Solar panels generate electricity when the sun shines. Battery storage lets you use that electricity when it doesn't. Getting the size right means your battery fills up from your panels on a typical day, holds enough charge to cover your evening and overnight usage, and doesn't leave you buying expensive grid electricity when you could be using your own stored energy.

Get it wrong in either direction and you lose money. An undersized battery fills early in the afternoon, starts exporting cheap electricity before your demand peaks, and runs dry by 9pm. An oversized battery rarely charges above 60%, stretches out your payback period by thousands of pounds, and delivers capacity you'll never use.

This guide walks you through the practical steps to size a solar battery correctly for your household, from reading your bills to calculating evening load, planning for an EV or heat pump, and understanding what physically fits in your home.

Quick Summary: How to Size a Solar Battery

The simple rule: your battery capacity in kWh should roughly equal your average evening and overnight electricity usage. For most UK households, that lands somewhere between 5 kWh and 10 kWh.

Typical choices by household type:

• Small home or flat (under 2,700 kWh/year): 5-7 kWh battery
• Average 3-4 bed home (3,400 kWh/year): 9-10 kWh battery
• High-usage home with EV or heat pump: 10-16 kWh or more

If you have a 4kW solar panel system, an 8-9 kWh battery is a common and well-matched pairing.

If you want a figure quickly and don't want to work through the full calculation, CRG Direct offers free site surveys that include a tailored battery sizing recommendation based on your actual bills and roof. Book one here.

Assess Your Electricity Usage Before Sizing

Battery sizing starts with your real consumption data, not estimates. Before you contact any installer, gather 12 months of electricity usage from your smart meter or energy supplier account. Most supplier apps and online portals show monthly kWh figures going back at least a year.

Calculate your average daily usage. Add up all 12 months and divide by 365. Most UK homes use between 8 kWh and 10 kWh per day on average, though individual households range from 4 kWh (small flat, two adults out during the day) to 12 kWh or more (large family home with electric heating or a home office running servers).

Identify evening and overnight kWh. This is the key number for battery sizing. Not your total daily consumption, but specifically what you use between the time your solar panels stop generating (typically around 4pm-6pm depending on season) and when they start again the next morning. For most households, this represents 60-70% of daily consumption.

Note seasonal variation. UK solar generation is high from April to September and drops sharply from October through February. Your battery needs to be sized for a useful contribution through winter, not just summer performance. In winter months, daily solar generation on a 4kW system in southern England might drop to 4-6 kWh, leaving little surplus to store. A battery that fully charges from solar every day in July may only reach 40-50% on a December day.

Calculate Evening Use and Peak Demand

Evening load is the number that drives your battery capacity decision. To measure it accurately:

Total your kWh used between 16:00 and 23:00. If you have a smart meter with half-hourly data (available through most supplier apps or through a device like Hildebrand Glow), export your data for a typical week and sum the afternoon and evening readings. If you don't have half-hourly data, a 7-day meter check works almost as well. Read your meter at 16:00 and again at 09:00 each day for a week, subtract morning from evening, and average the seven results. That average is your target storage window.

Record peak single-appliance draws. An electric oven draws 2-3 kW. A kettle draws 2-3 kW for three minutes. A washing machine on a hot cycle draws 1.5-2 kW. Your battery needs not just enough energy capacity (kWh) but enough power output (kW) to run these appliances at the same time. Check your chosen battery's continuous power rating, which is usually 3-5 kW for standard home batteries.

If your evening load calculation gives you a figure of, say, 8.5 kWh, a 9-10 kWh nominal battery covers it comfortably once you account for depth of discharge (explained below).

Include EV Charging and Future Loads

If you have an electric vehicle now, or plan to get one, you need to include it in your battery sizing.

List your EV charging needs in kWh per day. The average UK driver covers around 20-25 miles daily. A typical EV uses 3-4 miles per kWh, so 20 miles requires roughly 5-7 kWh of charge. If you charge at home most nights, this adds significantly to your evening load.

Estimate any other future loads:

• An air source heat pump adds 2,000-4,000 kWh per year to electricity consumption, concentrated in autumn and winter
• A second EV doubles the charging demand
• A hot tub or heated swimming pool can add 3,000-5,000 kWh per year

Add your future-load estimate to your baseline. If your current evening usage is 7 kWh and you plan to add an EV needing 6 kWh of daily charging, your effective target rises to 13 kWh. Sizing your battery at 10 kWh for current usage and expecting to expand later often costs more overall than starting with the right size. A common planning rule is to add 30-50% to your current baseline capacity if electrification is part of your five-year household plan.

