher-price period
- the length of the charging window
- the system’s permitted grid-charge rate
- round-trip loss
- standing household loads while charging
- any tariff, supplier or device-control condition
For backup, list the actual maintained loads and the time they need to run. Fridges, lighting, communications, pumps and heating controls create a different design from an electric shower, induction hob, EV charger or whole-home promise.
The model needs visible inputs and results for more than one season. An annual saving figure can be distorted by an unrealistic cycle count, excess battery capacity or no value for exported solar electricity. The solar self-consumption battery strategy and time-of-use battery arbitrage entries separate the energy and tariff parts of those calculations.
AC-coupled and DC-coupled architecture
An AC-coupled battery has its own inverter connected on the AC side of the installation. It can be a practical route where solar already has a separate inverter or where storage is being installed without solar. When solar energy is stored and later used by AC loads, it passes through the solar inverter and the battery conversion path, so the design needs a whole-system loss assumption.
A DC-coupled battery connects through a compatible hybrid inverter shared with the solar array. This can reduce conversion stages between the array and battery, but it ties the design to the hybrid inverter’s battery voltage, control protocol, charge limits and approved combinations. Future replacement and expansion depend on that compatibility.
Neither arrangement is universally better. Compare:
| Question | AC-coupled | DC-coupled |
|---|---|---|
| Existing solar | Can retain a separate solar inverter where the design permits | May require a compatible hybrid inverter and redesign |
| Solar-to-battery path | Uses the solar and battery conversion paths | Uses the shared DC side before AC conversion |
| Grid registration | Separate inverter capacity may add to existing generation | Shared inverter capacity still needs the complete system assessed |
| Future changes | Battery and solar remain more electrically separate | Battery choice and expansion remain tied to the hybrid platform |
The exact inverter, battery modules, controller, CT (current transformer) arrangement and communications define the architecture. “Works with solar” or “battery ready” is not a compatibility schedule. The AC-coupled, DC-coupled and retrofit battery storage entries explain the boundaries.
Battery location and site assessment
The location assessment covers the building, battery, power-conversion equipment and the work needed to install, commission and maintain them. Fitting the enclosure on a wall is only one part of that assessment.
The location assessment should cover:
- protected escape routes and the effect of a fire in the proposed position
- ventilation and the manufacturer’s temperature and clearance limits
- flooding, water ingress and foreseeable submersion
- accidental damage, including vehicle impact in a garage or beside a driveway
- wall or floor strength, point loads and the route used to move heavy equipment
- safe access for isolation, inspection, maintenance and later removal
- fire detection and the relationship with the building’s existing alarms
- cables, communications and separation from incompatible equipment
MIS 3012 requires suitable structural support, safe working space and a position that does not compromise escape routes. The site assessment should consider the current IET Code, PAS 63100 and BS 7671 as applicable, together with the manufacturer’s instructions. A model approved for outdoor use still needs a suitable outdoor position. An enclosure rating does not address flooding, direct impact, access or the surrounding construction.
The design record explains why the position is suitable and identifies any work needed before installation. The MIS 3012 and PAS 63100 reference pages explain the relevant requirements.
Grid connection and FIT metering
A battery inverter can operate in parallel with the public network, so the DNO connection route must include the storage and any existing generation. In Great Britain, the installer determines whether G98, G99 and any G100 export or import limitation apply.
The decision is not based on battery kWh. It considers the aggregate AC Registered Capacity, Fully Type Tested status, existing solar or storage, inverter architecture and the proposed mode of operation. A shared hybrid inverter and a separate AC-coupled inverter are not counted in the same way.
G100 can hold net flow at an agreed limit measured at the connection point. It does not remove the generating capacity installed behind that limit or automatically turn a G99 design into G98. The approved scheme also needs fail-safe behaviour if its measurement or communications fail. The export limitation with batteries entry covers that distinction.
Northern Ireland uses the separate G98/NI and G99/NI framework through NIE Networks. Its connection route and equipment schedule use the applicable Northern Ireland documents rather than the Great Britain forms.
If the property receives FIT payments for existing solar, contact the FIT licensee before the battery is added. Ofgem requires eligible generation and export to remain measured correctly and says storage additions must be notified. Keep the final single-line diagram and metering arrangement with the FIT records.
Backup circuits and island mode
A battery connected to the grid normally stops supplying the installation when grid conditions are lost unless a suitable island-mode arrangement has been designed. A product description that mentions EPS or backup does not state which circuits will run, for how long or at what power.
For backup operation, the technical schedule defines:
- the maintained circuits or the distribution included in whole-home backup
- continuous and peak output in island mode
- loads that are excluded or automatically shed
- the reserve available at the time of a cut
- whether solar can continue charging the battery while islanded
- transfer behaviour and any interruption to sensitive equipment
- isolation, neutral treatment, earthing and protection for island mode
- how the system returns to normal operation
Whole-home backup describes what remains connected, not what can run simultaneously. The house can still overload the inverter if the connected demand exceeds its island-mode rating. A short advertised transfer time also does not make the system a UPS for every sensitive device.
The whole-home backup changeover entry explains the electrical boundaries. The handover includes a circuit schedule and practical demonstration of the intended operating mode; the owner is not expected to test or alter protective equipment.
Product, system and installation boundaries
Battery data separates energy from power. Nominal capacity, usable capacity and reserve describe stored energy, while continuous charge, discharge and island-mode ratings describe how quickly energy can move. A time-limited peak is not a continuous rating.
The installed system adds the inverter, gateway, meter, CT, communications, AC- or DC-coupled architecture and relationship with existing generation. Location, fixing, environmental conditions, fire assessment, G98, G99 or G100 connection route and control behaviour belong to the installation rather than the battery module alone.
Certification attaches to an exact legal entity and defined scope of work. Product warranties, capacity-retention promises and workmanship protection cover different failures. The issued terms govern throughput limits, required servicing, internet connectivity, operating temperature, authorised control and transfer to a new owner.
Later expansion depends on whether additional modules must match the original type and age, whether the inverter has spare power or voltage range and whether added capacity changes the warranty, location assessment or DNO record.
Handover records and account control
The final pack identifies what was installed and how it was commissioned. It contains:
- equipment models, serial numbers and final single-line diagram
- electrical certificate and Building Regulations evidence where applicable
- MCS certificate where applicable
- DNO notification, authorisation and export-limitation records
- commissioning checklist and test references
- location and structural records
- maintained-circuit schedule and backup instructions
- product, capacity-retention, workmanship and financial-protection documents
- operating settings, reserve, charge schedule and export controls
- manuals, monitoring credentials and account-transfer instructions
The owner account is configured at handover, together with the normal readings, control permissions and fallback operating state if the internet, tariff feed, meter signal or installer portal is unavailable.
Screenshots are dated evidence of settings, not substitutes for certificates and manuals. The same equipment, network, warranty and account records are relevant when ownership of the home changes.