Home battery specifications often show several similar-looking figures:
- kWh
- kW
- kWp
- inverter power
- charge power
- discharge power
They do not mean the same thing.
The simplest distinction is:
kWh tells you how much energy the battery can store. kW tells you how quickly that energy can move.
That means a battery can be large in kWh and still be relatively modest in kW.
One figure tells you roughly how long the battery can help.
The other tells you how much it can help at once.
What Does kWh Mean on a Home Battery?
kWh stands for kilowatt-hour.
It is a measure of energy.
On a home battery, kWh tells you how much electricity can be stored.
For example, a battery with 10 kWh of usable capacity could theoretically supply:
- 1 kW for 10 hours
- 2 kW for 5 hours
- 5 kW for 2 hours
before allowing for losses, reserve settings, changing household demand and other system limits.
kWh tells you how much energy is available.
What Does kW Mean on a Home Battery?
kW stands for kilowatt.
It is a measure of power.
On a battery system, kW tells you how quickly energy can move into or out of the battery.
For example, a 10 kWh battery with 5 kW maximum discharge power means the battery may contain plenty of stored energy, but the system can only supply up to around 5 kW continuously if that is the system limit.
If the home suddenly needs more than that, the grid may supply the difference.
kW tells you how much demand the battery can cover at one time.
The Simplest Way to Remember kW vs kWh
- kWh — How Long?
- How much energy is stored.
- kW — How Much at Once?
- How quickly that energy can be delivered.
Another simple analogy is:
kWh = size of the tank
kW = size of the pipe
A large tank with a small pipe can hold a lot of energy but cannot deliver it very quickly.
A small tank with a large pipe can deliver energy quickly but may empty sooner.
kWh is the size of the store. kW is the rate at which energy can flow in or out.
What Does a 10 kWh Battery with a 5 kW Inverter Mean?
This is one of the most common battery configurations homeowners will see.
Imagine 10 kWh usable battery capacity and 5 kW continuous inverter output.
The two figures describe different things.
The 10 kWh figure tells you how much energy is available.
The 5 kW figure tells you how quickly that energy can be supplied to the home.
If the home is using 3 kW, the battery may be able to cover the entire load.
If the home is using 7 kW, the battery may supply 5 kW while the grid supplies the remaining 2 kW.
The battery still contains plenty of energy.
The limitation is simply how quickly that energy can be delivered.
Having enough stored energy does not guarantee enough instantaneous power.
A Bigger Battery Does Not Automatically Mean More Power
Example
Battery A — capacity 10 kWh, maximum output 5 kW.
Battery B — capacity 20 kWh, maximum output 5 kW.
Battery B stores twice as much energy. But if both systems use the same 5 kW inverter, their maximum continuous household output may be identical.
The larger battery may last longer. It does not automatically become more powerful.
This is an important distinction when comparing battery sizes.
A battery can become twice as large in kWh without becoming twice as powerful in kW.
Power-to-Capacity Ratio
Looking at battery power and capacity together can make the difference easier to understand.
A 10 kWh battery with 5 kW output, at maximum output: 10 kWh ÷ 5 kW = around 2 hours theoretically.
A 20 kWh battery with 5 kW output, at maximum output: 20 kWh ÷ 5 kW = around 4 hours.
Again, that is before losses, reserves and real operating conditions.
The second system lasts longer.
But it does not supply more power.
Adding kWh increases duration. Adding kW increases the rate at which the battery can help.
What Actually Limits Battery Power?
The battery modules are only one part of the system.
A typical energy path may be:
Battery Modules → Battery Electronics → Inverter → AC Supply → Home
The actual system limit may be set by:
- battery modules
- battery management system
- hybrid inverter
- dedicated battery inverter
- system configuration
- backup mode
- manufacturer limits
This is why a battery module’s capacity does not tell you the whole performance story.
Always check the power limit of the complete battery system, not just the capacity of the battery modules.
