Solar array capacity
Panel wattage multiplied by panel count gives the array's DC peak capacity in kWp. That is one of the starting points for inverter selection.
Solar inverter questions answered
What size solar inverter do I actually need?
One quotation includes a 3.68 kW inverter. Another recommends 5 or 6 kW. A third company wants to fit something larger again.
That does not automatically mean somebody is wrong. The inverter should follow the electrical design of the whole solar system — not simply the number written on the panels.
The short answer
There is no standard inverter size for a three-bedroom, four-bedroom or any other house.
Inverter size is not determined by bedroom count and it should not be selected from panel count alone.
A properly specified inverter has to make sense electrically with the solar array and practically with the household.
- How much solar PV is being installed?
- How are the panels split across the roof?
- How many orientations and strings are required?
- How many MPPT inputs does the design require?
- What will the DNO permit?
- Is battery storage being installed?
- What charge and discharge power is required?
- Are future EV or heat-pump loads expected?
What actually determines inverter size?
Six things matter more than the size of the house.
Roof orientation
A single south-facing array behaves differently from panels divided east and west, north-east and south-west, or over several roof faces.
Strings and MPPTs
Electrically different groups of panels may require separate strings and MPPT inputs. Total kW alone does not tell us whether an inverter is suitable.
Grid connection
The DNO connection route matters. The difference between G98 and G99 can influence both the design and how quickly a system can proceed.
Battery requirements
Hybrid inverter rating, battery charge and discharge power, usable storage and backup capability are related but they are not all the same specification.
Household demand
EVs, heat pumps, electric hot water and heavy daytime loads can make additional generation useful — but they do not, by themselves, dictate inverter size.
Panels and inverters use different ratings
6 kWp of solar panels does not automatically require a 6 kW inverter.
Solar panels are normally described by their peak DC output. Inverter output is normally described by the AC power it can deliver.
Those two figures do not have to be identical.
10 × 450 W
4.50 kWp of installed solar panel capacity.
14 × 450 W
6.30 kWp of installed solar panel capacity.
20 × 475 W
9.50 kWp of installed solar panel capacity.
The appropriate inverter for each example still depends on roof layout, permitted DC input, voltage and current limits, MPPT arrangement, grid connection and the rest of the system.
Panel kWp and inverter kW are related — but they are not
interchangeable numbers.
The inverter manufacturer's electrical limits must always be respected.
The 3.68 kW question
Why do so many UK solar quotations contain a 3.68 kW inverter?
In Great Britain, G98 covers small generating installations within defined limits.
The figure homeowners commonly encounter is 16 amps per phase. At a nominal 230 volts, 16 amps is approximately 3.68 kW.
That is why 3.68 kW appears so frequently on single-phase domestic solar quotations.
It does not mean every house should have a 3.68 kW inverter, and it does not necessarily mean the solar panels themselves must total only 3.68 kWp.
Where a proposed installation falls outside the relevant G98 limits, the project will normally move into the G99 process with the local Distribution Network Operator.
G98, G99 and export limits
Inverter rating and permitted grid export are not necessarily the same thing.
| Issue | What it means | Why it matters |
|---|---|---|
| G98 | The connection route used for qualifying small-scale generation within the relevant limits. | Commonly associated with the 16 A per-phase limit and approximately 3.68 kW on a 230 V single-phase supply. |
| G99 | The connection process generally required for generation above the relevant G98 limits. | The DNO may need to assess the proposed system before unrestricted operation is permitted. |
| Export limitation | Equipment can sometimes restrict how much electricity is exported to the grid. | A larger generation system does not automatically mean the full inverter output may be exported at all times. |
| G100 | Engineering requirements associated with export-limiting schemes. | It can be relevant where generation capacity and permitted export are intentionally different. |
The actual connection route is property-specific and is ultimately determined by the electrical design and the DNO requirements for that installation.
Oversizing and clipping
A little inverter clipping is not automatically a design fault.
Solar panels are rated under standard test conditions. In the real world, they spend much of the year operating below their theoretical peak output.
This means a designer may deliberately specify more DC panel capacity than the inverter's maximum AC output, provided the inverter manufacturer permits that electrical configuration.
On the brightest periods, the panels may occasionally be capable of producing more than the inverter can convert. The inverter then limits its output. This is generally referred to as clipping.
The goal is not zero clipping at any cost.
The goal is a sensible annual system design using equipment within its permitted operating limits.
Conversely, installing an unnecessarily large inverter purely to avoid every possible moment of clipping can add cost without necessarily adding useful annual energy.
MPPTs and roof layout
Sometimes the important inverter number is not kW. It is the number of usable inputs.
