Sizing a solar array to your consumption

When people start considering solar panels, one of the first questions is usually:

Roof space is important, but it should not be the only starting point.

A well-designed solar array should take account of:

• How much electricity the property uses
• When that electricity is used
• How much suitable roof space is available
• The expected solar generation at the property
• Whether a battery, electric vehicle or heat pump is planned
• The customer’s financial and energy goals

The objective is not automatically to install the largest possible array. It is to design a system that fits the property’s present and future energy needs.

1. Start with annual electricity consumption

Your electricity bill or supplier account should show how much electricity your property uses over a year.

This is normally measured in kilowatt-hours, written as kWh.

For example, a property might use:

• 3,500 kWh of electricity each year
• 4,500 kWh of electricity each year
• 7,000 kWh of electricity each year

This annual figure provides a useful starting point, but it does not determine the correct solar-array size by itself.

Two properties could both consume 4,000 kWh per year while using electricity at completely different times of day.

2. Look at when electricity is being used

Solar panels generate during daylight hours, with output changing throughout the day and across the seasons.

That means the timing of your electricity consumption matters.

A household that uses more electricity during the day may be able to use a greater proportion of its solar generation directly.

A property that is empty during most daylight hours may export more of its generation unless it has:

• A home battery
• Smart appliance scheduling
• An electric vehicle available to charge
• Other flexible electrical demand

This does not mean solar is unsuitable for an evening-use household. It means the wider system may need to be designed differently.

3. Understand the difference between kW and kWh

These two terms are often confused.

A solar array is usually described by its maximum rated power in kilowatt-peak, written as kWp.

Its expected annual generation is estimated in kWh.

4. Estimate how much electricity the array could generate

The expected output of a solar array depends on more than its headline size.

A solar assessment should consider factors including:

• Property location
• Roof direction
• Roof pitch
• Shading
• Panel specification
• Inverter design
• System losses
• Available usable roof area

A system’s rated size should therefore not be treated as a guarantee of annual generation.

Two arrays with the same kWp rating can produce different amounts of electricity because they are installed in different conditions.

5. Compare expected generation with consumption

Once annual consumption and estimated solar generation are understood, they can be compared.

However, matching annual solar generation to annual electricity consumption does not mean the property will stop using the grid.

Solar generation and electricity demand do not occur evenly throughout the year.

A property might generate surplus electricity during a bright summer afternoon but still need grid electricity on a winter evening.

This is why good system design looks beyond a single annual number.

A simple sizing example

6. Should the array be smaller than annual consumption?

Sometimes a smaller array may be appropriate.

Possible reasons include:

• Limited suitable roof space
• High levels of shading
• A restricted budget
• Low daytime electricity consumption
• Planning or network constraints
• A customer prioritising direct solar use over total generation

A smaller system can still deliver meaningful benefits when it is properly designed.

The decision should be based on the customer’s objectives—not an arbitrary target.

7. Can the array be larger than annual consumption?

Yes. There can also be valid reasons to install an array expected to generate more than the property’s present annual electricity consumption.

These might include:

• Future electric-vehicle charging
• A planned heat pump
• Air-conditioning demand
• A home battery
• Increased home working
• Growing household electricity use
• Exporting surplus electricity through an eligible tariff

The important word is planned.

A larger array should be supported by a clear reason and realistic modelling—not simply added because more panels can fit.

Solar panels are a long-term asset. Designing only around today’s electricity bill could overlook future demand that is already reasonably predictable.

8. Where does a battery fit into the calculation?

A battery does not generate electricity.

Its role is to store electricity so it can be used at a different time.

A compatible battery may allow the property to store some daytime solar surplus for use later, which can change how the overall system is designed.

Battery sizing should consider:

• The property’s electricity-demand profile
• The amount of expected solar surplus
• Evening and overnight consumption
• Electricity tariffs
• Charging and discharging power
• Usable battery capacity
• Whether backup power is required

Avoid assuming that solar-array size and battery capacity should automatically match.

9. Roof space still matters

Consumption may help establish what the property could benefit from, but the roof determines what can physically be installed.

A roof assessment should consider:

• Available dimensions
• Roof direction and pitch
• Shading from trees, chimneys and nearby buildings
• Roof condition
• Safe panel positioning
• Required clearances
• Access and installation requirements
• Whether more than one roof elevation could be used

The final design must balance energy requirements with technical suitability.

10. Your priorities influence the answer

There is no single “perfect” solar-array size for every property.

Different customers may prioritise different outcomes:

• Reducing electricity purchased from the grid
• Improving protection from future energy-price increases
• Charging an electric vehicle
• Supporting a battery
• Increasing use of their own renewable electricity
• Exporting surplus electricity
• Preparing for future electrification
• Working within a particular budget

A good recommendation should explain the relationship between these priorities, the proposed system size and the expected outcome.

It should not simply present a panel count.

Questions to ask before accepting a design