Charge Power vs Discharge Power
- Charge Power
- How quickly energy can enter the battery.
- Discharge Power
- How quickly energy can leave the battery and supply the home.
Battery power is not always one number.
A system may have different limits for charging and discharging.
These figures may be the same. They may also be different.
This becomes particularly important when comparing battery systems for:
- smart tariff charging
- solar storage
- high household demand
- backup
Battery power is not always one number. Charging and discharging can have different limits.
Continuous Power vs Peak Power
- Continuous Power
- The output the system can normally sustain.
- Peak or Surge Power
- A higher output that may only be available for a short period. Can help with loads that briefly require more power, such as motors, pumps or compressors.
Battery specifications may also show continuous power, peak power and surge power.
These are not interchangeable.
Peak power should not be treated as the normal everyday output of the system.
A short peak-power figure should not be compared with another battery’s continuous output.
Backup Power Can Be Different Again
Backup operation can introduce another power figure.
A battery may have:
- normal grid-connected output
- backup output
- short-duration peak backup output
These are not always identical.
A battery may therefore behave differently during normal operation and during a grid power cut.
If backup is important, check the actual backup specification rather than assuming the normal battery output applies.
Do not assume normal output, peak output and backup output are the same figure.
How Long Will a Home Battery Last?
A simple way to estimate battery runtime is:
Approximate Runtime = Usable Battery Capacity ÷ Average Load
For example, a 10 kWh usable battery supplying an average 2 kW load gives 10 ÷ 2 = around 5 hours theoretically.
In reality, the result can be affected by:
- battery reserve
- conversion losses
- changing household demand
- battery protection limits
- operating conditions
There is also one important requirement: the battery must have enough power to supply the load.
If the battery can only supply 1.5 kW continuously, it cannot independently support a 2 kW load even if enough energy is stored.
Runtime calculations only work if the battery has enough power to support the load in the first place.
How Fast Can a Battery Charge?
The same relationship works in the other direction.
A simple estimate is:
Approximate Charging Time = Energy Required ÷ Charge Power
For example, energy required of 10 kWh at a charge power of 5 kW gives a theoretical charge time of around 2 hours.
Real charging may take longer because of conversion losses, battery management, charge tapering, temperature and household demand.
But the relationship is still useful when comparing systems.
Cheap Tariff Windows Make kW Important
Example
Battery capacity: 15 kWh
Charge power: 3 kW
Cheap tariff window: 4 hours
The theoretical maximum energy that can be added during that period is 3 kW × 4 hours = 12 kWh, before losses and operating limits.
So even though the battery can store 15 kWh, the cheap period may not be long enough to fill it from empty.
Charge power becomes especially important if you intend to fill the battery using a cheap overnight electricity tariff.
Battery capacity tells you how much you could store. Charge power and available time tell you how much you can actually put in.
Solar Charging Has the Same Constraint
Charge power matters when storing surplus solar too.
Imagine a solar surplus of 7 kW, a battery maximum charge power of 3 kW and a battery state of charge of 50%.
The battery may have plenty of spare capacity.
But if the system can only charge at 3 kW, it cannot absorb the full 7 kW solar surplus at that moment.
The remaining electricity may supply other household loads or be exported to the grid, depending on how the system is configured.
Having spare battery capacity does not mean the battery can absorb unlimited solar surplus.
What Happens If the House Uses More Power Than the Battery Can Supply?
Usually, the grid simply supplies the difference.
For example, a house demand of 7 kW with a battery maximum output of 5 kW means a grid contribution of 2 kW.
The battery is still doing useful work.
It is supplying most of the household demand and reducing grid import substantially.
This is an important distinction.
The battery does not need to cover every single instantaneous load to be valuable.
A battery can reduce grid import substantially without eliminating every instantaneous import.
Does Battery Power Need to Match the Maximum House Load?
Not necessarily.
A home might normally use 1–3 kW but occasionally reach 8–10 kW when several large appliances overlap.
