Solar PV System Design Basics
Good PV design is a balancing act: array size, inverter window, roof orientation and the household's actual demand all have to line up.
Before a panel goes on the roof, the system has to be designed — how many panels, in what string configuration, with what inverter, on which roof. Get the design right and the system performs; get it wrong and you clip generation or the inverter shuts down on cold days.
This guide covers the design fundamentals: string sizing against the inverter window, orientation and shading, and matching the array to the load so the customer sees the benefit.
Key takeaways
- String voltage must stay within the inverter's MPPT window across the full temperature range.
- Cold weather raises panel voltage — size strings so they don't exceed the inverter's maximum.
- South-facing at around 30–40° is optimal, but east/west splits can suit self-consumption better.
- Even partial shading on one panel can drag down a whole string.
- Match array size to the household load and any battery, not just the available roof space.
String sizing and the inverter window
Each panel produces a voltage that varies with temperature — higher when cold, lower when hot. Panels are wired in series into strings, and the string voltage must stay inside the inverter's MPPT operating window across the whole temperature range the site sees.
The critical check is the coldest-day maximum: panel open-circuit voltage rises as temperature falls, and a string that exceeds the inverter's maximum input voltage on a cold clear morning can damage or shut it down.
Design for the cold day
Always check string voltage at the lowest expected temperature — that is where an over-long string trips the inverter's maximum input.
Orientation and tilt
In the UK, a south-facing roof at roughly 30–40° gives the highest annual yield. But orientation should follow the goal: an east/west split spreads generation across morning and evening, which can suit a household that uses more power at those times and improves self-consumption.
Factor in the roof's actual pitch and direction rather than assuming ideal conditions.
Shading
Shading is the design killer. Because panels in a string share a current path, shade on one panel can pull down the output of the whole string. A chimney, a tree or a neighbouring building matters more than its size suggests.
Where shading is unavoidable, power optimisers or microinverters let each panel work independently, protecting the rest of the array.
Matching to the load
The best-value system is sized to what the home actually uses, especially if there is no battery. Oversizing generation that gets exported cheaply may earn less than a smaller system with high self-consumption.
Look at the household's consumption pattern, whether a battery is included, and any EV charging, then size the array to suit rather than filling the roof by default.
Size, then certify
Once the design is set and installed, TradePlanr's PV commissioning certificate records the array, inverter and test results in one document.
Frequently asked questions
Why does cold weather matter for PV design?
Panel voltage rises as temperature drops. If a string is too long, its open-circuit voltage on a cold clear day can exceed the inverter's maximum input voltage and damage or shut it down, so strings are sized against the coldest expected temperature.
Is south-facing always best?
South at 30–40° gives the highest annual yield, but an east/west split can be better for self-consumption if the household uses more power in the mornings and evenings. The best orientation depends on the goal.
How much does shading affect output?
A lot. Panels in a series string share a current path, so shade on one panel can reduce the whole string's output. Optimisers or microinverters mitigate it by letting panels work independently.
From guidance to action
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