Batteries: The Missing Piece of the Renewable Puzzle
Batteries: The Missing Piece of the Renewable Puzzle
“Generating electricity is only half the story—storing it changes everything.”
Solar panels are remarkably good at producing electricity. Unfortunately, they do not always produce it when we most need it.
On a bright summer afternoon, a solar installation may generate far more electricity than a house is using. A few hours later, when the sun has gone down, the oven is cooking dinner, the television is on and the heat pump is still running, the house may once again be importing electricity from the grid.
That is the fundamental weakness of solar power without storage.
A home battery changes the relationship between generation and consumption. Instead of being forced to use solar electricity immediately or export it, we can save it and decide when it will be most valuable.
For us, battery storage has become just as important as the 26 solar panels on the roof. Our system has approximately 50 kWh of storage, which sounds enormous compared with the batteries installed in many homes. During summer it can feel enormous. During a cold, dark winter spell, with the heat pump running and very little solar generation, it can feel surprisingly finite.
That experience has taught me one particularly important lesson:
Never size a battery or batteries from your best summer day. Size them from your most demanding winter days.
Solar Generation and Household Demand Rarely Match
Most solar electricity is generated around the middle of the day.
Many households, however, use the greatest amount of electricity in the morning and evening. People return home, cook meals, heat rooms, use computers, watch television, run washing machines and charge vehicles.
Without a battery, there are three possible destinations for solar electricity:
It is used immediately by the house.
It is exported to the grid.
It is wasted because the system has been restricted from exporting further.
Battery storage adds a fourth and much more useful option:
Save the electricity now and use it later.
The Energy Saving Trust describes this as one of the principal advantages of battery storage: surplus solar electricity can be stored during the day and then used at night or during cloudy periods. Batteries can also be charged from the grid when electricity is cheaper.
This is what makes solar electricity genuinely flexible.
Moving Sunshine into the Evening
Imagine that a solar installation generates 25 kWh during a bright day.
The house might consume only 8 kWh while that electricity is being generated. Without storage, much of the remaining 17 kWh will be exported.
At 6.00 pm, solar generation falls rapidly, but household demand rises. Dinner is being cooked, lights are switched on and the heat pump may begin working harder as the outdoor temperature falls.
A battery allows some of that afternoon generation to be moved into the evening.
It is not physically moving sunshine through time, of course, but that is effectively what it achieves. Electricity generated when it is plentiful becomes available when it is useful.
This is especially valuable for homes with:
Heat pumps
Electric vehicles
Electric cooking
Home offices
Workshops
Medical or communications equipment
High evening electricity consumption
The battery does not increase the amount of energy produced by the solar panels. It increases the value of that energy by making it available at a better time.
Avoiding Expensive Peak-Rate Electricity
Battery storage can be valuable even when there is very little solar generation.
Modern time-of-use tariffs charge different prices at different times of the day. Electricity may be relatively inexpensive during an overnight charging window but considerably more expensive during the morning or early evening peak.
A battery can be programmed to charge during the cheaper period and then supply the house when grid electricity becomes more expensive.
The Energy Saving Trust notes that smart time-of-use tariffs normally require a smart meter and reward households that can shift consumption away from peak periods. Batteries are particularly useful because they allow the household to buy electricity when it is cheap and use it later.
The process might look like this:
Charge the battery overnight at the cheapest rate.
Use stored electricity during the morning peak.
Recharge from solar during the day.
Use the remaining stored electricity during the evening peak.
Export electricity only when the battery is full or when the export rate is especially attractive.
This is much more sophisticated than simply generating solar electricity and hoping to use it at the right moment.
It turns the home into a small energy-management system.
The Battery Can Be Valuable in Winter Too
There is a common assumption that a solar battery is useful only in summer.
In summer, the battery certainly captures surplus solar generation. However, smart tariffs can make it useful throughout the winter.
On a poor December day, the solar panels may produce only a small proportion of the electricity required by the house. The battery can instead be charged overnight using cheaper grid electricity.
That stored electricity can then run the house during more expensive periods.
This means a home battery can have two different seasonal roles:
Summer role
Store surplus solar electricity.
Reduce exports at low rates.
Power the house after sunset.
Charge an electric vehicle from stored solar power.
Winter role
Charge from cheaper overnight electricity.
Avoid expensive peak-period imports.
Supplement limited solar production.
Reduce the cost of running a heat pump.
A well-managed battery is therefore not merely a solar accessory. It is a year-round tool for controlling when electricity is purchased and used.
Start with the Worst Winter Days
Battery installers often discuss average daily electricity consumption. Averages are useful, but they can also be misleading.
