How do you go off-grid with solar power in Belgium?

Going off-grid with solar power and a battery only works reliably in Belgium if you size the system on December, not on June. According to PVGIS data, 1 kWp in East Flanders delivers on average a quarter of its June yield in December, and there are winter weeks in which the panels generate almost nothing. A good off-grid solar system therefore consists of four building blocks that you calculate together. The first two are enough kWp at a well-chosen tilt angle, and a battery for night-time consumption and a short autonomy. The other two are an inverter-charger that forms the grid itself, and a generator with automatic start for the long shortfalls. The basis is always your measured quarter-hour consumption.

Maybe you already have a first design, sized on your yearly consumption and a sunny day. That works until the first grey week of December, when the battery is empty by the evening and the generator has to run far more than planned. The hidden cost of sizing on June is paid in January.

What parts make up an off-grid solar system with a battery?

An off-grid installation has no grid to fall back on, so every part must be able to carry the full load:

• Solar panels (kWp): the primary source. The number follows from winter consumption, not from yearly consumption.
• MPPT charge controller: connects the panels directly to the battery on the DC side (DC coupling).
• Off-grid battery (kWh): usually lithium iron phosphate (LFP). Bridges the night and short dark periods.
• Off-grid inverter-charger: converts DC to 230/400 V AC, forms the local grid and charges the battery from the generator.
• Generator: the back-up for long periods without sun, ideally with automatic start.
• Control and monitoring: in a Victron off-grid system, the GX device controls the generator, among other things. Through monitoring you know at any moment how the system is doing.

Are you still weighing a fully independent system against a middle way? Then first read our article on what off-grid means and which rules apply.

How much do solar panels really produce in a Belgian winter?

We calculated it with PVGIS, the solar energy tool of the European Commission's Joint Research Centre. Starting point: 1 kWp in Hamme (East Flanders), facing due south, 14% system losses, PVGIS-SARAH3 radiation data (averages 2005 to 2023).

Wallonia is no different. With the same settings, 1 kWp in Namur delivers about 31 kWh in December and 127 kWh in June, or 1,053 kWh over the year. Three conclusions:

• December delivers about 1.1 kWh per kWp per day, June about 4.3 kWh. A system that has plenty in June falls short in December.
• Averages hide the real challenge. In the PVGIS hourly series for 2014 to 2023, 1 kWp at 35 degrees in Hamme delivered less than 0.5 kWh on 43% of December days. The weakest five consecutive days (December 2017) brought barely 0.63 kWh per kWp in total. The weakest month was December 2023, with 18.5 kWh per kWp. Snow on the panels is not in these figures.
• A steeper angle helps in winter. At 60 degrees, the same kWp delivers about 20% more in December, at the cost of about 5% less over the year. In an off-grid system that trade is often worth it, because summer surplus is lost anyway.

How big should an off-grid battery be, and how many days of autonomy do you need?

A battery that bridges a full dark winter week is rarely a sensible choice in Belgium. If you want to cover that with storage alone, you pay for capacity that sits idle most of the year. We therefore size the battery on:

• consumption outside the sunny hours on a representative winter day, measured per quarter hour;
• a short autonomy, typically in the order of one day, so the generator does not have to start on every cloudy day;
• usable capacity and service life. Victron, for example, gives 2,500 cycles at 80% depth of discharge and 5,000 cycles at 50% for its LFP batteries, each time down to at least 80% of rated capacity.

Watch the temperature too. The same Victron batteries may only charge between +5 °C and +50 °C. A battery in an unheated shed may therefore not be able to charge in January, exactly when you need it most. Place the battery in a frost-free, ventilated room.

Should you choose a Victron MultiPlus-II or a Quattro as off-grid inverter?

The inverter-charger is the heart of the system. As a Victron Energy Certified Partner, we work with devices such as the Victron MultiPlus-II and the Quattro. The main difference is in the AC inputs:

According to the MultiPlus-II datasheet, you can connect loads to that second output that must not drain the battery, such as an electric water heater. Also check:

• Peak power per phase. Measure it per quarter hour and also look at the inrush currents of motors, compressors and heat pumps.
• Continuous power, not the model number. The number in the name is a VA value. Victron itself calculates with continuous power at 25 °C: a MultiPlus-II 48/3000 delivers 2,400 W continuously.
• PowerAssist. Under a heavy load, the inverter tops up the generator's power with energy from the battery. That way the generator does not have to carry every peak on its own.

In our worked off-grid case, three Quattro 48/8000 units each feed one phase of a 3 x 400 V home.

Do you connect off-grid solar panels through an MPPT (DC) or through the inverter (AC)?

With DC coupling, an MPPT charge controller charges the battery directly.

With AC coupling, an ordinary solar inverter is connected to the output of the off-grid inverter. That is handy when an installation is already in place, but there is a hard limit. Under Victron's factor 1.0 rule, the solar power may not exceed the continuous power of the inverter-charger at 25 °C. The off-grid inverter then turns the solar inverter down by raising the frequency. Victron also sets a minimum battery capacity per kWp of AC-coupled solar. Both can also be combined: existing panels AC-coupled, extensions through an MPPT.

Why does an off-grid system in Belgium need a generator?

Because December sets the design. A generator with automatic start makes the difference between a system that works most of the time and a system that always works. A Victron GX device can start and stop the generator through a relay based on, among other things, state of charge, battery voltage, AC load or battery current. For the state of charge you set a start level and a separate stop level. That way the generator charges the battery a good way up each time, instead of switching on and off for short runs. How to size the generator itself is explained in our article on a generator with a battery for off-grid.

Do you still have a grid connection? Then the grid can take over the role of the generator in a semi off-grid system. According to the Synergrid rule C10/11, the switch between on-grid and off-grid must then follow the break-before-make principle.

How many solar panels do you need to go off-grid? A worked example for a home without gas

Illustrative figures, rounded. A real design starts from your own quarter-hour measurements.

A detached home with a heat pump uses, according to a measurement, an average of 18 kWh per day in December and 8 kWh per day in June. The roof has room for 12 kWp at 35 degrees, facing due south.

• Battery: 25 kWh of LFP, used down to 80% depth of discharge, gives 20 kWh usable. That is a little more than one average December day.
• Weakest five days: the 12 kWp deliver about 7.6 kWh together, against 90 kWh of consumption. Without a generator, this cannot be bridged.
• Generator: if it delivers about 6 kW net to the system, it covers the shortfall in an average December in roughly 27 running hours. In the weakest December that is about 56. Add charging and conversion losses.
• Tilt angle: at 60 degrees, the December yield rises to about 472 kWh and the average shortfall drops to about 86 kWh, almost half the generator hours.

The summer surplus is large. In an off-grid system it is lost, unless you shift consumption to the summer. The monitoring software shows you when that surplus is there.

Want to see what such a system looks like in real life? Take a look at our off-grid references, such as the off-grid stable and cottage in Kasterlee or the off-grid school in Incourt, with solar panels, a battery and a generator.