A solar battery does not generate more electricity. It changes when you use the electricity that is already being generated. By storing surplus midday generation and releasing it in the evening, a battery replaces electricity that would otherwise be imported from the grid at your full unit rate. The financial case rests on the gap between your import rate and your Smart Export Guarantee rate: if you import at 24p and export at 8p, every unit the battery captures and stores is worth 16p more than it would have been as export. Whether that gap justifies the battery cost depends on how much surplus generation you actually have, and that varies significantly by month.
This calculator models your solar system’s monthly generation profile against your household consumption to show how much surplus is available to charge a battery each month, how much evening demand can be served from storage, and what additional annual saving a battery produces at different sizes. It also models the time-of-use tariff case, where the battery is additionally charged from cheap overnight electricity on days when solar generation is low. All outputs are illustrative and based on UK average irradiance data. Squared Money is an introducer, not a lender. Checking eligibility will not affect your credit score.
At a Glance
- The financial case for battery storage depends on the gap between your import rate and SEG rate. At 24p import and 8p SEG, each unit captured by the battery is worth 16p more. At 24p import and 15p SEG, the gap narrows to 9p and the battery case weakens significantly: the solar battery size calculator.
- Summer months provide most of the battery charging opportunity. In June and July a well-placed 4kWp system may generate four to five times the monthly output of January. Most of the surplus available to charge a battery occurs in these months. A battery that sizes well for summer will be underused in winter: the solar battery size calculator.
- Diminishing returns begin relatively quickly as battery size increases. The first 5 kWh of battery capacity typically captures the most valuable surplus. Beyond 10 kWh for a standard 4kWp system, the additional saving per extra kWh of battery decreases substantially. The monthly breakdown table makes this visible: the solar battery size calculator.
- A time-of-use tariff significantly improves the battery financial case. On an off-peak overnight rate of around 7p to 12p, the battery can be charged cheaply overnight for evening use on days when solar generation is low. This adds a separate, year-round saving on top of the solar capture benefit.
- Battery degradation is not modelled but is real. Most batteries lose 2% to 3% of capacity per year. A 10 kWh battery has approximately 7 to 8 kWh of usable capacity after ten years. Warranties typically guarantee 80% capacity at ten years. The payback calculation in this tool uses the initial capacity throughout, which slightly overstates the long-term return.
Solar battery size calculator
Model the monthly generation and storage profile for your solar system to see how much additional saving a battery produces and whether the financial case stacks up. All figures are illustrative estimates.
Your solar system
Your household
The gap between your import rate and SEG rate is the value of each unit the battery captures instead of exporting. A wider gap means a stronger battery case.
Battery to model
The off-peak rate allows the battery to be charged cheaply overnight and discharged in the evening, adding a separate saving independent of solar generation. This saving applies year-round, not just in summer.
About This Calculator
The monthly model
Why seasonal variation matters for battery sizing
UK solar generation is highly uneven across the year. A south-facing 4kWp system generates roughly three to four times as much electricity in June as it does in January. Most of the surplus available to charge a battery occurs in the summer months: in winter, some days produce less than the battery’s daily capacity, meaning the battery never charges fully. A battery sized for summer surplus will be underused in winter and vice versa. The monthly table shows this profile explicitly so you can see where the battery is working hard and where it is idle.
The self-consumption rate
What it means and why it affects the battery case
The self-consumption rate is the proportion of solar generation you use directly as it is produced. A high rate (above 50%) means there is less export for the battery to capture, which reduces the additional saving. A low rate (below 30%) means there is more surplus available and a battery can add more value. The battery case is strongest for households with low self-consumption: those who are out during the day and currently export most of their generation at the lower SEG rate.
Diminishing returns
Why bigger is not always better
The first few kWh of battery capacity capture the most valuable surplus: the peak generation that exceeds immediate consumption and evening demand. As battery size increases, additional capacity either sits unused (because generation does not fill it) or captures export that is approaching the SEG rate in value rather than the import rate. The battery size comparison section shows the additional saving at each size from 3 kWh to 15 kWh so you can identify where the return per extra kWh starts declining.
What is not modelled
Limitations to be aware of
Battery degradation is not modelled: capacity reduces by approximately 2% to 3% per year, so actual long-term savings will be lower than the figures shown. The model uses UK average monthly irradiance profiles and will not reflect local weather variation, panel efficiency degradation, or inverter losses. Self-consumption rates are assumed constant across the year, whereas in practice summer and winter rates differ. The time-of-use saving assumes consistent overnight charging, which depends on battery management system capability and tariff terms.
How to Use This Calculator
Enter your solar system details
Set the system size, roof orientation, and shading level. If you already have solar panels installed, these should match your existing installation. If you are planning a combined solar and battery installation, use the planned system specification. Your self-consumption rate is the proportion of your solar generation you currently use directly: check your inverter monitoring app if available, or use 30% to 35% as a typical starting point for a household where people are out during the day.
Set your household consumption and tariff
Your annual electricity consumption is on your energy bills or in your smart meter account. Your import rate is the unit rate you pay per kWh. Your SEG export rate is on your energy account: this is the rate your supplier pays you for electricity exported to the grid. The gap between these two rates is the financial value the battery captures per unit stored. A wider gap means a stronger battery case.