Match Battery Capacity to Your Solar System

A battery can only store what your panels generate in excess of your daytime consumption. If your solar array doesn't produce enough surplus to charge the battery daily, you'll be drawing grid electricity to top it up, which changes the economics significantly.

Check your solar panels' expected daily generation. Your installer should have provided a generation estimate by month. If not, a 4kWp system in southern England generates roughly:

• June/July: 18-22 kWh per day
• April/May and August/September: 12-16 kWh per day
• October/March: 6-10 kWh per day
• November to February: 3-6 kWh per day

Calculate average daily solar surplus. Subtract your daytime consumption (what you use while the panels are generating, typically 30-40% of daily total for a household out during the day, 50-60% for people at home) from the daily generation figure. The remainder is what's available to charge your battery.

If a 4kWp system generates 14 kWh on an average spring day and you use 5 kWh during daylight hours, 9 kWh is available to store. A 10 kWh nominal battery captures almost all of that.

Compare surplus to desired battery capacity. If your panels regularly generate only 7 kWh of surplus but you're considering a 15 kWh battery, you'll never fill it from solar alone and will rely heavily on cheap overnight grid charging. That changes the sizing logic, because you're now optimising for time-of-use arbitrage rather than solar self-consumption.

Estimate Usable Battery Capacity and Depth of Discharge

Battery manufacturers quote nominal capacity, the total energy the battery can hold. Usable capacity is what you can actually store and discharge.

Depth of Discharge (DoD) determines how much of the nominal capacity is accessible. Most modern lithium iron phosphate (LFP) batteries allow a DoD of 90-95%, meaning a 10 kWh battery delivers 9-9.5 kWh of usable storage. Some older lithium-ion (NMC) systems have lower DoD figures, down to 80%. Always check the product datasheet, not just the headline kWh figure. We cover this in more detail in our guide to depth of discharge and battery lifespan.

Inverter power rating affects usable output. A battery's power rating (in kW) determines how quickly energy can be discharged. A 5 kWh battery with a 2 kW continuous output can run a 2 kW load for 2.5 hours. The same battery with a 3.6 kW output handles an electric oven more comfortably but drains faster under heavy load. If your household has a high simultaneous peak demand, confirm the battery's continuous power output is sufficient before committing.

Apply a Practical Rule of Thumb for Sizing

When the detailed calculation feels complex, these benchmarks work for most UK households.

Battery capacity ≈ evening and overnight kWh usage. If your household uses 8 kWh between 4pm and 9am, a 9-10 kWh nominal battery (with 90% DoD) gives you approximately 8.5-9 kWh of usable storage, a good match.

Battery capacity ≤ daily solar surplus if your panels are small. Fitting a 13 kWh battery to a 3kWp solar array on a north-east-facing roof doesn't make economic sense. Size down to match what the panels can reasonably fill.

Typical UK benchmark ranges:

Annual Consumption	Recommended Battery
Under 2,500 kWh	5-7 kWh
2,500-3,500 kWh	7-10 kWh
3,500-5,000 kWh	10-13 kWh
Above 5,000 kWh or EV	13-20 kWh

Most UK households choosing battery storage land on either a 5 kWh system (smaller homes, lower usage) or a 9.5-10 kWh system (3-4 bedroom family homes). The 10 kWh size has become the most common choice, matching well against average daily consumption and available solar generation.

Sizing for Emergency Backup Power

Not every buyer needs backup, but for households in rural areas with frequent outages, or those with medical equipment or home offices requiring uninterrupted power, backup is a genuine requirement that affects sizing.

Decide on whole-home or partial backup. Whole-home backup powers every circuit in the property. Partial backup covers a critical load panel, typically lighting, fridge, a few sockets, and perhaps broadband. Whole-home backup requires a larger battery and a higher-rated inverter. Partial backup can be achieved with a smaller system if the critical circuits are identified in advance.

List the circuits you need to keep running in an outage and add up their consumption over the duration of an expected outage. If you want 12 hours of backup covering fridge (1.5 kWh), lighting (0.5 kWh), broadband (0.2 kWh), and a medical device (2 kWh), you need 4.2 kWh of reserve capacity, reserved from your usable storage and not counted toward evening-use coverage.