Suppose a household uses an average of 14 kWh per day across the year. It might be tempting to install a battery with approximately that capacity.
However, the same house might consume 30 kWh on a cold winter day because:
The heat pump runs for longer.
More lighting is required.
People spend more time indoors.
The tumble dryer is used.
Hot-water demand increases.
Solar generation contributes very little.
An electric vehicle needs charging.
A battery sized around the annual average may perform well for much of the year but empty rapidly during the very periods when stored electricity is most valuable.
Our own experience illustrates this. A 50 kWh storage system sounds exceptionally large, but a heat pump can consume a substantial amount of electricity during prolonged cold weather. Add normal household demand, office equipment, cooking and other loads, and even 50 kWh can be depleted.
That does not mean the battery has failed. It means winter demand has to be taken seriously.
A Practical Method for Sizing a Battery
Before asking an installer what size battery you need, examine your own consumption.
Smart-meter data, inverter records or energy-monitoring software can provide half-hourly or daily figures. Ideally, collect at least a full year of information so that seasonal variations are visible.
Step 1: Find your highest realistic winter consumption
Look at the coldest, darkest winter days.
Do not base the calculation on a single abnormal incident, such as a day when several electric heaters were accidentally left running. Instead, identify several genuinely demanding but representative winter days.
For example:
Winter day one: 32 kWh
Winter day two: 35 kWh
Winter day three: 37 kWh
Winter day four: 34 kWh
In this case, 37 kWh is a sensible starting point.
Step 2: Add at least 10% for growth and change
Electricity consumption rarely remains constant.
You might later add:
An electric vehicle
A larger heat pump
Air conditioning
An induction hob
Workshop equipment
A home office
More occupants
Additional refrigeration or freezer capacity
Adding 10% to a 37 kWh requirement gives:
37 kWh × 1.10 = 40.7 kWh
The starting target would therefore be approximately 41 kWh of usable storage.
For a household without electric heating, the calculation may be much smaller:
Worst winter day: 14 kWh
Add 10%: 14 × 1.10 = 15.4 kWh
That household might begin by considering around 15–16 kWh of usable capacity.
The important point is that the figure comes from measured consumption rather than guesswork.
Step 3: Check usable capacity, not just advertised capacity
A battery advertised as having 10 kWh of capacity may not provide the full 10 kWh for everyday use.
Some capacity may be reserved to protect the battery, while the owner may choose to retain a further percentage for emergency backup.
Ask the installer:
What is the total capacity?
What is the usable capacity?
What minimum state of charge is recommended?
Can an emergency reserve be set?
Does the warranty restrict the depth of discharge?
These questions matter far more than the number printed prominently on the brochure.
Step 4: Allow for conversion losses
Batteries are not 100% efficient.
Electricity is converted when it enters the battery and converted again when it leaves. Some energy is lost as heat and through the inverter and control electronics. UK Power Networks notes that batteries have associated losses and must be operated efficiently to provide their full benefit.
A system that must reliably deliver 15 kWh should therefore have more than exactly 15 kWh of nominal storage.
Step 5: Consider the battery’s power output
Capacity is measured in kilowatt-hours, or kWh. Power is measured in kilowatts, or kW.
These are not the same thing.
A 20 kWh battery may store plenty of energy, but if its inverter can supply only 3 kW, it may not be able to run several high-power appliances simultaneously.
For example:
Kettle: approximately 3 kW
Oven: perhaps 2–3 kW
Heat pump: variable, but potentially several kilowatts
Tumble dryer: dependent on type
Electric shower: often a very high load
EV charger: commonly 7 kW
If the battery can supply only 3 kW and the house suddenly demands 8 kW, the remaining power will normally have to come from the grid.
When comparing systems, ask about both:
How many kWh can it store?
and:
How many kW can it deliver?
Battery Size Is Not a Competition
It is easy to assume that a larger battery is always better.
It is not.
An oversized battery may spend much of the year partially empty or unused. If it rarely completes a useful charge-and-discharge cycle, the additional capacity may never repay its extra cost.
An undersized battery creates the opposite problem. It may fill before midday in summer, forcing the remaining solar generation onto the grid. In winter it may empty before the expensive evening period has finished.
The correct size is therefore not the largest system that will fit in the building. It is the system that matches:
Household demand
Solar generation
Tariff structure
Future plans
Required backup time
Available charging windows
Inverter power
Installation budget
Modular batteries can be attractive because additional units may be added later, provided the inverter and battery-management system have been designed for expansion.
Can a Battery Keep the House Running During a Power Cut?