Set the battery size and cost, and review the size comparison
Set the slider to the battery size you are considering. The battery size comparison section below the summary cards shows additional annual savings at 3 kWh through to 15 kWh, making the diminishing returns visible. Use this to identify whether the battery size in your installer quote is appropriate for your system and usage profile, or whether a smaller and less expensive battery would achieve most of the financial benefit.
Toggle the time-of-use section if applicable
If you are on or considering a time-of-use tariff that offers a lower off-peak overnight rate, toggle this section on and enter the off-peak rate. This adds a separate calculation for the saving from charging the battery cheaply overnight on days when solar generation is low. The combined saving (solar capture plus overnight charging) is shown in the summary cards when this is active.
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Checking won’t harm your credit scoreFrequently Asked Questions
What battery size is right for a standard 4kWp solar system?
For a 4kWp system with typical UK usage, a battery in the range of 5 kWh to 10 kWh is commonly recommended. The lower end captures most of the valuable daily surplus during the summer months without paying for capacity that sits idle in winter. The upper end is worth considering for households with higher evening demand, those planning to charge an electric vehicle from the battery, or those on a time-of-use tariff where the overnight charging benefit partly fills the battery every night regardless of solar generation.
The battery size comparison section in the calculator above shows the additional annual saving at each size from 3 kWh to 15 kWh for your specific system and usage inputs. Run this comparison before accepting an installer’s default recommendation: installers sometimes propose the largest battery in their range when a smaller unit would achieve most of the financial benefit at significantly lower cost. If the additional saving between a 7.5 kWh and a 10 kWh battery is only £60 per year, and the cost difference is £1,000, the smaller battery is likely the better financial decision.
Can I add a battery to an existing solar installation?
Yes, in most cases. Battery storage can be retrofitted to existing solar installations, though the process and cost depend on the type of inverter already installed. Hybrid inverters, which handle both solar generation and battery storage, are the most straightforward to connect a battery to. Standard string inverters can be paired with AC-coupled batteries, which connect to the household’s electrical system independently of the existing solar inverter. DC-coupled batteries, which connect directly to the solar panels, typically require a hybrid inverter and may involve replacing the existing inverter, which adds to the cost.
When obtaining quotes for a battery retrofit, ask the installer whether your existing inverter is compatible with their battery product or whether any additional equipment is required. The inverter compatibility question significantly affects the total installation cost and should be confirmed before comparing battery prices. An MCS-certified installer should be able to assess your existing installation and confirm the most cost-effective retrofit approach.
Does battery storage affect my Smart Export Guarantee eligibility?
In principle, yes. The Smart Export Guarantee requires that exported electricity is generated from qualifying renewable sources. Electricity stored in a battery and then exported to the grid could in theory have been partially charged from the grid (overnight charging on a time-of-use tariff), which raises a question about whether that export qualifies for SEG payment. In practice, most battery management systems distinguish between solar-charged and grid-charged electricity and only export solar-sourced electricity. However, the SEG eligibility rules for battery storage are nuanced and vary by supplier.
Before installing a battery, confirm with your SEG supplier whether the addition of battery storage affects your current tariff terms. Some suppliers require notification of system changes. If you are considering switching to a time-of-use tariff that includes SEG export payments, confirm explicitly that the supplier’s terms accommodate a combined solar-and-battery system including overnight charging. The rules in this area continue to evolve as battery adoption increases.
How long does a solar battery last and does degradation affect the financial case?
Most lithium battery systems used in domestic solar installations are warranted for ten years at approximately 70% to 80% of original capacity. Degradation in practice tends to be around 2% to 3% per year, meaning a 10 kWh battery has approximately 7 to 8 kWh of usable capacity at year ten. The battery continues to function after the warranty period, typically for twelve to fifteen years in total, though with progressively reduced capacity.
The financial implication of degradation is that the annual saving declines over time as capacity reduces. This calculator uses the initial battery capacity throughout the payback period, which means it slightly overstates the return for longer payback calculations. For a battery with a seven or eight year payback, the degradation effect over that period (perhaps 15% to 20% capacity reduction) means the actual payback is closer to eight or nine years. This does not change the financial case significantly for batteries with shorter paybacks, but it matters for marginal cases where the undegraded payback is already approaching or exceeding ten years.
Squaring Up
The battery financial case is strongest where the gap between the import rate and the SEG export rate is wide, the self-consumption rate is currently low (plenty of surplus to capture), and the system size generates meaningful daily surplus in summer. The monthly profile makes clear that battery charging is a summer-heavy activity: the same battery that is cycling daily in July may rarely fill in December. Sizing the battery to the summer profile rather than the annual average is the financially efficient approach, and the size comparison table in this calculator shows where the return per extra kWh starts declining.
The time-of-use tariff addition changes the calculation materially by adding a year-round overnight charging benefit that does not depend on solar generation. For households who can shift to a time-of-use tariff, the combined solar capture and overnight charging saving often justifies a somewhat larger battery than the solar-only case would support. The toggle in this calculator makes that comparison visible.
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Checking won’t harm your credit score Check eligibilityThis tool is for illustrative purposes only and does not constitute financial or energy advice. All generation estimates use UK average irradiance data adjusted for orientation and shading and will differ from actual output. Monthly consumption profiles are based on UK average seasonal patterns and will vary by household. Battery performance figures assume initial rated capacity and do not account for degradation over time. Smart Export Guarantee eligibility for battery-equipped systems varies by supplier and tariff: confirm terms with your supplier before installation. Your home may be at risk if you do not keep up repayments on a secured loan.
