A home battery is not just a number of kilowatt hours.
Capacity matters, but so do power, controls, tariffs, backup capability and the way the battery interacts with the rest of your home.
The right system depends first on what you want the battery to do.
Start With What You Want the Battery to Do
Before comparing battery sizes or brands, decide what job you want the battery to perform.
You might want it to:
- store excess solar electricity
- charge from cheaper off-peak electricity
- reduce peak-rate grid imports
- provide backup during a power cut
- support EV charging
- support a heat pump
- increase solar self-consumption
- take advantage of export tariffs
- combine several of these functions
Two homes with similar electricity use may need very different batteries if their objectives are different.
A battery designed mainly to store afternoon solar for use in the evening is a different proposition from one expected to provide whole-home backup or support large electrical loads.
Choose the job first. Then choose the battery.
What Does a Home Battery Actually Do?
A home battery stores electrical energy so it can be used later.
Depending on the system, it may charge from:
- solar PV
- the electricity grid
- both solar and the grid
It can then discharge electricity back into the home when required.
A typical solar-and-battery system might behave like this:
Solar → Home → Battery → Grid
During another period, the energy flow might be:
Grid → Battery → Home
And in some systems:
Battery → Home → Grid Export
The exact behaviour depends on the equipment, settings, tariff and how the installation is designed.
That is why a battery should be thought of as part of the wider home-energy system rather than simply as a box that stores electricity.
Battery Capacity vs Battery Power
This is one of the most important things to understand when comparing home batteries.
Capacity — kWh
Battery capacity is measured in kilowatt hours — kWh.
It tells you how much energy the battery can store.
A 10 kWh battery can theoretically store twice as much energy as a 5 kWh battery.
But that tells you nothing about how quickly the energy can be delivered.
Power — kW
Battery power is measured in kilowatts — kW.
It tells you how quickly the battery can charge or discharge.
This determines how much electrical demand it can support at one time.
A home might simultaneously have:
- a heat pump running
- a kettle boiling
- an oven operating
- appliances running
- an EV charging
Whether the battery can support that demand depends on its power, not just its stored capacity.
Capacity tells you how much energy you have. Power tells you how quickly you can use it.
A Bigger Battery Is Not Always a More Powerful Battery
Example
Imagine two batteries.
Battery A
- 10 kWh usable capacity
- 3 kW continuous output
Battery B
- 8 kWh usable capacity
- 6 kW continuous output
Battery A stores more energy. Battery B can supply more power at one time.
Which is better depends entirely on what you want the system to do.
For evening solar storage, capacity might be the main priority.
For backup power or supporting larger loads, output power may become equally important.
This is why comparing batteries purely by the largest kWh number can be misleading.
Usable Capacity, Reserve and Battery Losses
The capacity printed on a product specification is not always the amount of energy available to use.
Several terms can appear.
- Nominal Capacity
- The total theoretical capacity of the battery.
- Usable Capacity
- The amount of stored energy made available for normal use.
- State of Charge
- The percentage of energy currently stored in the battery.
- Battery Reserve
- A percentage deliberately kept available rather than discharged during normal operation.
- Depth of Discharge
- How much of the battery's capacity can be used before reaching its lower operating limit.
There are also losses when energy is stored and recovered.
Electricity passes through power electronics and the battery itself during charging and discharging.
As a result:
10 kWh put into a battery does not necessarily mean 10 kWh will later come back out.
This matters when considering solar storage, tariff charging and potential financial savings.
How Big Should Your Battery Be?
There is no single battery size that suits every home.
Useful battery capacity depends on factors including:
- daily electricity consumption
- when electricity is used
- solar generation
- how much solar is exported
- electricity tariffs
- export tariffs
- desired backup duration
- battery charge and discharge power
- EV charging
- heat-pump demand
- future electrical loads
A household using 15 kWh per day does not automatically need a 15 kWh battery.
Some of that electricity may be used while solar is generating.
Some loads may be too large for the battery to supply fully.