An MPPT — maximum power point tracker — allows the inverter to manage the electrical behaviour of connected solar panels.
Consider a house with panels on two roof faces pointing in different directions. Their generation profiles are different. Three roof faces can add another layer of complexity.
Shading, string voltage, panel current and panel count also have to stay within the inverter's electrical limits.
- One roof orientation may be straightforward.
- Two orientations may require separate string design.
- Three distinct groups may require additional MPPT capability.
- Different inverter models can have very different input limits.
So an apparently attractive inverter with the correct headline kW rating may still be the wrong inverter for the actual roof.
Hybrid inverters and batteries
A 6 kW hybrid inverter does not mean every battery connected to it can deliver 6 kW.
Battery systems introduce several different power ratings which are easy to confuse.
- Solar PV input capability
- Inverter AC output
- Battery charging power
- Battery discharging power
- Usable battery capacity in kWh
- Backup or EPS output capability
A battery measured in kWh describes stored energy. A power rating measured in kW describes how quickly electricity can flow.
Those are different questions.
That is why battery capacity should be considered alongside inverter power rather than using one figure as a substitute for the other.
For more on storage capacity, see What Size Solar Battery Do I Actually Need?
EVs, heat pumps and future demand
A high-demand home can justify more solar — but demand alone does not choose the inverter.
A home with gas heating and modest electricity consumption can behave very differently from an otherwise identical house with:
- A heat pump
- Electric hot water
- One or two electric vehicles
- Home working
- Heavy daytime electrical loads
Higher demand may make additional solar generation useful because more electricity can potentially be consumed on site.
But this still does not mean: "large electricity bill = fit the largest inverter."
The proposed solar array, roof, battery, grid connection and electrical limits must all agree with one another.
Why quotations disagree
Two inverter recommendations can both look plausible while being based on completely different assumptions.
Installer A stays within G98
The proposal may deliberately use a 3.68 kW inverter to keep the connection straightforward.
Installer B applies through G99
The design may use a larger inverter because the proposed array, household demand and grid connection justify it.
Installer C changes the whole design
Different panel count, roof layout, battery size, MPPT strategy or export assumptions can lead to another inverter entirely.
Do not compare inverter sizes until you understand why each
installer chose theirs.
The number on the inverter is part of the design — not a quality score.
What should I ask the installer?
A good quotation should be able to explain the inverter choice in plain English.
Ask these questions before deciding whether one inverter is "better" than another:
- What is the total solar array capacity in kWp?
- Why was this particular inverter size selected?
- What is the DC-to-AC design relationship?
- How many MPPTs are required for my roof?
- How will the panels be divided into strings?
- Is this a G98 or G99 installation?
- Is export being limited?
- What is the battery charge and discharge power?
- What backup power is actually available?
- Can the inverter accommodate the proposed panel voltage and current?
- Has likely future demand been considered?
If the answer is simply "that's the inverter we normally fit", you still do not know whether it is the right inverter for your property.
Common inverter questions
Quick answers.
What size solar inverter does the average home need?
There is no single inverter size for an average home. Array capacity, roof layout, MPPT and string requirements, grid connection, battery design and household demand all matter.
Why would an installer use a 3.68 kW inverter on a larger array?
3.68 kW is commonly associated with the G98 single-phase 16 A limit. It can also be technically acceptable for the panel array to have a higher DC peak rating than the inverter AC rating where the inverter manufacturer permits that design.
Is a bigger solar inverter always better?
No. The inverter has to suit the panel array, string design, grid connection and battery system. A larger unit is useful only where the rest of the design can make sensible use of it.
What is inverter clipping?
Clipping occurs when available solar panel power exceeds what the inverter can convert to AC at that moment. Occasional clipping can be a deliberate and reasonable design trade-off.
What is the difference between G98 and G99?
G98 is the connection route for qualifying small generation installations within defined limits. Systems above the relevant limits generally require the G99 process with the Distribution Network Operator.
Does the number of MPPTs matter?
Yes. Multiple orientations, different shading conditions or electrically different groups of panels may require separate string and MPPT arrangements.
Does battery size determine inverter size?
No. Battery capacity in kWh, battery charge and discharge power, solar PV input and inverter AC output are separate specifications which should be considered together.
Do an EV or heat pump mean I need a bigger inverter?
Not automatically. They increase electricity demand and may justify additional generation, but the inverter still has to suit the solar array, electrical design and permitted grid connection.
Related Tom Solar guides
Look at the system as a whole.
Start with the property
The right inverter should be the result of the survey — not the starting point of the sales pitch.
Tom Solar looks at the roof, panel layout, household electricity demand, battery requirement, cable routes, grid connection and practical installation issues before arriving at a sensible system design.