How many solar panels do I need? The calculation

Step 1: take your annual statement and find your electricity use in kilowatt-hours. That number is the base; what a kWh is exactly, we explain separately. Step 2: divide that use by 0.85 to 0.9. That gives the power in watt-peak you need, because in the Netherlands an installation produces roughly 0.85 to 0.9 kWh per watt-peak per year, following the assumptions of Milieu Centraal (Dutch). Step 3: divide that power by the power per panel and round up.

Common panels now sit around 435 watt-peak. That makes the calculation for a south-facing roof look like this:

The calculation gives a starting point, not a final answer. On an east-west roof each panel yields a bit less, so the count comes out slightly higher. How much each orientation differs is covered in our piece on solar panel yield.

Generation equal to consumption is a logical starting point. While the net metering scheme runs, exported power is offset against your draw and every generated kilowatt-hour is worth the same. After that scheme ends, what counts most is how much of your own power you use directly; exporting earns less.

That is no reason to install fewer panels, but it is a reason to calculate smarter. Exporting a lot can trigger feed-in charges with some suppliers, while a larger system leaves room for a heat pump, EV charger or home battery later. So during the survey we look beyond last year's consumption at what will change in the coming years.

A common panel measures about 1.75 by 1.15 metres, so count on roughly 2 square metres per panel. Eight panels need about 16 to 18 square metres of uninterrupted surface on a pitched roof. Roof windows, a chimney or a dormer break up that surface; that is why we always draw the layout first.

A flat roof works differently. Panels sit in ballasted frames there, and in a south-facing layout we keep spacing between rows so they do not shade each other. That quickly means well over double the roof area per panel. An east-west layout packs tighter and often uses a flat roof better. The edges count too: we keep clearance along the roof edge because of wind load.

Whether your roof is suitable in terms of load, orientation and shade can be checked upfront via the Milieu Centraal guide on roof suitability (Dutch). During the survey we assess it on site, including the cable route to the fuse box.

Shade from a tree, chimney or neighbouring building cuts yield harder than many people expect. Sometimes a different layout solves it, sometimes micro-inverters are smarter because each panel then performs independently. The trade-off is covered in our comparison of string inverter versus micro-inverters.

The panel count also drives the inverter choice. An inverter has an operating range: too many or too few panels on a string and the system underperforms. If you want to expand later, say so. Then we pick an inverter with headroom, or a micro system that grows per panel.

Your annual statement looks backward, but the installation will sit there for 25 years. If a heat pump, an electric car or a home office is coming, your use rises and with it the ideal panel count. That is why we ask about your plans during the survey, not just your meter readings.

From our practice: the scaffolding, the mounting and the connection are largely one-off costs. Adding a second round of panels later is more expensive per panel than sizing properly the first time. If the roof allows it and it fits your plans, going slightly larger is often the more logical choice. But that should follow from the calculation, not from sales talk.

We start with your yearly use and your plans, inspect the roof for surface, shade and cable route, and draw out the layout. That produces a proposal with the panel count, the total power in watt-peak and the expected yearly yield for your roof, plus the matching inverter.

You get a no-obligation quote within 24 hours, no deposit required, and after approval we usually install within three weeks. Want to run your situation past us first? Send us your consumption and a photo of your roof or read on about what to look for when buying solar panels.