And if the battery is mainly being used to shift cheap overnight electricity into expensive daytime periods, the ideal size may depend on tariff windows rather than solar generation.
The correct starting point is therefore not:
“How much electricity does the house use?”
It is:
“When does the house use electricity, and what do we want the battery to achieve?”
Battery Storage With Solar PV
Battery storage is commonly installed alongside solar PV.
Without a battery, solar electricity is typically:
used by the home first → surplus exported
With a battery:
used by the home first → surplus charges battery → remaining surplus exported
The stored solar energy can then be used later when generation falls.
This can increase the proportion of solar electricity used within the property.
But installing solar does not automatically mean a battery is essential.
Whether battery storage makes sense depends on:
- when you use electricity
- how much solar you generate
- export rates
- import tariffs
- battery cost
- how the battery will be controlled
You Can Have Battery Storage Without Solar
A home battery does not need solar panels to be useful.
A standalone battery can charge from the grid when electricity is cheaper and discharge later when electricity is more expensive.
The energy flow might look like:
Cheap-rate Grid Electricity → Battery → Home During Peak Period
This can be particularly relevant for homes on time-of-use tariffs.
It can also allow battery storage to be installed before solar PV, with solar added later if the system has been planned appropriately.
Battery storage is therefore increasingly becoming an energy-management technology, rather than simply an accessory for solar panels.
AC vs DC Coupled Battery Storage
Home batteries can be connected to a property in different ways.
Two common arrangements are AC coupling and DC coupling.
DC Coupled
Solar generation and battery storage are connected on the DC side of a compatible hybrid inverter.
A simplified arrangement is:
Solar Panels → Hybrid Inverter ↔ Battery → Home
This is common on systems where solar and battery storage are designed together.
AC Coupled
The battery has its own inverter connected to the home’s AC electrical system.
A simplified arrangement is:
Solar Inverter → Home AC System ← Battery Inverter
This can be useful when adding a battery to an existing solar installation.
Neither architecture is automatically better.
The right choice depends on:
- whether solar already exists
- inverter compatibility
- backup requirements
- system efficiency
- future expansion
- manufacturer ecosystem
- installation constraints
Smart Tariffs Can Change How a Battery Is Used
A modern battery does not have to wait for excess solar before charging.
It may also charge from the grid when electricity is cheaper.
For example:
Overnight cheap electricity → Battery → Daytime household demand
Battery strategy can therefore involve:
- charging during low-cost periods
- avoiding expensive peak periods
- reserving capacity for solar later in the day
- preserving backup capacity
- exporting when export prices are attractive
- automatic optimisation based on tariffs
This means the same battery can perform very differently depending on its settings.
Battery hardware matters. Battery strategy matters too.
How Does the Battery Know When to Charge or Discharge?
A battery system needs to understand what is happening electrically around the home.
It may use:
- an energy meter
- CT clamps
- inverter measurements
- smart-meter information
- software controls
A CT clamp is commonly used to measure the direction and amount of electricity flowing through a cable.
This helps the battery understand whether the property is:
- importing electricity
- exporting electricity
- generating surplus solar
- experiencing additional electrical demand
The battery then responds according to its settings.
This leads to an important principle:
A battery does not know what you intended. It reacts to what the system can measure and the rules it has been given.
Why Does My Battery Sometimes Do Something I Don't Expect?
Battery behaviour can sometimes seem strange until the controls are understood.
Examples include:
- charging from the grid unexpectedly
- refusing to discharge
- stopping at a particular state of charge
- exporting electricity
- charging at a different time than expected
- discharging heavily when an EV begins charging
There can be many reasons.
The system may be:
- following a tariff schedule
- protecting a backup reserve
- responding to a CT measurement
- obeying export settings
- reaching an inverter limit
- following automated optimisation
Before assuming the battery is faulty, it is useful to understand what the system is actually measuring and how it has been configured.
Battery Storage and EV Charging
An electric vehicle can represent one of the largest electrical loads in a home.
