Solar Inverter Sizing Calculator (kW)
Calculates the ideal solar inverter size in kW for your PV system from the array power and sizing ratio, giving minimum and recommended power.
For a 4 kWp solar array with a sizing ratio of 0.85, the minimum inverter power is 3.4 kW, which this calculator rounds up to a 3.5 kW model. This page explains where that figure comes from and when a different ratio makes sense for a UK home.
An inverter converts the direct current (DC) from the panels into alternating current (AC) that the home can use and that can be exported to the grid. Most households fix the inverter size once, at installation, so it pays to get the ratio right before the quote is signed. An inverter that is too small caps output on the sunniest days, while one that is too large adds cost without adding much generation over the year.
The size of an array is quoted in kWp, the rated power of the panels under Standard Test Conditions of 1,000 W/m² and 25 °C. Real operation rarely matches those conditions. Irradiance stays below 1,000 W/m² for most of the day, and panels run hotter than 25 °C in summer sun, which reduces output. The sizing ratio reflects this gap, so the inverter can be smaller than the array's rated total without a large loss of energy.
In the UK, cold, bright days can push panel output above its rated value, so the gap between rated and real output changes with the weather. The 0.85 value in this calculator is a default that you can change between 0.70 and 1.00. The right figure depends on roof orientation, shading, daytime household use and whether a battery is planned.
The calculator gives a mathematical minimum and a recommended size. Treat the recommended figure as a planning value, and check the inverter's maximum DC input voltage and the export limit set by the network operator before ordering equipment.
The explanation below uses metric units; the calculator works in either system.
The result updates automatically.
Total peak output of all the panels. E.g. 10 panels × 400 W = 4 kWp
0.85 is a typical value. It reduces the inverter cost with minimal impact on annual generation.
Enter the price per kW from the installer's quote to estimate the inverter cost.
Recommended inverter output
rounded up to the nearest half kW to match an available model
- Calculated minimum output
- 4.25 kW
- Inverter-to-panel ratio
- 90%
How it works
Formula
How the calculation works
Solar Inverter Sizing: How to Get It Right in the UK
A solar inverter converts the direct current (DC) produced by the panels into alternating current (AC) that the home can use and that can be exported to the grid. An inverter that is too small caps output on the sunniest days, while one that is too large adds cost without adding much generation over the year. This calculator estimates the inverter size that balances those two effects, using the rated power of the array and a sizing ratio.
How the sizing ratio works
The formula is P_inv = P_panels × k, where P_inv is the inverter power in kW AC, P_panels is the total array power in kWp DC, and k is the sizing ratio. The ratio is also called the sizing factor. For a 5 kWp array and k = 0.85, the minimum inverter is 4.25 kW. The calculator rounds this up to the next half kilowatt, 4.5 kW, which is a size usually available as a standard model.
A ratio below 1 is a practical choice rather than a compromise. Panel ratings are measured under Standard Test Conditions (STC), an irradiance of 1,000 W/m² and a cell temperature of 25 °C. Real operation rarely matches those conditions. Irradiance is usually lower than 1,000 W/m² for much of the day, and cell temperature rises well above 25 °C in summer sun, which reduces output. The temperature coefficient on the panel datasheet shows how much power is lost for each degree of temperature rise. A smaller inverter takes advantage of this gap, because the array seldom needs its full rated output.
Reference by system size
| System size (kWp) | Minimum inverter (k = 0.85) | Typical profile |
|---|---|---|
| 3 kWp | 2.55 kW → 3 kW | Small flat or one to two bedroom home |
| 5 kWp | 4.25 kW → 4.5 kW | Three bedroom semi-detached house |
| 8 kWp | 6.8 kW → 7 kW | Four bedroom detached house |
| 10 kWp | 8.5 kW → 8.5 kW | Large detached house |
| 15 kWp | 12.75 kW → 13 kW | Small commercial premises |
The profiles are illustrative only. Roof area, orientation and household electricity use matter more than the number of bedrooms, so the table is a quick check rather than a design basis.
Clipping
Clipping happens when the panels produce more DC power than the inverter can convert. The inverter limits its output to its rated power, and the surplus is not converted. It occurs mainly around midday on cool, bright days, when panels run efficiently and the sun is high. How often it happens depends on how the array's peak output compares with the inverter rating, so roof orientation and tilt both affect it.
A lower ratio means the inverter reaches its limit more often, so clipping increases as the ratio falls. The saving on the inverter has to be weighed against the energy lost to clipping. The calculator accepts ratios from 0.70 to 1.00, and the installer can model the expected annual yield at any value in that range.
Cold weather and voltage limits
The calculator checks power only. Voltage needs a separate check. The open-circuit voltage of a string of panels rises as the temperature falls, so a string designed on a mild afternoon can exceed the inverter's maximum DC input voltage on a frosty morning. The installer should calculate the string voltage at the coldest expected temperature, using the voltage temperature coefficient from the panel datasheet, and compare it with the inverter's limits. The inverter's MPPT voltage window also needs to suit the operating voltage of the string.
Cold, bright conditions also raise panel output, so the array can briefly produce more than its rated kWp. This is one reason why the sizing ratio is a planning value and not a guarantee of any particular output.
Types of inverter
- String inverters are a single unit serving the whole array. Residential models are widely available in single-phase versions, and larger three-phase units serve commercial roofs.