How Much Extra Capacity for Emergency Backup Power

Add 20-40% to your baseline sizing to hold a backup reserve. If your evening usage calculation suggests a 9 kWh battery and you want a 3-4 kWh emergency reserve on top, a 13 kWh battery covers both comfortably.

Size your inverter for the emergency load in kW. If you plan to run a fridge, lighting, and a boiler pump during an outage at the same time (totalling around 1.5 kW), a standard 3.6 kW battery inverter handles this easily. Running a full electric cooker during an outage (8-10 kW draw) requires a much more powerful hybrid inverter, so confirm this with your installer before specifying.

Confirm the battery supports seamless backup switching. Some battery systems pause for 20-30 milliseconds when switching to backup mode. This interruption reboots most routers, clocks, and some sensitive electronics. True uninterruptible backup (zero transfer time) requires a system specifically rated for it. GivEnergy, SolarEdge, and some Victron setups support this, but cheaper DC-coupled systems often don't. If continuity matters to you, state this requirement at the survey stage.

Consider EV Charging Strategies With Battery Storage

Whether you should use your home battery to charge your EV, or charge the EV directly from cheap overnight grid electricity, depends on your tariff setup.

If you have a time-of-use tariff with cheap overnight electricity (some tariffs fall to 7-10p per kWh overnight), charging your EV from the grid at night is usually cheaper than charging from your home battery. The battery then better serves your household loads. In this case, the EV charging demand doesn't need to be built into battery sizing.

If your battery charges from solar and you want solar-powered EV charging, you need enough battery capacity to cover both household evening use and EV charging. A household using 8 kWh in the evening and wanting to add 7 kWh of EV charge from stored solar needs a battery of at least 16-18 kWh nominal capacity.

Model the daily kWh impact. Use your smart charger's app or estimate from your EV's efficiency rating and typical mileage. Know this number before your survey so the installer can factor it in.

Cost, Payback, and Financing Choices

Battery capital cost per kWh typically runs £400-£700 per usable kWh installed in 2026, depending on brand, chemistry, and installation complexity. A 10 kWh nominal LFP system costs roughly £4,000-£6,000 including installation. Smaller 5 kWh systems typically start around £3,000-£4,600. For a fuller breakdown, see our guide to solar battery costs in the UK.

Model payback using your current tariffs. If your battery saves you from buying 8 kWh of electricity at 26p per kWh each day, that's £2.08 per day or around £760 per year, depending on your usage, generation and energy costs. On a £5,000 battery, simple payback would be around 6.5 years, faster if electricity prices rise, slower if you oversize and don't fill the battery daily.

Battery installations in the UK attract 0% VAT, reducing the headline cost compared to what you'd pay on a standard-rated product.

Financing options:

• Cash purchase: fastest payback, no interest cost
• 0% finance: CRG Direct offers interest-free finance for qualifying customers. Repayments typically offset the electricity savings in year one, making the net cash impact close to neutral
• Personal loan: typical APRs of 6-9% over 3-7 years, still often cost-effective when modelled against rising electricity prices
• Battery-only retrofit vs combined solar-and-battery: installing a battery alongside a new solar system is cheaper than retrofitting later. If you're getting solar now and considering a battery, do it in one installation

Physical Constraints: Installation, Space, and Inverter Compatibility

Check available indoor space. A typical home battery system measures roughly 100cm x 60cm x 25cm and mounts on a wall bracket. Common locations are the loft, garage, utility room, or understairs cupboard. LFP batteries can be installed in unheated spaces within a reasonable temperature range (typically -10°C to 50°C, though performance degrades below 0°C). Check the manufacturer's specification for your intended location.

Confirm your inverter matches the battery chemistry. LFP and NMC batteries require different charging profiles. If you already have a solar inverter and want to retrofit a battery, the inverter must be compatible, either a hybrid inverter designed to manage both, or a compatible AC-coupled system. Not every inverter supports every battery brand. This is a detail your installer must verify before specifying equipment.

Check gateway and export control compatibility. If your DNO requires an export limitation device (common for larger systems), confirm the battery management system integrates with it. Some battery systems natively support export control, while others need a third-party gateway. Mismatched systems can prevent grid connection approval.