A typical home EV charger may draw around 7 kW.
If a battery sees that load in the same way as normal household demand, it may attempt to support the EV charger.
That could result in:
Battery → EV
and potentially empty a home battery surprisingly quickly.
Sometimes that is exactly what the homeowner wants.
Sometimes it is not.
The desired behaviour depends on:
- electricity tariff
- solar generation
- battery size
- battery output power
- EV charging schedule
- charger controls
- metering and CT arrangement
This is why solar, batteries and EV chargers should be considered together rather than as unrelated products.
Battery Storage and Heat Pumps
A heat pump can substantially increase a property’s electrical demand, particularly during winter.
That affects battery design in several ways.
Consider:
- winter solar generation is lower
- heating demand is generally higher
- heat pumps can operate for long periods
- electrical demand may coincide with expensive tariff periods
- backup power requirements become more demanding
A battery may help shift cheaper electricity into more expensive periods or provide some support for heating demand.
But both capacity and output power need to be considered.
A large battery with limited discharge power may still be unable to supply all simultaneous household loads.
Storage Is Not the Same as Backup
Important
Installing a battery does not automatically mean your home will remain powered during a power cut.
This is one of the biggest misconceptions around home batteries.
Most grid-connected electrical systems must disconnect safely from the electricity network when a grid outage occurs.
To provide backup power, the system may require additional equipment such as:
- EPS capability
- backup gateway
- automatic switching
- grid isolation
- dedicated backup circuits
- suitable inverter functionality
Backup should therefore be considered during system design rather than assumed simply because a battery is present.
Different Levels of Backup
Backup can mean different things.
Essential-Loads Backup
Selected circuits may remain powered.
Typical examples might include:
- lighting
- fridge and freezer
- broadband
- selected sockets
- controls
This reduces the electrical demand placed on the battery during an outage.
Partial-Home Backup
A larger group of circuits remains available, while particularly large loads are excluded.
Whole-Home Backup
The backup system can supply most or all of the property’s electrical installation.
But this does not mean unlimited power.
Even a large battery can still be restricted by:
- inverter output
- battery discharge power
- simultaneous electrical demand
- starting currents
- remaining state of charge
Whole-home backup does not mean infinite power.
What Can a Home Battery Actually Run?
To answer this properly, two questions need to be considered:
- Does the battery have enough stored energy?
- Can it deliver enough power?
Small loads such as:
- broadband
- lighting
- refrigeration
may be relatively easy to support.
Larger loads such as:
- kettles
- ovens
- immersion heaters
- heat pumps
- EV chargers
can require substantially more power.
Several large loads operating simultaneously can exceed the battery or inverter’s output even when plenty of energy remains stored.
This is another reason why the kWh figure alone does not tell you what a battery can do.
What Happens During a Long Power Cut?
Backup behaviour varies between systems.
Questions worth asking include:
- how much battery capacity is reserved for backup?
- can the solar panels continue generating during an outage?
- can solar recharge the battery while disconnected from the grid?
- what happens if the battery becomes completely depleted?
- can the system restart itself once solar generation returns?
Some systems have more advanced backup and restart capabilities than others.
If backup is important, these details should be understood before the system is purchased.
Where Can a Home Battery Be Installed?
Battery location should be considered as part of the system design.
Possible locations might include:
- garage
- utility area
- dedicated plant space
- suitable external wall
- other manufacturer-approved locations
Factors to consider include:
- battery dimensions
- weight
- wall or floor mounting
- operating temperature
- weather protection
- flooding risk
- access for servicing
- cable routes
- inverter position
- required clearances
- manufacturer requirements
The easiest place to physically fit the battery is not necessarily the best place for the complete installation.
Can More Battery Capacity Be Added Later?
Many modern home batteries are modular or expandable.
That can allow additional battery modules to be installed later.
But expandable does not necessarily mean unlimited.
