What Size Solar Panel Do I Need?
Author: Steve Fairless
Originally Published: 25th September 2024 · Updated: 28th August 2026
The size of solar PV system you need should be based on your annual electricity consumption, when you use that electricity, the usable roof area, orientation and shading, future loads such as an EV or heat pump, inverter and grid-connection limits, and whether battery storage is part of the design. Matching annual generation to annual consumption is a useful reference point, but it is not the same as maximising bill savings.
At Sustainable Energy Engineering, we size systems from the property backwards. We do not start with a fixed "4 kW package" and try to make every home fit it. The same 4.5 kWp array can be excellent for one household and poorly matched for another because consumption timing, roof geometry and future electrification can be completely different.
This update replaces the old sunlight-hours shortcut with a more realistic UK sizing method using annual kWh, kWp, yield, self-consumption, export, grid connection and storage.
Updated: 28th August 2026 using the latest Ofgem Typical Domestic Consumption Values, current price-cap unit rates, current MCS solar PV requirements and current grid-connection guidance.
What Size Solar Panel System Do I Need? The Short Answer
- Start with at least 12 months of electricity use in kWh. In July 2026 Ofgem's medium single-rate electricity TDCV became 2,500 kWh/year, but your own bill data is much more useful than a national median.
- Then model generation in kWh per kWp for the actual roof. Location, orientation, pitch and shade change annual yield.
- Do not aim for a perfect annual kWh match without checking timing. Summer surplus cannot automatically supply winter demand unless it is exported and later re-imported through the grid.
- Check roof area and module geometry. A theoretical system size is irrelevant if it does not physically fit.
- Plan future electrical loads now. EV charging, heat pumps, electric hot water and home working can materially change electricity demand.
- Separate array size (kWp), inverter power (kW) and battery capacity (kWh). They are related but not interchangeable.
First: "What Size Solar Panel?" Usually Means "What Size Solar PV System?"
An individual module might be rated at 430 W, 450 W or 500 W. A home normally needs several modules connected as an array. The array size is the sum of the module ratings.
For example, ten 450 W panels create a 4.5 kWp DC array. That 4.5 kWp figure is not the annual generation. Annual generation is measured in kWh and depends on how productive that 4.5 kWp array is on the specific roof.
Use Your Own Electricity Data Before Any National Average
Ofgem updated its Typical Domestic Consumption Values from 1 July 2026. For a standard single-rate electricity meter, the revised reference values are 1,600 kWh/year for a low user, 2,500 kWh/year for a medium user and 3,800 kWh/year for a high user. The medium multi-rate reference is 3,400 kWh/year.
| Usage level | Single-rate electricity | What it tells us |
|---|---|---|
| Low | 1,600 kWh/year | A useful benchmark for a low-consuming household, not a design target. |
| Medium | 2,500 kWh/year | The 2026 median reference value used for standardised communications. |
| High | 3,800 kWh/year | A higher-use reference, still well below many electrified homes with EVs or electric heating. |
Your own annual total can sit far outside those figures. More importantly, two homes using exactly 3,800 kWh in a year can have completely different load shapes. One may use most electricity during daylight hours; the other may use it mainly after 5pm.
Why the Old "Divide by Sunlight Hours" Formula Is Too Crude for UK Design
A common online shortcut divides daily energy use by a guessed number of "sun hours". That can be useful for a rough off-grid exercise, but it is not how we would size a grid-connected UK roof. Solar yield changes by month, orientation, tilt, weather, shading, horizon and system losses. A proper model uses annual or monthly irradiance data and a recognised performance method.
In practice, we want an annual generation forecast in kWh for the exact proposed array, then we compare that with the customer's electricity profile.
A Practical Starting Table for 450 W Panels
The table below is not a recommendation for every home. It simply shows how modern panel count translates into array kWp and approximate module face area if each panel is about 2.0 m². Real roof allowance is greater because of gaps, edges and obstructions.
| Panels | Array rating | Approx. module face area | Who might consider it? |
|---|---|---|---|
| 8 | 3.60 kWp | About 16 m² | Lower electricity use, smaller roof or a design prioritising self-consumption. |
| 10 | 4.50 kWp | About 20 m² | Common scale for a medium-size modern roof if geometry and demand support it. |
| 12 | 5.40 kWp | About 24 m² | Higher annual demand, strong roof potential or future EV/heat-pump plans. |
| 15 | 6.75 kWp | About 30 m² | Larger roof and higher demand, subject to inverter and DNO design. |
How Annual Yield Changes the Array Size Needed to Generate a Given Amount
Suppose we want to generate approximately 3,800 kWh/year, broadly equivalent to Ofgem's current high single-rate reference consumption. The required kWp depends on site yield.
Illustrative array size to generate about 3,800 kWh/year
Calculation: target annual energy divided by annual site yield. These yields are illustrative only; an installation forecast should be calculated for the actual property.
Annual Generation Matching Is Not the Same as Energy Independence
If a 4 kWp system generates 4,000 kWh in a year and the home uses 4,000 kWh, it is tempting to say the home is "100% solar powered". That is not what the annual numbers mean. Solar generation is concentrated in daylight and heavily weighted towards spring and summer. Consumption may peak on winter evenings.
The grid therefore remains the balancing mechanism for a normal grid-connected home. The customer may export large summer surpluses and import during dark periods even if the two annual totals happen to match.