Chemistry, Warranty, and Lifespan Considerations

Lithium iron phosphate (LFP) vs lithium-ion NMC:

Factor	LFP	NMC
Thermal safety	Excellent	Good
Energy density	Lower	Higher
Cycle life	4,000-6,000 cycles	2,000-3,000 cycles
Typical lifespan	12-15 years	10-12 years
Cost	Slightly higher per kWh	Slightly lower per kWh
Indoor install	Preferred	Acceptable

For UK home installations, LFP is the dominant choice and generally the safer recommendation for a loft or utility room. NMC makes more sense where space is genuinely constrained and higher energy density matters. A reputable MCS-accredited installer like CRG Direct can advise on the best chemistry for your property.

Check warranty terms carefully:

• Years of coverage: 10 years is standard, and some manufacturers offer 12
• Throughput guarantee: expressed in MWh of total energy cycled. A 10 kWh battery with a 36 MWh throughput guarantee covers approximately 10 full cycles per year for 10 years, adequate for most home use
• End-of-warranty capacity: most manufacturers guarantee 70-80% of original capacity at warranty end

Plan for replacement. At 10-15 years, battery capacity degrades to a point where replacement becomes worth considering. At that stage, the cost of a replacement battery will reflect the technology prices of that time, likely lower than today. Budget for this in your long-term planning.

Future-Proofing and Expandability

Decide whether you want to allow for modular expansion. Some battery systems are modular, so you install one unit now and stack additional modules later as your usage grows. Others are fixed-capacity units that require complete replacement to increase storage. If you expect your consumption to rise significantly (EV, heat pump, growing family), a modular system avoids the cost of replacing an undersized unit in five years.

Check your inverter's allowance for stacked batteries. Hybrid inverters have a maximum battery capacity they can manage. A 5 kWh battery installed on a hybrid inverter rated to 20 kWh can have additional modules connected later without inverter replacement. An inverter rated to 10 kWh maximum is a ceiling you can't pass without replacing the inverter too.

Plan for EV and heat pump adoption timelines. If an EV is two years away and a heat pump is five years away, factor both into your battery specification now rather than retrofitting capacity twice. The combined incremental cost of specifying slightly more storage at installation is usually far lower than two separate expansion projects, particularly when working with regional experts in solar and battery installation across Hampshire, Surrey and West Sussex.

Can You Add More Battery Later?

Yes, but it costs more than getting the size right first time.

Adding a module to an expandable system typically costs £1,500-£3,000 per additional kWh once the hardware, an installation visit, and any updated commissioning and certification are factored in. If the same capacity had been installed at the original project, the incremental cost would have been lower because the fixed costs (scaffolding, installer visit, certification, commissioning) are already absorbed. We go deeper on this in our guide to adding a battery to an existing solar system.

The practical guidance: if you're between two sizes, say a 9.5 kWh and a 13.5 kWh, and an EV or heat pump is genuinely likely within five years, go larger now. The additional upfront cost is usually recoverable within 2-3 years of the larger system's improved performance.

If the larger system is significantly more expensive and the future load is speculative, install the right size for current needs on a system that supports expansion, and revisit when the load arrives.

Sizing Examples and Benchmarks

Example 1: Small terrace, 2,700 kWh/year consumption

Annual consumption: 2,700 kWh. Average daily usage: 7.4 kWh. Evening and overnight usage (65%): 4.8 kWh. Solar surplus from 3kWp system (spring/summer average): 6 kWh. No EV, no heat pump.

Recommended battery: 7-8 kWh nominal. A 7.4 kWh LFP system covers evening load with a small reserve. Nominal 7.5 kWh at 90% DoD delivers 6.75 kWh usable, sufficient for most evenings with headroom for seasonal variation.

Example 2: Three-bed semi, 3,400 kWh/year consumption

Annual consumption: 3,400 kWh. Average daily usage: 9.3 kWh. Evening and overnight usage (65%): 6 kWh. Solar surplus from 4kWp system: 8-9 kWh on spring/summer days. No EV currently, but planned within three years.

Recommended battery: 9-10 kWh nominal, on an expandable system. This covers current evening usage comfortably and leaves 1-2 modules of expansion headroom when the EV arrives. A 9.5 kWh LFP system at 90% DoD gives 8.5 kWh usable, covering the 6 kWh evening load and holding a useful reserve.