Potentially—but never assume that it will.
Many people understandably believe that a house with solar panels and a battery will continue operating when the grid fails. In reality, a standard grid-connected system may shut down during a power cut to prevent electricity being fed into cables that engineers expect to be dead.
To provide backup power, the installation usually needs specific emergency-power or islanding equipment.
Depending on the system, this might support:
A small emergency socket
A dedicated essential-load circuit
Selected lighting and refrigeration
Internet and communications equipment
The whole house, within the inverter’s limits
An Energy Saving Trust case study highlights a household that expected its battery to work during a power cut but discovered that its installation did not provide off-grid backup.
Before purchasing a system, ask the installer:
Does the system provide backup during a grid failure?
Is the switchover automatic?
Which circuits will remain powered?
Can the solar panels recharge the battery while the grid is down?
What is the maximum backup output?
How much battery capacity is reserved for an emergency?
Will the heat pump, freezer, internet and essential lighting continue operating?
A large battery is not automatically an uninterruptible power supply.
The design of the installation matters just as much as its capacity.
Deciding What Must Stay On
During a prolonged power cut, it may be unwise to operate the house normally.
A better approach is to identify essential loads.
These might include:
Refrigerator and freezer
Internet router
A few lighting circuits
Heating controls
Circulation pumps
Security systems
Medical equipment
Phone and radio charging
Selected sockets
Cooking with an electric oven, charging an EV or running an electric shower could consume stored energy very quickly.
Suppose the essential circuits use an average of 500 watts.
A battery containing 10 kWh of usable energy could theoretically run them for:
10 kWh ÷ 0.5 kW = 20 hours
In practice, conversion losses, changing loads and the battery’s reserved capacity would reduce that figure.
If the essential load rises to 2 kW, the same battery might provide only around five hours before allowing for losses.
Backup duration is therefore not determined by battery size alone. It depends on what the household chooses to power.
Smart Tariffs Change the Calculation
A battery’s financial value depends heavily on the difference between the price paid to charge it and the value of the electricity when it is discharged.
Consider an illustrative tariff:
Off-peak electricity: 10p per kWh
Peak electricity: 30p per kWh
Ignoring losses for a moment, moving 10 kWh from the off-peak period to the peak period creates a gross saving of:
10 kWh × 20p = £2 per cycle
If this happened on 250 days of the year:
£2 × 250 = £500 per year
However, the real calculation must also account for:
Battery losses
Days when the battery is not fully used
Changes in tariff rates
Standing charges
Battery degradation
Maintenance
Finance costs
Income lost by not exporting the electricity
Export payments received at other times
Time-of-use tariffs can save money, but the Energy Saving Trust warns that their value depends on the tariff structure and how much consumption can genuinely be shifted away from peak periods.
A battery does not create cheap electricity. It creates the opportunity to purchase, generate, store and use electricity more intelligently.
Exporting May Sometimes Be Better Than Storing
The traditional assumption is that self-consuming solar electricity is always better than exporting it.
That is no longer automatically true.
Some export tariffs pay relatively attractive rates. Under certain tariff combinations, it might be more profitable to:
Charge the battery cheaply overnight.
Use solar electricity during the day.
Export surplus solar electricity.
Discharge the battery during an expensive peak period.
Alternatively, a tariff may reward exporting stored energy when the grid is under pressure.
The Smart Export Guarantee requires participating suppliers to pay eligible small-scale generators for renewable electricity exported to the grid, although householders must actively sign up and may need an eligible system, smart meter and appropriate certification.
The best strategy depends on the current import and export rates.
This is why good battery software is becoming increasingly important. The system needs to consider:
Tomorrow’s weather forecast
Expected solar generation
Household consumption patterns
Import prices
Export prices
Required emergency reserve
Battery state of charge
A badly programmed battery can charge at the wrong time, export too cheaply or remain empty when peak electricity becomes expensive.
A well-programmed battery can make decisions that would be tedious for a person to manage manually every day.
The Economics Must Be Calculated Honestly
Battery prices have fallen, but storage still represents a major investment.
The Energy Saving Trust currently estimates that battery storage systems typically cost around £5,000 to £8,000, although the final price depends on capacity, installation complexity and associated equipment. It also suggests that a typical battery may last around 10 to 12 years.
The simplest payback calculation is:
Installation cost ÷ annual financial saving = simple payback period
For example:
Battery installation cost: £6,000
Estimated annual saving: £600
Simple payback: 10 years
But even this may be too simple.
