Annual electricity consumption
Annual kWh gives us the broad scale of household demand. A home using 3,000 kWh a year is a very different proposition from one using 9,000 kWh.
Solar battery questions answered
What size solar battery do I actually need?
One installer recommends 5 kWh. Another says 10 kWh. Somebody else wants to sell you nearly 20 kWh of storage.
The correct battery size is not determined by a favourite product or a standard package. It should follow the electricity demand of the household and what you actually want the battery to do.
Electricity demand
Solar generation
Tariffs
Battery power
The short answer
There is no universal battery size for a three-bedroom house, four-bedroom house or any other house.
Two physically similar homes can have completely different electricity requirements.
One household may use relatively little electricity during the evening. Another may have electric heating, an immersion heater, an EV, home working and several people at home throughout the day.
That means statements such as:
✓ Every home needs a 5 kWh battery
✓ Every four-bedroom house needs 10 kWh
✓ You should always install the biggest battery you can afford
are too simplistic to be useful.
The battery should be sized around the household rather than the bedroom count.
What affects the answer?
Eight things I would want to know before deciding battery size.
When electricity is used
Battery storage is largely about moving electricity from one time to another. Evening and overnight consumption can therefore matter more than the annual total alone.
Solar-array size
A large battery paired with a small solar array may not regularly fill from solar. Array capacity and realistic generation need to be considered alongside storage.
Roof orientation
East, west and south-facing arrays produce electricity at different times of day. The generation profile can affect how much energy is available to charge the battery.
Electricity tariff
If the battery can charge cheaply overnight and discharge during more expensive periods, its role is no longer limited to storing surplus daytime solar.
Heat-pump demand
Electrified heating can increase consumption significantly, particularly during winter when solar generation is also lower. That seasonal mismatch matters.
Electric vehicles
An EV can add substantial electricity demand, but it may be more sensible to charge the car directly from cheap-rate electricity or surplus solar rather than route every kWh through the house battery.
Battery power
Capacity in kWh tells you how much electricity the battery can store. Charge and discharge power in kW tells you how quickly it can move that electricity. Both matter.
Capacity versus power
A 10 kWh battery does not necessarily mean your house can draw 10 kW from it.
This distinction is easy to miss when comparing quotations.
Battery capacity is normally expressed in kilowatt-hours — kWh.
Battery power is expressed in kilowatts — kW.
Think of capacity as the amount of energy available and power as how quickly that energy can be delivered.
✓ 5 kWh, 10 kWh or 20 kWh describes stored energy
✓ 3 kW, 5 kW or 7 kW describes power flow
A large-capacity battery with limited discharge power may still require grid electricity when several high-load appliances are operating simultaneously.
That is why I would compare both figures rather than looking only at the size printed on the battery.
Nominal versus usable storage
Check how much of the advertised battery capacity is actually available.
Battery quotations may refer to nominal capacity, usable capacity, or sometimes simply a rounded marketing figure.
Those numbers are not always identical.
When comparing two systems, ask:
✓ What is the nominal capacity?
✓ What is the usable capacity?
✓ What depth of discharge is permitted?
✓ Does the warranty depend on operating limits?
Two batteries carrying similar headline numbers can therefore offer slightly different amounts of usable energy.
A useful starting point
Look at what happens between sunset and the next useful charging opportunity.
If the main purpose of the battery is to store daytime solar for use later, one useful question is:
How much electricity does this household typically use after solar generation falls away and before useful solar generation returns?
That can provide a better starting point than simply dividing annual electricity consumption by 365 and buying a battery of that size.
But even this is not the entire calculation.
Cheap overnight tariffs can allow the battery to be charged from the grid, while winter electricity use can be dramatically different from summer use.
So battery sizing is normally a balancing exercise rather than one simple formula.
Typical questions
What would make me consider more — or less — storage?
Reasons to consider more capacity
✓ High household electricity consumption
✓ Significant evening and overnight demand
✓ Large solar array
✓ Heat pump or electric heating
✓ Electric hot water
✓ Useful cheap-rate grid charging
✓ Future electricity demand expected to rise
Reasons less may be enough
✓ Low household consumption
✓ Much of the electricity is already used during daylight
✓ Small solar array
✓ Little evening demand
✓ EV charges independently overnight
✓ Limited opportunity to cycle a larger battery
✓ Additional capacity would regularly remain unused
Question
Is a 5 kWh battery enough?
It can be.
For a lower-consumption household, or one where a relatively modest amount of electricity needs to be shifted into the evening, a battery around this size may perform a useful role.
But the same capacity could be exhausted quickly in a household with:
✓ Electric heating
✓ Large evening loads
✓ Electric cooking
✓ Electric hot water
✓ High overnight consumption
So the answer is not whether 5 kWh is a good battery size.
It is whether approximately 5 kWh is a good battery size for that particular household.