There may be restrictions around:
- maximum battery modules
- inverter capacity
- charge and discharge power
- compatible battery generations
- firmware
- age differences between battery modules
- warranty conditions
It is also important to distinguish between adding capacity and adding power.
Adding another battery module may increase the amount of stored energy without increasing the maximum amount of power the system can supply.
Plan for Future Electricity Demand
Homes are becoming increasingly electrified.
A property might currently have a relatively modest electrical load but later add:
- electric vehicle
- second EV
- heat pump
- air conditioning
- electric hot water
- additional solar PV
Those changes can significantly alter how useful a battery is and how it should be controlled.
Future-proofing does not necessarily mean buying the largest battery today.
It means considering where the home is likely to go before making choices that could unnecessarily limit future options.
Grid Connection and Export
Grid-connected battery systems operate alongside the electricity network.
Depending on the system design, the battery may:
- import electricity for charging
- reduce grid imports
- work alongside solar generation
- export electricity
Grid requirements can depend on:
- inverter capacity
- existing solar generation
- additional generation
- export settings
- DNO requirements
Some installations may require notification, approval or export limitation.
The details vary, but the key point for homeowners is simple:
The battery is part of the property’s electrical system and grid connection, not an isolated appliance.
Is Home Battery Storage Worth It?
There is no universal answer.
The financial value of battery storage depends heavily on how it is used.
Important factors include:
- battery purchase price
- usable capacity
- electricity import rates
- export rates
- solar generation
- household load profile
- round-trip losses
- charging strategy
- battery lifespan
- degradation
- warranty
- future electricity demand
A battery that simply stores a small amount of excess solar may have a very different financial outcome from the same battery actively shifting cheap-rate electricity every day.
This is why battery value should be assessed around how the system will operate, not just how much the equipment costs.
What Should a Good Battery Proposal Tell You?
A battery quotation should contain enough information for you to understand what is actually being offered.
Useful information includes:
- battery manufacturer and model
- nominal capacity
- usable capacity
- continuous output power
- peak power where relevant
- inverter model and rating
- AC or DC coupled architecture
- solar compatibility
- backup capability
- which circuits are backed up
- expansion options
- monitoring and controls
- warranty
- expected operating strategy
- grid and export arrangements
Two quotations for batteries with a similar capacity may still represent very different systems.
Understanding the specification makes them much easier to compare.
Battery Tools & Calculators
Energy Flow Hub is developing practical tools to help homeowners understand battery storage.
- Battery Size Calculator
- Explore how electricity use, solar generation and battery strategy affect suitable capacity.
- Capacity vs Power Visualiser
- See why battery kWh and battery kW are two different things.
- What Can My Battery Run?
- Explore how different household loads affect battery output and runtime.
- AC vs DC Battery Visualiser
- See how different battery architectures connect to solar and the home.
- Home Energy Flow Simulator
- Watch solar, battery, grid, EV and household demand interact in different scenarios.
Common Home Battery Questions
Do I need solar panels to install a battery?
No. A battery can also charge from the electricity grid and discharge later.
What size battery do I need?
It depends on when you use electricity and what you want the battery to achieve, not simply your annual consumption.
Can I add more batteries later?
Many systems are expandable, but manufacturer and inverter limits can apply.
Will my battery work during a power cut?
Not automatically. Backup requires suitable inverter capability, switching equipment and a clearly designed backup circuit or whole-home arrangement.
Can a battery charge from the grid?
Many systems can. Grid charging can support smart-tariff shifting and may be useful even without solar panels.
What is the difference between kW and kWh?
kWh describes stored energy. kW describes how much power the battery can charge or discharge at one time.
Is a bigger battery always better?
No. Extra capacity only creates value when the home can charge it and use or export the additional stored energy often enough.
Start With the Battery's Job
A useful battery design begins by deciding what the battery needs to achieve: capture solar surplus, shift electricity through a tariff, provide backup, or combine those jobs.
Once the job is clear, capacity, power, coupling, controls and expansion can be assessed as one system.