A proper calculation should include:
Expected battery degradation
Inverter replacement
Warranty length
Finance interest
Changes in import prices
Changes in export rates
Maintenance costs
The value of backup power
Future electricity consumption
Some benefits are difficult to price.
How much is it worth to keep the freezer, heating controls and internet operating during a power cut? How much value do we place on using more of our own renewable electricity? What is the benefit of being less exposed to future peak electricity prices?
A battery should not be sold using an unrealistically short payback based on perfect daily cycling. The calculation must reflect how the household will actually use it.
A Useful Battery Economics Formula
A more informative annual estimate is:
Useful annual battery throughput × net value per kWh = gross annual benefit
Suppose a 10 kWh battery completes the equivalent of 220 full useful cycles in a year:
10 kWh × 220 = 2,200 kWh annual throughput
If each stored kilowatt-hour provides an average net benefit of 18p after considering import prices, export income and losses:
2,200 × £0.18 = £396 per year
That figure can then be compared with:
The installed cost
The warranty period
Expected battery life
Alternative investments
Future energy requirements
This approach is much more realistic than assuming the battery will empty and refill completely every day.
Our 50 kWh System: Large, but Not Unlimited
Our own battery system is much larger than the installation found in a typical home.
It works alongside:
Twenty-six solar panels
An air-source heat pump
Solar hot-water systems
Electric appliances
Home office and technical equipment
A household with significant electricity consumption
During a bright summer day, the panels can recharge the batteries while supplying the house. The stored electricity then carries us through the evening and overnight.
Winter is a different story.
There can be several consecutive days of low solar production while the heat pump is working harder than usual. If the batteries begin the period partly charged, they may eventually reach their minimum reserve.
A 50 kWh figure creates an impression of complete energy independence. The reality is more nuanced.
It provides substantial flexibility. It allows us to store large amounts of solar electricity, make use of cheaper charging periods and reduce peak imports. But it cannot manufacture energy during a week of poor weather.
Storage solves the timing problem.
It does not entirely solve the seasonal generation problem.
Batteries Should Follow Energy Efficiency
Before spending thousands of pounds on storage, reduce unnecessary consumption.
It is usually cheaper to avoid wasting a kilowatt-hour than to install equipment to store it.
Useful measures include:
Improving insulation
Draught-proofing
Installing LED lighting
Replacing inefficient refrigeration
Using a heat-pump tumble dryer
Improving heat-pump settings
Reducing unnecessary standby consumption
Scheduling appliances intelligently
Maintaining freezers and refrigerators
Heating water at the most appropriate time
If efficiency improvements reduce a winter day from 24 kWh to 20 kWh, the required battery may be smaller and cheaper.
The correct order is often:
Reduce demand.
Generate clean electricity.
Store the surplus.
Manage when electricity is imported and used.
Questions to Ask Before Buying
Before accepting a battery quotation, ask:
What is our measured winter electricity consumption?
What is the battery’s usable capacity?
What is its maximum continuous power output?
What is its short-duration peak output?
Can additional modules be added later?
What happens during a power cut?
Can solar panels recharge it while off-grid?
What circuits receive backup power?
How long is the battery warranty?
Is the warranty based on years, cycles or energy throughput?
What capacity is guaranteed at the end of the warranty?
Does the system work with different energy suppliers?
Can it respond automatically to smart tariffs?
Can the owner control charging and discharging?
What happens if the manufacturer’s online service closes?
Is the installer MCS certified?
Has the distribution network operator been notified where required?
The Energy Saving Trust recommends obtaining quotes from at least three MCS-certified installers.
The cheapest quotation is not necessarily the best. The battery, inverter, software, emergency-power equipment and installation must work together as one system.
The Missing Piece Is Control
Solar panels generate electricity.
A battery gives us control over when that electricity is used.
It can turn midday generation into evening power. It can replace expensive peak-rate imports with cheaper overnight electricity. It can reduce dependence on the grid and, when designed correctly, provide limited resilience during power cuts.
But batteries need careful sizing.
Begin with measured consumption from the worst realistic winter days. Add at least 10% for growth and changing circumstances. Check the usable capacity, not merely the advertised figure. Examine the inverter output, emergency-power arrangements, warranty and tariff compatibility.
Most importantly, do not assume that a battery will make a house permanently self-sufficient.
Our 50 kWh system has shown us just how valuable storage can be—but it has also demonstrated that even a large battery has limits when winter solar generation is low and the heat pump is working hard.
Renewable generation gives us cleaner electricity.
Storage allows us to decide when that electricity matters most.
Generating electricity is only half the story. Storing it—and managing it intelligently—is what finally brings the renewable system together.
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