Installation 9 min read

What to consider before installing solar panels

Orientation, inverter sizing, and the questions installers don't always ask

The brochures all look the same. Glossy panels, a smiling family, a roof tilted just so against a blue sky, and a number at the bottom that says how many years until you have earned your money back. None of this is wrong, exactly, but it skips most of the decisions that actually matter. By the time the panels go on the roof, you have already made choices that will shape your energy production for the next twenty-five years. Some of those choices are obvious. Others are buried in the small print, or never raised at all.

This article is for the reader who wants to understand those choices before signing anything. Most of it is aimed at someone considering their first solar installation, but if you already have panels, the second half on inverter sizing and orientation strategy is worth a read too. There is a fair chance your installer made decisions you were never told about.

Start with how much electricity you actually use

Almost every solar conversation starts with the question "how many panels can fit on the roof?" That is the wrong question. The right one is "how much electricity does this household consume in a year, and at what times of day?"

Pull out your last three or four annual electricity bills. Look at the kilowatt-hours, not the euros. A typical Belgian or Dutch household with electric cooking and a heat pump might use somewhere between 4,500 and 7,500 kWh per year. A household still on gas heating, with no electric vehicle, might use closer to 2,500 to 4,000. The number matters because it sets the upper bound on how much solar capacity is sensible to install. Producing far more than you can use, in an era where feed-in compensation has dropped sharply across most of Europe, is no longer the win it used to be.

The second number to think about is when you use that electricity. If you work from home and the dishwasher, washing machine, and heat pump all run during daylight hours, your self-consumption rate will naturally be high. If the house is empty between 8am and 6pm and almost everything happens in the evening, less of your solar production will be used directly, which changes the economics. We will get back to this when we talk about orientation.

Roof orientation: the south-facing myth

Conventional wisdom says solar panels should face due south. That is technically correct, in the sense that south-facing panels with the right tilt produce the most kilowatt-hours over a year. In Belgium, where the optimum tilt is somewhere between 30 and 40 degrees, a perfectly south-facing roof will give you 100% of the theoretical maximum yield. But here is the thing: that maximum is a benchmark, not a requirement.

Panels facing east or west typically produce around 75 to 85 percent of what a south-facing array would deliver. That sounds like a lot of lost energy on paper, until you look at when the energy is produced. A south-facing array piles most of its production into a sharp peak around midday. An east-west split produces a flatter, broader curve that runs from sunrise to sunset.

For households that consume electricity in the morning and evening, an east-west configuration is often the better economic choice, even though it produces less total energy. Fronius has a good piece on this from their inverter perspective. The key insight is that with current European tariff structures, a kilowatt-hour you use yourself is worth far more than one you sell back to the grid. An east-west roof that supplies the coffee machine in the morning and the dishwasher in the evening can outperform a south-facing roof that produces a midday peak you cannot use.

There is also a third scenario that comes up regularly in Belgian rowhouses: panels split across two non-ideal orientations, like south-east and south-west, or even north-east and south-west. Modern inverters with multiple MPPT inputs handle this well, and the financial outcome is usually better than the simple percentage loss suggests. Do not let an installer tell you that a complicated roof is a deal-breaker. It rarely is, although it does mean the inverter selection becomes more important.

For a quick check on your specific roof, the European Commission runs a free tool called PVGIS that estimates annual production for any orientation and tilt at any location in Europe. It takes about five minutes and gives you a number you can hold against your installer's quote.

Inverter sizing: the decision nobody explains

Here is the question installers almost never ask: how big should your inverter be relative to your panel capacity?

In residential solar, there is a metric called the DC-to-AC ratio, also written as the inverter loading ratio or ILR. It is simply the total panel capacity in kilowatts of DC power divided by the inverter's rated AC output. A 5 kW inverter paired with 6 kW of panels has a DC-to-AC ratio of 1.2.

Most people assume the inverter should be exactly the same size as the panel array. That assumption is wrong, and it is worth understanding why. Solar panels almost never produce their full rated power. Standard test conditions assume 1,000 watts per square meter of irradiance, a 25°C cell temperature, and a perfectly clean panel facing the sun head-on. In real life on a Belgian roof, panels typically operate between 60 and 85 percent of their rated power for most of the day. The handful of moments where they hit full output are usually around solar noon on a clear day in late spring.

If the inverter exactly matches the panel capacity, it sits underused for almost the entire year. If the panels are slightly oversized relative to the inverter, the inverter operates closer to its sweet spot for many more hours per day, and total annual production goes up. The price you pay is a few hours of "clipping" on the brightest summer days, where the inverter caps the output at its rated maximum and any additional DC power is simply lost.

In modern residential design, a DC-to-AC ratio between 1.15 and 1.25 is the consensus sweet spot. In northern European climates, where peak irradiance is rare, you can push slightly higher. SolarPower Europe's industry guidance and most installer training materials line up on this. If your installer quotes a 5 kW inverter with 5 kW of panels, ask why. The honest answer is usually "that is what we had in stock", which is fine if the price is right, but you should know the trade-off.

Choosing an inverter brand

Not all inverters are equal, and the choice matters more than the panel brand for long-term reliability. The European market is dominated by a handful of names. SMA, the German manufacturer based in Kassel, is widely considered the premium choice in residential installations and holds roughly a 22 percent share of the European residential and small commercial segment. Fronius, from Austria, is the other quality European option, especially popular with installers who care about service and documentation. SolarEdge, based in Israel, is the dominant choice when you want optimisers on every panel, which makes sense on roofs with shading or multiple orientations. GoodWe, Huawei and Sungrow are competitive on price and have improved significantly in recent years. Enphase microinverters are a different architecture entirely, with one small inverter per panel, and are worth considering on small or shaded installations.

A premium inverter costs perhaps 200 to 500 euros more than a budget alternative. Spread across a 25-year lifespan, that is loose change. The difference shows up in efficiency curves at low irradiance, in firmware updates a decade from now, and in whether the manufacturer is still around to handle a warranty claim in 2040. If your installer pushes a brand you have never heard of, it is fair to ask why.

The questions to ask before you sign

A short list of things to bring up with any installer, written down so you do not forget under the friendly sales-pitch pressure:

The last one is the killer question. A good installer will quote a number derived from PVGIS or a comparable simulation, taking shading and orientation into account. A bad installer will quote a generic number that has nothing to do with your specific roof. The difference between those two installers is usually a difference of several hundred euros per year in actual production, and it shows up in your bills for the next quarter-century.

A note on monitoring, because this is what we do

Once the panels are on the roof, the inverter logs production data and serves it through some sort of app. Manufacturer apps vary wildly in quality, and they all share one inconvenient truth: the data they show you belongs to the manufacturer, not to you. If you switch inverter brands later, that history typically does not come with you.

This is where independent platforms like PVOutput become useful. They let you upload your production data to a service that is yours, store it long-term, and access it through any tool you like. HelioPeak, the iOS app I work on, sits on top of PVOutput and gives that data a more readable home on iPhone and iPad. Other tools exist for Android and the web. The point is that the question of where your data lives is worth thinking about before installation, not five years later when you discover the manufacturer is shutting down their cloud service.

But that is a conversation for after the panels are on the roof. The harder decisions are the ones you make beforehand: how big, which way, what inverter, which installer. Get those right, and the next twenty-five years take care of themselves.

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