- Hybrid inverters also manage a battery. They suit systems where a battery is planned, and the battery charge power should be checked against the array output.
- Panel-level electronics, such as microinverters and power optimisers, are fitted to individual panels. They limit the effect of shading on a single panel, but they must be matched to the manufacturer's system.
Grid connection and certification in the UK
Connection to the network is governed by the Energy Networks Association Engineering Recommendations G98 and G99. A single-phase inverter rated up to 3.68 kW per phase is normally covered by G98, which is a notification process. Larger inverters, including a single-phase unit above 3.68 kW, fall under G99, which requires an application to the local distribution network operator before connection. Three-phase connections are assessed on their total rating and follow their own thresholds.
The inverter's anti-islanding protection, which disconnects it when the grid is lost, is tested to IEC 62116, the international test procedure. Electrical installation must follow BS 7671, the Requirements for Electrical Installations. In England and Wales, Part P of the Building Regulations covers electrical work in dwellings, and in Scotland the Building (Scotland) Regulations 2004 apply. Installers certified under the Microgeneration Certification Scheme (MCS) are the usual route for domestic systems.
Common mistakes
The most frequent error is sizing the inverter to match the array's kWp exactly. This adds cost, and the extra capacity only earns a return if the array regularly exceeds the inverter's rating. The opposite error is choosing a very low ratio, close to the bottom of the calculator's 0.70 to 1.00 range, which clips more energy on the sunniest days.
Another mistake is ignoring the cold-weather voltage check described above. A third is confusing the inverter's AC rating with the power that can be exported. The network operator can set an export limit, and the inverter may need to be configured to respect it. Choosing an inverter on price alone, without checking the warranty, local support and availability of spare parts, is also a common source of later problems.
When to use this calculator
Use this calculator to pre-size a grid-connected system with a single array orientation. For arrays split across roof planes with different orientations, a site with shading, or a system with a planned battery, the result is only a starting point and a full design is needed. The calculator is not designed for off-grid systems, where the inverter must cover peak household demand and battery charging. It does not replace a site survey or a string design.
Working with your installer
Ask the installer for a design that states the array power in kWp, the inverter AC rating, the string layout, the maximum DC voltage at the coldest expected temperature, the export limit and the expected annual yield. Compare these figures with the calculator result. If an installer proposes a much larger inverter, ask why, and check whether the extra capacity is needed for a battery or a future expansion. A clear, written design makes it easier to confirm that the inverter suits the roof and the household.
Frequently asked questions
What sizing ratio should I use in the UK?
The calculator uses 0.85 as its default value. The ratio keeps the inverter smaller than the array, which lowers the inverter cost, and the energy lost to clipping is weighed against that saving. The right ratio depends on roof orientation, shading and whether a battery is planned, so ask your installer to model the expected annual yield for your roof before you choose a value.
What is clipping in a solar PV system?
Clipping happens when the panels produce more DC power than the inverter can convert to AC. The inverter limits its output to its rated power, and the surplus energy is not exported. It usually occurs around midday on cool, bright days when panels run efficiently. A lower sizing ratio means clipping occurs more often, so the saving on the inverter has to be weighed against the energy it costs. Ask your installer for a yield estimate at the chosen ratio before you decide.
What is the difference between kWp and kW when sizing an inverter?
kWp (kilowatt peak) is the rated power of the panels under Standard Test Conditions of 1,000 W/m² and 25 °C, and it is the reference used when designing the array. kW is the AC output power of the inverter in operation. Because panels rarely reach their rated kWp in real conditions, the inverter can have a lower kW rating than the array total without a large loss in annual yield. The sizing ratio sets the relationship between the two figures.
Can I connect a 6 kW single-phase inverter to a 7 kWp array?
Yes, the combination is technically feasible. A 6 kW inverter with a 7 kWp array gives a ratio of 0.86, which is within the calculator's 0.70 to 1.00 range. Check that the inverter's maximum DC input voltage and current suit the string design at the coldest expected temperature, and that the AC output is compatible with the export limit agreed with the network operator. A single-phase inverter above 3.68 kW generally needs G99 approval from the network operator before connection, which takes longer than G98 notification.
Does the inverter need to be the same make as the panels?
No. Inverters and panels from different manufacturers are technically compatible as long as the electrical parameters match, including maximum DC voltage, maximum current and the MPPT voltage window. The exception is systems with panel-level electronics such as power optimisers or microinverters, which are designed to work within a single manufacturer's ecosystem. Check in writing that the chosen combination is supported by the inverter manufacturer and the installer before you commit to it.
Do I need a certified installer to fit a solar inverter in the UK?
For a grid-connected system, use an installer certified under the Microgeneration Certification Scheme (MCS), which is the usual route for domestic systems. The electrical work must follow BS 7671, and in England and Wales Part P of the Building Regulations applies to electrical work in dwellings. Before the system is switched on, the grid connection must be notified or approved with the network operator under G98 or G99, depending on the inverter rating. A certified installer can handle these steps and provide the documents you need.
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By RivoCalc · Reviewed under our editorial methodology · Updated 16 June 2026
These results are estimates for pre-sizing only. Real output depends on panel orientation and tilt, shading, module temperature, the inverter datasheet and local network connection rules, so a qualified installer should confirm the final design.