Question
Do I need 10 kWh of battery storage?
Around 10 kWh can represent a useful middle ground for many higher-use households, but bedroom count alone does not justify it.
I would still want to know:
✓ How much electricity the household uses annually
✓ How much is used overnight
✓ How large the solar array is
✓ Whether cheap-rate grid charging is available
✓ What loads the battery is expected to support
If a quotation contains 10 kWh simply because that is the installer's standard package, I would ask for the calculation behind it.
Question
Do I need two batteries?
Sometimes, absolutely.
A second battery can make sense where there is enough electricity demand and enough opportunity to charge the additional capacity.
The mistake is assuming:
two batteries must always be twice as useful as one.
If the first battery already covers almost all useful daily energy shifting, the second may add comparatively little value.
Conversely, a high-demand electrified home may make regular use of considerably more than one battery can provide.
Cheap overnight electricity
A battery can do more than store solar.
Modern time-of-use electricity tariffs can change the role of home storage considerably.
Where compatible equipment and tariffs permit it, the battery can be charged during a low-cost overnight period and discharged later when imported electricity is more expensive.
That means a battery may have two jobs:
✓ Store surplus solar electricity
✓ Shift cheaper grid electricity into expensive periods
This can make additional capacity useful even during periods when solar generation alone would not fill the battery.
But tariff rules, equipment compatibility and the difference between cheap and expensive rates all need to be considered.
EVs
Don't automatically size the house battery to charge the entire car.
An electric vehicle may contain a battery several times larger than the home storage system.
It would therefore be easy to conclude that an EV owner needs an enormous home battery.
But that is often the wrong way to look at it.
Depending on the household, the car may instead be charged:
✓ Directly from surplus solar
✓ During a cheap overnight tariff
✓ At times when the home battery is not needed
The EV should be considered in the overall energy design, but it does not automatically follow that every kWh entering the car should first pass through the home battery.
Heat pumps
A heat pump can make battery sizing more interesting — especially in Scotland.
Heat pumps can increase household electricity consumption substantially.
But much of that extra demand occurs during the colder months when solar generation is also at its lowest.
Simply installing an enormous battery does not create additional winter solar generation.
A heat-pump household therefore needs the whole energy picture considered:
✓ Winter electricity demand
✓ Winter solar generation
✓ Overnight tariffs
✓ Battery charge rate
✓ Battery discharge rate
✓ Household peak loads
In some cases, cheap overnight charging becomes particularly important to the battery strategy.
Real example
Why nearly 19 kWh of storage made sense for one home.
In one anonymised Central Scotland assessment, the household used approximately 8,725 kWh of electricity per year.
The property also had:
✓ Air-source heat pump
✓ Electric hot water
✓ Electric vehicle
✓ High overall household electricity demand
✓ 20-panel proposed solar array
In that situation, approximately 18.8 kWh of proposed battery storage could be considered against genuinely substantial demand.
That does not mean 18.8 kWh should become a standard recommendation for every home.
Put the same battery into a low-consumption household and the economics and usage could look completely different.
Warning signs
Ask how the battery recommendation was calculated.
I would want more explanation if:
✓ Battery size was chosen before electricity use was known
✓ Nobody asked when electricity is normally used
✓ Heat-pump or EV demand was ignored
✓ Solar-array size was not considered
✓ The installer cannot explain usable capacity
✓ Charge and discharge power are never mentioned
✓ Tariff strategy is ignored
✓ The only justification is "bigger is better"
None of those automatically means the proposed battery is wrong.
They simply mean the homeowner deserves a better explanation before spending thousands of pounds.
Battery comparison checklist
Don't compare batteries using capacity alone.
Usable capacity
How many kWh can actually be used during normal operation?
Discharge power
How much household load can the battery support at one time?
Charge power
How quickly can surplus solar or cheap grid electricity refill it?
Warranty
What period, throughput, cycle or capacity-retention conditions apply?
Expandability
Can more storage be added later if electricity demand grows?
System compatibility
Does it work properly with the proposed inverter, monitoring system and intended tariff strategy?
The answer
The right battery is the one your household can make useful use of.
Battery sizing should not be a competition to install the biggest box possible.
Nor should the battery automatically be kept small simply to reduce the quotation price.
The objective is to establish a sensible amount of storage from the evidence:
✓ What the house consumes
✓ When it consumes it
✓ What the solar system can generate
✓ What cheap-rate charging opportunities exist
✓ What future demand is likely
✓ What the battery can actually deliver
Once those are known, a 5 kWh, 10 kWh or 20 kWh recommendation can be judged on its merits rather than taken on trust.
Still unsure?
Work out what the house needs before choosing the battery.
If several solar companies have proposed completely different storage capacities, the useful question is not simply which battery brand is best.
First establish roughly how much storage the property and household can realistically use.