Solar storage, tariff charging and backup
Write down the intended jobs and rank them:
- storing daytime solar for later household use
- charging from a time-of-use tariff
- reducing import during an expensive period
- preserving a reserve for outages
- supplying selected circuits during a power cut
- supporting whole-home backup within a load limit
- participating in a supplier or flexibility service
One system may support several of these, but the settings can conflict. A large protected backup reserve reduces energy available for daily tariff or solar use. Filling the battery from the grid overnight can leave less headroom for morning PV.
Set practical targets. For example, identify the household load to cover between the end of solar generation and the next cheap period, or the essential circuits needed during an outage. Avoid starting with the capacity of a popular product.
For installation, DNO connection and handover, see the home battery storage guide.
Household demand, PV and tariff data
Annual electricity consumption cannot show the timing that controls a battery.
Use half-hourly or finer data where available:
- household import
- PV generation
- grid export
- major loads
- EV charging
- heat-pump demand
- tariff periods
Separate weekday, weekend and seasonal behaviour. A battery selected from a summer solar day can be underused or insufficient at another time of year.
Where solar is not yet installed, use the site-specific PV estimate and an interval profile rather than assuming a fixed percentage will be available for storage. Keep electricity imported directly from the grid separate from solar generation.
The data may come from several meters. Check whether each figure is total generation, net grid flow or a derived portal value. A reversed or misplaced CT (current transformer) can produce a convincing but incorrect household profile.
Usable and nominal capacity
Battery energy capacity is stated in kWh. Nominal capacity is the nameplate amount under the manufacturer’s definition. Usable capacity is the part made available for the specified operation after the product’s limits are applied.
The energy available for a particular job can be lower again because of:
- protected BMS limits
- configured minimum SoC (state of charge)
- backup reserve
- temperature
- age and state of health
- conversion loss
- inverter operating limits
The displayed SoC is an estimate within the controlled operating window, not a direct reading of every cell’s total chemical energy.
Ask the supplier to state:
- nominal capacity
- usable capacity
- usable capacity at beginning of life
- normal operating minimum
- backup reserve
- expansion-module effect
- capacity basis used by the warranty
If one quotation lists nominal kWh and another usable kWh, do not compare the numbers directly.
Charge and discharge power
Power is stated in kW. It determines how quickly energy can enter or leave the system.
Keep these ratings separate:
- continuous battery charge power
- continuous battery discharge power
- short-duration surge power
- inverter AC rating
- backup or EPS output
- per-phase limit
- configured import or export limit
The lowest applicable battery, BMS, inverter, cable or configured limit controls working power. Adding battery modules may increase stored energy without increasing inverter power.
Compare power with the intended load. A battery can have enough kWh for several hours but still be unable to run a group of appliances at the same time. Conversely, a high-power battery may empty quickly if its usable energy is small.
Temperature, SoC and age can also reduce available power. Ask for the product conditions rather than assuming the headline maximum is always present.
For tariff charging, check whether the allowed charge power and window are sufficient to reach the target after normal household demand. For solar, check whether the battery can accept the expected surplus without forcing avoidable export.
AC-coupled systems
An AC-coupled battery has its own battery inverter connected to the home’s AC installation. It can be added to an existing PV system without replacing the solar inverter where the complete design supports it.
Potential advantages include:
- clearer separation from existing solar
- independent replacement of the solar and battery inverter
- practical retrofit to several PV architectures
- battery-only installation without solar
Solar energy stored in an AC-coupled battery passes through the PV inverter from DC to AC, then the battery inverter from AC to DC. It returns through another DC-to-AC conversion when discharged to household loads. Compare efficiency using the declared system boundary rather than a cell-only figure.
The additional inverter contributes to the generation and storage capacity considered by the DNO. A site export limit is separate from that installed capacity.
If backup is required, confirm how the AC-coupled system behaves in island mode and whether the existing PV inverter can continue. The answer depends on the exact gateway, controls, PV inverter and protection.
DC-coupled systems
A DC-coupled battery connects through a compatible hybrid inverter or another DC-side controller shared with PV. Solar can charge the battery without first being converted to AC.
Potential advantages include:
- a common inverter and control platform
- fewer conversion stages for direct PV-to-battery energy
- coordinated PV and battery power control
- possible use of PV energy that would otherwise be clipped, where the exact design supports it
Compatibility is more tightly linked. Check:
- battery voltage range
- communication protocol
- supported modules and quantities
- firmware
- charge and discharge limits
- PV MPPT limits
- combined PV, battery and AC power restrictions
- replacement options if one component is withdrawn
A hybrid inverter’s separate PV, battery and AC maxima may not be available simultaneously. Ask for the operating limits in the proposed configuration.
Replacing a hybrid inverter later can affect both solar and battery operation. A long battery warranty is less useful if the required inverter or communications support is unavailable.
Solar and battery power flow
Ask for a schematic showing the PV, battery, inverters, meters, CTs, consumer units and grid connection.