Example 3: Four-bed detached, EV household

Annual household consumption: 5,200 kWh (including an existing EV adding approximately 1,800 kWh/year). Daily household evening usage: 7.5 kWh. Daily EV charge from home battery: 6 kWh. Total target storage: 13.5 kWh. Solar array: 6kWp, generating 10-12 kWh surplus on an average spring day.

Recommended battery: 13-16 kWh nominal. A 15 kWh LFP system at 90% DoD gives 13.5 kWh usable, covering both loads. If the household also has a heat pump, revisit this calculation for winter usage when heating demand peaks.

How to Work With an Installer

Request a free site survey from CRG Direct. Bring your 12-month electricity usage data, any smart meter half-hourly exports you can pull from your supplier app, and a note of planned future loads. The survey covers roof assessment, shading analysis, inverter and battery compatibility, and DNO requirements. CRG Direct produces a written system specification and cost breakdown at no charge and no obligation.

Ask for detailed generation modelling. A credible installer won't just quote you a battery size. They'll show you estimated monthly solar generation, expected self-consumption rates with and without the battery, and projected annual bill savings. If a quote doesn't include this, ask for it.

Request multiple quotes and compare equipment. Quotes for similar-sounding systems can vary by battery chemistry, inverter quality, warranty terms, and included installation scope. Ask each installer to specify: battery brand and model, nominal and usable kWh, continuous power output (kW), DoD, warranty duration and throughput guarantee, and whether the system is expandable. Compare these line by line rather than on headline price alone.

Checklist: Steps to Choose the Right Size Solar Battery

• Collect 12 months of electricity bills or smart meter data
• Calculate average daily kWh and separate out evening/overnight usage
• Measure your solar panels' expected average daily generation by season
• Decide whether you need emergency backup power, and if so, list critical circuits
• List current or planned EV charging and heat pump loads in daily kWh
• Apply the rule of thumb: battery ≈ evening kWh use, adjusted for future loads
• Check DoD and usable capacity of shortlisted batteries
• Confirm the battery's continuous power output (kW) meets peak household demand
• Check whether your existing or planned inverter supports the battery chemistry
• Verify physical installation space and location temperature range
• Ask whether the system supports modular expansion
• Get at least two itemised quotes including equipment specifications
• Book a CRG Direct free survey for a tailored recommendation

Common Questions About Solar Battery Sizing

What size battery do I need for a typical UK home? Most 3-4 bedroom UK homes use 8-10 kWh of electricity per day. If 60-65% of that falls in the evening and overnight window, you need 5-7 kWh of usable storage. A 9-10 kWh nominal battery (at 90% DoD) gives you roughly 8.5-9 kWh usable, which covers most evenings comfortably. For smaller homes using under 2,500 kWh per year, a 5-7 kWh battery is usually sufficient.

What does battery capacity actually mean? A 10 kWh battery holds a maximum of 10 kilowatt-hours of electrical energy. A 1 kW appliance running for 10 hours would drain it completely. In practice, DoD limits mean you access 90-95% of that in a standard home battery, so around 9-9.5 kWh. That's enough to run a typical household's evening loads (lighting, TV, fridge, occasional kettle or washing machine) for most of the night.

Why does oversizing or undersizing reduce value? An undersized battery fills early in the afternoon, starts exporting electricity at low SEG rates before your evening demand peaks, then runs dry by late evening. You buy grid electricity at full price for the hours the battery could have covered. An oversized battery rarely charges to full capacity from your panels, may rely on expensive grid electricity to charge, stretches out your payback period, and provides no additional benefit over a correctly sized unit. Both scenarios mean you pay more than necessary for the outcome you actually achieve.

Final Notes and Next Steps

Battery sizing is straightforward when you have your actual consumption data. The calculation takes 20-30 minutes if you have 12 months of bill data and a smart meter report. What installers add is generation modelling, equipment knowledge, and an understanding of how local grid conditions, DNO rules, and tariff structures in your area affect the best configuration.

CRG Direct offers free, no-obligation site surveys that produce a tailored battery sizing recommendation alongside a full system specification and cost breakdown. Whether you're buying solar and battery together or adding storage to an existing solar system, the survey gives you the numbers to make an informed decision.

Book at crgdirect.co.uk/quote or call +44 330 133 2497.

Sources

• Ofgem, Smart Export Guarantee (SEG): official scheme page
• HMRC, 0% VAT on energy-saving materials: official guidance
• MCS Certified Installer Register: mcscertified.com