For example:
- heat pump
- oven
- kettle
- immersion heater
- other household loads
If those peaks only last a few minutes, designing the entire battery system around the absolute highest recorded demand may add considerable cost for relatively little benefit.
A better question is:
How often does household demand exceed the battery power limit, and how much energy is actually imported during those periods?
Battery power does not need to match the highest electrical peak the home has ever recorded to be useful.
Heat Pumps and Battery Power
A heat pump can add a sustained electrical load to the home.
For example, a heat pump at 2.5 kW, general household load of 1 kW and cooking at 3 kW gives a combined demand of 6.5 kW.
A 5 kW battery may still cover the majority of that demand, with the grid supplying the difference during the overlap.
That may be completely acceptable.
The correct battery-power requirement depends on whether the homeowner wants to:
- reduce grid imports
- cover most normal demand
- eliminate almost all grid import
- provide backup
These are different objectives.
A heat pump raises the power requirement, but it does not automatically mean the battery must cover the home’s theoretical maximum demand.
EV Chargers and Battery Power
A 7 kW EV charger does not automatically mean the home battery must have at least 7 kW of output.
If the EV charges directly during a cheap electricity period, Cheap Grid → EV may be the most sensible strategy.
There may be little benefit in doing Cheap Grid → Home Battery → EV, because that introduces extra battery cycling and additional energy conversions.
The battery can instead be used to support household demand during more expensive periods.
Not every large electrical load needs to become a battery load.
kW vs kWh vs kWp
- kWp — Solar Array Size
- Kilowatt-peak. Describes the rated peak capacity of the solar PV array under standard test conditions.
- kWh — Battery Capacity
- Describes how much energy the battery can store.
- kW — Power
- Describes how quickly electricity can flow through the system at a particular moment.
Solar and battery quotations can contain several similar-looking measurements. For example: a 6 kWp solar array, a 10 kWh battery and a 5 kW inverter.
Three similar-looking numbers can describe three completely different parts of the system.
Battery Inverter Power vs DNO Export Limit
The battery or inverter may be capable of one level of power while the property has a different permitted grid export limit.
For example, an inverter capability of 10 kW with permitted export of 5 kW.
These are separate specifications.
The inverter may potentially move more power internally around the property while export control prevents more than the agreed amount from entering the grid.
Inverter power and DNO export limit are separate specifications.
Does Adding More Battery Capacity Increase Power?
Sometimes. But not always.
Adding another battery module may:
- increase usable kWh only
- increase both kWh and available battery-side power
- increase battery capability while the inverter still limits AC output
The answer depends on the system architecture.
For example, you might increase from 10 kWh to 20 kWh while the same 5 kW inverter remains in place.
The battery lasts longer. But the maximum household output remains unchanged.
Other systems may scale both capacity and power as modules are added.
Adding battery capacity and adding battery power are not automatically the same upgrade.
What Should You Check on a Battery Quote?
A useful battery proposal should make the main performance figures clear.
Look for:
- nominal battery capacity
- usable battery capacity
- maximum charge power
- maximum discharge power
- continuous AC output
- peak output
- how long peak output is available
- backup output
- inverter rating
- whether adding modules increases power
- which component limits the complete system
If the proposal only tells you “10 kWh battery”, you still do not know how quickly that battery can charge or how much of the home’s demand it can supply at once.
A quote that only tells you the battery’s kWh capacity is not giving you the full performance picture.
Which Number Matters for Which Question?
| Question | Main Figure to Check |
|---|---|
| How much energy can the battery store? | kWh |
| How long will the battery last? | Usable kWh + load |
| How much can it supply at once? | Discharge kW |
| How quickly can it recharge? | Charge kW |
| Can it fill during my cheap tariff window? | Charge kW + available time |
| Can it run several large loads? | Continuous discharge kW |
| How long will backup last? | Usable backup kWh |
| What can run during backup? | Backup kW |
| How large is the solar array? | kWp |
Worked Example: 10 kWh Battery + 5 kW Inverter
Consider a battery with usable capacity 10 kWh and continuous output 5 kW.