The diagram should make clear:
- what measures grid import and export
- what measures PV generation
- which controller decides battery power
- whether grid charging is enabled
- whether battery export is enabled
- the DNO export or import limit
- how the system behaves after sensor failure
A shared app does not prove that the equipment uses one control loop. Independent solar, battery, EV and immersion controllers can all react to the same grid reading and compete.
If a legacy FIT (Feed-in Tariff) installation is present, the position of the battery relative to the generation meter matters. The modification must preserve accurate eligible generation and export measurement.
Location and building suitability
Assess the proposed battery location before choosing the equipment.
The assessment considers:
- fire and separation
- escape and firefighting routes
- ventilation
- stored flammables and fuel
- flooding and water
- structural load
- vehicle or accidental impact
- environmental conditions
- cable routes
- service and replacement access
- manufacturer clearances
PAS 63100, MIS 3012, BS 7671, the current IET Code and the product instructions can all affect the result. An enclosure rating or permission for outdoor use does not make every outside position suitable.
Ask for the written location assessment and any detection, protection, enclosure or structural work included in the quote. Do not accept a product brochure photograph as site evidence.
The inverter and battery can have different location requirements. Assess each item against the rules and product instructions for its proposed position.
DNO connection and limits
Battery storage is generation equipment for the Great Britain connection process because its inverter can operate in parallel with the grid.
The installer assesses:
- existing PV and other generation
- battery inverter Registered Capacity
- single-phase or three-phase arrangement
- Fully Type Tested status
- G98 or G99 route
- export and import limits
- G100 limitation where applicable
A G100 scheme limits net import or export at the connection point and must fail safely. It does not remove the installed inverter capacity from the G98 or G99 assessment.
Compare quotations against the same DNO scope. One may include a prior application, limitation hardware and commissioning while another assumes connect-and-notify.
Northern Ireland uses the separate G98/NI and G99/NI framework through NIE Networks.
Backup capability
A battery that stores energy does not necessarily provide power during a network outage. Normal grid-connected inverters must stop energising the grid when supply is lost.
Backup needs:
- compatible EPS or island output
- source separation and changeover
- island-mode earthing
- protection verified for inverter fault current
- backed-up circuits or whole-home distribution
- load and surge assessment
- reserve and low-battery behaviour
- commissioned grid-loss and restoration tests
Whole-home backup means the distribution is connected to the backup source. It does not mean every load can run simultaneously.
Ask for the continuous output, surge capability, phase arrangement, transfer behaviour and appliances or circuits included. PV continuation and black start are product-specific.
UPS (uninterruptible power supply) performance should not be assumed from a home battery backup specification. Critical equipment needs its own assessment of continuity and supply quality.
The home battery backup guide covers these checks in more detail.
Operating modes and external control
Review the modes the household will actually use:
- self-use
- time-of-use charge and discharge
- backup reserve
- solar export priority
- dynamic tariff or price control
- supplier-managed flexibility
- manual emergency mode
Ask who can change each setting and what happens without cloud access. Some integrations depend on the inverter account, supplier app, third-party service or an API.
Use one primary scheduler for normal operation. Two services can issue contradictory charge, discharge or reserve targets.
Confirm the fail-safe state after loss of the CT, meter, gateway, internet or cloud service. The system must retain the electrical and DNO limits even if tariff optimisation stops.
Battery warranty limits and separate cover
Battery warranties can use:
- elapsed years
- equivalent cycles
- aggregate energy throughput
- retained capacity
- application or operating-mode restrictions
- registration
- internet connectivity
- firmware access
- installer or service requirements
Compare the exact UK warranty for the model and installation date. A headline term on a product page may not show which limit ends cover first.
Separate:
- battery-module warranty
- inverter and gateway warranty
- capacity-retention promise
- labour and access
- installer workmanship
- insurance-backed protection
Warranty expiry does not mean the battery stops working. It changes the contractual cover.
Identify the legal entity carrying each obligation. If a manufacturer or installer ceases trading, confirm whether any other company has formally accepted the hardware warranty. Keep the contract, serials, registration and issued terms.
Statutory rights against the trader are separate from a manufacturer guarantee. Their application depends on the individual circumstances and should not be treated as a replacement warranty promise.
Battery monitoring data and account ownership
Useful monitoring shows:
- PV generation
- household consumption
- grid import and export
- battery charge and discharge
- SoC
- operating mode
- alarms and event history
Check which measurements come from a meter and which are calculated. Ask for data export, local access, account transfer and the handling of cloud subscriptions.
The owner should receive administrator access. The installation should not become unmanageable when the original installer account is removed.
For a supplier or flexibility service, check who can command the battery, how to opt out and whether the contract changes the product warranty or export tariff.
Battery quotation comparison
Put these rows into the final comparison:
- intended jobs
- usable kWh for normal operation
- protected backup reserve
- continuous charge and discharge kW
- backup power and circuits
- AC or DC architecture
- exact compatible equipment
- location and safety work
- DNO route and limitation
- monitoring and external control
- warranty limits and labour
- installer protection
- commissioning and handover
Ask the installer to amend the quotation where an answer changes the equipment or work. A verbal assurance that backup, battery expansion or a future tariff “should work” is not a system specification.