What you bring
- Recent electricity bills or meter readings
- A rough idea of when your household is at home
- Notes on your roof direction, shade and condition
- Your budget and what you want from solar
Bright Ray / Your home, considered
How solar system size is decided
A bigger system is not automatically a better one. Size comes from how you use electricity, how much suitable roof you have and what may change at home. This guide explains the terms and gives you questions to ask.
What a provider assesses
- How much of your roof is usable
- Shade and direction across the roof
- How your usage pattern fits the proposed system
- What the roof and electrical set-up can accommodate
What you compare
- Panel capacity, stated in kW
- Estimated electricity produced, stated in kWh
- Battery capacity, if included, stated separately
- The assumptions behind each proposed size
- Whether future changes at home were considered
- Panel capacity (kW)
- The combined rated power of the panels, in kilowatts. It describes the system's size on paper rather than the electricity it will produce over a year.
- Electricity production (kWh)
- The amount of electricity a system generates over a period, in kilowatt-hours. Estimates depend on roof direction, shade, weather and equipment, so ask for the assumptions.
- Battery capacity (kWh)
- How much electricity a battery can store, in kilowatt-hours. It is separate from panel capacity and from the rate at which the battery can charge or supply your home.
A system for a changing household
Electric vehicle
If I buy an electric car, how would that affect the size you would suggest?
Heat pump
If I replace my boiler with a heat pump, should the system be sized differently?
Working from home
How does being at home during the day change what you would propose?
Household size
If more or fewer people live here in future, how flexible is the proposed system?
What drives system size?
Sizing is a balance between what you need and what your home can accommodate. Demand, usable roof space, roof direction and shade, budget and planned changes at home all play a part.
You bring knowledge of how your household lives; a provider assesses the roof and works through the numbers; you then compare what has been proposed and why.
Your electricity use
When and how much electricity your household uses is the starting point. Recent bills or meter readings help.
What usage information do you need from me?
Usable roof space
Roof area, obstructions and shade limit how many panels can be fitted.
How much of my roof did you count as usable?
Orientation and shade
Slope direction and shade influence how well different parts of a roof suit panels.
Budget and priorities
Your budget and what you want from the system, such as lower bills or room to grow, affect what is sensible.
How does the proposed size reflect my budget and priorities?
What do kW, kWh and capacity mean?
Quotes often use similar-looking units for very different things. Power, measured in kilowatts (kW), is a rate: how much electricity is being produced or used at a moment. Energy, measured in kilowatt-hours (kWh), is an amount over time.
Panel capacity, electricity produced and battery capacity are three separate ideas. Keeping them apart makes proposals much easier to read.
Panel capacity (kW)
The rated size of the panels together. It describes the system's rating, not how much electricity your roof will actually produce.
What is the total panel capacity, and how many panels does that mean?
Electricity produced (kWh)
The amount of electricity generated over a period such as a year. It depends on the roof, shade, weather and equipment.
What assumptions sit behind any production estimate?
Battery capacity (kWh)
How much electricity a battery can hold. It is separate from panel capacity and from how fast it can charge or discharge.
Is battery capacity stated separately from panel capacity?
What might change in your home?
Your electricity use today may not match your use in five years. An electric vehicle, a heat pump, working from home or a change in household size can all shift demand.
You do not need to predict the future precisely. It helps to tell a provider what is likely, possible or unlikely so they can discuss whether to allow for it.
Electric vehicle
Charging a car at home can add a significant amount of demand, depending on how it is used.
Heat pump
Replacing a gas boiler with electric heating changes how much electricity the household uses, especially in winter.
Working from home
Being home in the daytime moves more of your use into the hours when panels produce electricity.
Household size
Children arriving or leaving, or others moving in or out, changes daily habits and demand.
Could the system be extended later if my needs change?
How do you compare proposed sizes?
When two proposals differ in size, do not simply favour the largest number. Ask what each assumes about your use, your roof and your future plans, and whether the reasoning makes sense for your household.
A good comparison asks why that size was chosen, what it leaves out and what would change if your assumptions turned out wrong.
What usage was assumed?
Check the proposal reflects your actual bills or readings, not a generic household.
What electricity use did you assume for my home?
How was roof space counted?
Ask which slopes were used and how shade and obstructions were allowed for.
Which parts of my roof are included, and which were left out?
Why not bigger or smaller?
A clear explanation of the alternatives shows the size was chosen deliberately.
Why did you choose this size rather than a larger or smaller one?
How were estimates produced?
Production figures should come with their assumptions, such as direction, shade and weather data.
What would change the estimate, and by how much?
Start with one email
Ready to talk about your assumptions?
Email us your usage, roof notes and likely changes at home, and ask what Bright Ray can help with.
