How to design a solar system step by step

Six decisions in a specific order, because each one constrains the next.

The individual tools each answer one question. This joins them, so the answers stay consistent — a system sized for one figure and laid out for another is the usual source of confusion when comparing quotes.

The order, and why it matters

1. Consumption. Everything downstream depends on it. A full year of bills.
2. Array size. What that consumption implies, before the roof is considered.
3. Roof layout. What actually fits. This is where ambition meets the chimney, and the number usually falls.
4. Strings. How panels group electrically, constrained by the inverter's voltage window.
5. Inverter. Sized against the array, with the ratio a deliberate choice.
6. Payback. Whether the result is worth doing.

Working out of order causes rework. Choosing an inverter before knowing the string layout is the classic example, since string length is bounded by the inverter's voltage limits.

Expect to loop

Design is iterative. The roof may not fit what consumption suggests; payback may argue for a different size; string constraints may force a panel count that is not what you first drew. Each pass gets closer.

What this is for

Not to replace a qualified installer — the final design must be done by someone who will sign it off and who understands local regulations, structural loading and grid connection rules.

It is to let you arrive at that conversation understanding your own numbers. An installer's proposal is much easier to evaluate when you already know roughly what your roof supports and what payback is plausible.

Frequently asked questions

Do I need an installer if I use this?

Yes. Final design, structural assessment, grid connection and sign-off require someone qualified. This is for understanding your own numbers before and during that conversation.

What if the roof will not fit what I need?

That is the normal outcome and the reason layout comes before finalising anything. Either accept a smaller system, use additional roof planes, or reduce consumption.

Why does the order matter?

Because each step constrains the next. Choosing an inverter before working out strings is the common mistake, since permissible string length depends on the inverter's voltage window.