The Number We Care About Is Useful Solar, Not Just Maximum Solar
For bill reduction, the value of a kWh used directly in the property can differ from the value of a kWh exported. A larger array can still be a strong choice, especially with a good export tariff or future electric loads, but the economics should be modelled rather than assumed.
How Current Electricity Prices Affect the Value of Self-Consumption
For 1 July to 30 September 2026, Ofgem's GB-average price-cap electricity unit rate for a standard variable Direct Debit tariff is 26.11 p/kWh. From 1 October, the published average rises slightly to 26.32 p/kWh. Actual tariffs vary by region, payment method and contract.
At 26.11 p/kWh, avoiding 1,000 kWh of grid import is worth about £261 before considering export opportunities, tariff differences or battery losses. That calculation shows why load timing matters: a solar kWh used directly can be more valuable than the same kWh exported on a lower-rate tariff.
Roof Orientation and Shade Can Change the Required kWp
A south-facing roof with little shade is a strong generation surface, but east- and west-facing roofs can still work very well. Current consumer guidance commonly estimates an east- or west-facing roof may generate around 15-20% less than an equivalent directly south-facing system, though the exact difference depends on location, pitch and horizon.
That does not make east-west poor. It can spread generation across more of the day, which can improve alignment with morning and afternoon household demand. We size the array against both annual yield and when the energy is useful.
Future Demand Can Be More Important Than Current Demand
If you plan to buy an EV, replace gas heating with a heat pump, add electric hot water, build an extension, work from home more often or install air conditioning, today's electricity bill may understate tomorrow's demand.
Adding panels later is possible in many cases, but an extension can involve scaffolding, new DNO paperwork, a new inverter or altered string design. If the future load is credible and the roof space is available, it can be better to plan for it from the outset.
What About Battery Storage?
Battery capacity should not be used as a substitute for correct array sizing. A battery cannot create energy; it shifts energy through time. The design question is how much surplus solar is available to charge it, how much evening/night demand exists, how much battery power is needed and whether grid charging is part of the tariff strategy.
When we design solar PV with battery storage, we therefore compare PV kWp, battery kWh and battery/inverter kW separately.
Grid Connection Can Influence Inverter and Export Design
UK distribution rules distinguish small fully type-tested generation up to 16 A per phase under G98 from larger connections that normally fall into G99 processes. Sixteen amps corresponds to 3.68 kW on a single-phase supply and 11.04 kW on a three-phase supply.
That threshold does not mean a single-phase home can never have more than 3.68 kWp of solar modules. DC array size, inverter AC output and export limitation are different concepts. Larger DC arrays and export-controlled designs can be possible, but the DNO process and equipment design need to be correct.
Why Inverter Size Is Not Automatically Equal to Panel kWp
Panels are rated on the DC side. The inverter has an AC output rating and defined DC input limits. It can be appropriate for the DC array rating to be higher than the inverter's nominal AC rating because the array rarely operates at nameplate power for long periods in UK conditions. The correct ratio depends on orientation, inverter limits, clipping, battery integration and manufacturer design rules.
We check maximum open-circuit voltage in cold conditions, MPPT voltage range, input current, string configuration and the total inverter loading before approving the design.
How Much Roof Space Does the "Right" System Need?
Current consumer guidance describes an average domestic system of around 3.5 kWp using roughly 6-12 panels and around 10-20 m² of roof surface area. Modern panel power varies enough that panel count alone is increasingly unhelpful.
A roof with 20 m² of gross area may not provide 20 m² of usable module area. Dormers, roof windows, chimneys, ridge/eaves geometry, fire/access requirements and shading all reduce layout freedom.
Can a Solar System Be Too Big?
Yes, in the sense that extra modules can become poor value if they sit in heavy shade, require expensive electrical changes or create large volumes of low-value export that the customer did not plan to use. But "oversized" cannot be judged from consumption alone.
A high-export system may be intentional. A customer may expect an EV next year. A battery may absorb surplus. A larger array may improve winter generation. The design needs a reason.
Can a Solar System Be Too Small?
Also yes. A small package may look cheaper but can underuse a good roof, leave future demand uncovered and make later expansion relatively expensive. If scaffolding, survey, electrical work and commissioning are already required, the marginal cost of extra modules can be lower than adding a second project later.
Our Solar Sizing Process
What We Need Before We Recommend kWp
- At least 12 months of electricity use from bills or smart-meter data.
- Half-hourly data where available so we can see daytime, evening and overnight load.
- Roof dimensions, orientation and pitch.
- Shade/horizon information rather than a visual guess from ground level.
- Future load plans such as EV, heat pump, hot tub, electric heating or home working.
- Supply details including single/three phase and existing generation/storage.
- Battery objective if storage is being considered.
- Tariff/export strategy so the model values import avoidance and export sensibly.
- Equipment constraints including inverter MPPT windows, current limits and warranty rules.
- DNO connection route so the proposed AC/export arrangement is compliant.
A Better Target Than "Cover 100% of My Annual Use"
We prefer to ask: what array makes the best use of this roof and this customer's present and future electricity profile? Sometimes that produces annual generation below annual use. Sometimes it produces more. The result should be deliberate.
If you are considering a domestic solar PV system, the first useful document is not a panel brochure - it is your electricity history.
Send Us Your Annual kWh and We Will Size from There
We can compare your consumption, usable roof space, likely yield, future loads, inverter arrangement and battery options before recommending a system size.
Request a tailored solar system quote and include your annual electricity usage so the design starts with real numbers.