- Home Demand = 3 kW
- The battery can potentially supply 3 kW and the grid does not need to contribute. At a constant 3 kW demand, the theoretical runtime would be 10 ÷ 3 = around 3.3 hours before losses and reserve.
- Home Demand = 7 kW
- The battery can supply up to 5 kW and the grid supplies 2 kW. The battery is not failing — it is operating at its maximum available power.
- Home Demand = 2 kW
- The battery could theoretically supply that load for 10 ÷ 2 = around 5 hours before losses and reserve.
This one example shows why both figures matter.
10 kWh tells us how much energy is available.
5 kW tells us how quickly we can use it.
Worked Example: 20 kWh Battery + the Same 5 kW Inverter
Now double the battery capacity: usable capacity 20 kWh, continuous output 5 kW.
The system can store twice as much energy.
At a constant 2 kW load: 20 ÷ 2 = around 10 hours theoretically.
But if household demand rises to 7 kW, the battery still only supplies 5 kW and the grid still supplies 2 kW.
The system lasts longer, but it does not become more powerful.
What Makes a Good Battery Power Specification?
There is no single correct kW figure for every home.
A useful system should have enough power to:
- absorb a meaningful amount of solar surplus
- charge effectively during a cheap tariff window
- cover a useful proportion of household demand
- support the intended backup loads
- work with future energy plans
It does not necessarily need to eliminate every rare high-power grid import.
The right battery power is the one that covers the loads that matter most often.
Common Home Battery kW vs kWh Questions
What is the difference between kW and kWh?
kWh measures stored energy. kW measures power, or how quickly that energy can move.
Is a 10 kWh battery also 10 kW?
No. A 10 kWh battery may have a completely different charge or discharge power rating.
What does a 10 kWh battery with a 5 kW inverter mean?
The battery stores around 10 kWh of usable energy while the inverter may limit continuous household power to around 5 kW, depending on the exact system.
How long will a 10 kWh battery last?
It depends on household load. At an average 2 kW demand, 10 kWh would theoretically last around five hours before losses and reserve are considered.
Can a 5 kW battery run a whole house?
It may cover most normal household demand, but if simultaneous demand exceeds 5 kW the grid may need to supply the difference.
What happens if household demand exceeds battery output?
The battery supplies up to its available power limit and the grid can supply the remaining demand during normal grid-connected operation.
Is charge power the same as discharge power?
Not necessarily. Battery systems can have different maximum charging and discharging limits.
What is continuous battery output?
It is the power the battery system can sustain during normal operation.
What is peak battery output?
It is a higher power level that may only be available for a short period.
Does adding another battery module increase power?
Sometimes, but not always. Capacity may increase while inverter output remains unchanged.
What kW battery do I need for a heat pump?
It depends on the heat pump’s electrical demand, other simultaneous loads and whether you want the battery to cover all demand or simply reduce grid import.
Do I need 7 kW battery output for a 7 kW EV charger?
Not necessarily. The EV may charge directly from the grid during cheap tariff periods rather than being supplied through the home battery.
Is backup output the same as normal battery output?
Not always. Check the specific backup power rating of the system.
What is the difference between kW, kWh and kWp?
kW measures power, kWh measures energy and kWp describes the rated peak size of a solar PV array.
Understand Both Sides of the Battery Specification
A home battery needs more than a large kWh number.
You need to understand:
- how much energy is stored
- how quickly it can charge
- how quickly it can discharge
- how much output is continuous
- what happens during short power peaks
- what happens during backup
- whether additional battery modules also increase power
kWh tells you how much energy the battery can store. kW tells you how quickly that energy can move.
A battery can become larger in kWh without becoming more powerful in kW.
A good battery specification needs to tell you both how much energy is available and how quickly the system can use it.