Does your off-grid generator really have to run 24/7?

Does your site run on a generator because there is no grid, the grid is too far away or the connection is too small? The answer is usually the same: do not replace the generator, demote it. Let solar panels and a battery carry the daily load, and let an off grid generator start automatically only when the battery needs it. The rest of this article explains why that works, what continuous running costs you, how to size the system and which rules apply in Belgium.

On your farm, remote site or workshop, the generator runs day and night. At night it turns for a fridge, a few lights and the router, and you hear it from the house. The fuel has to come to the site and the oil has to be changed. And in the back of your mind is the night it stops.

The expensive part is not the engine. It is the hours. A generator is sized for your biggest machine, yet most of the day and all night it delivers only a fraction of that. Every one of those hours costs fuel and brings the next service closer. And when it is your only power source, one breakdown stops the whole site.

That is how a farm in St-Georges-sur-Meuse, in the province of Liège, used to run. The street had no electricity connection, so everything on the farm ran on a generator, day and night. YouPower installed solar panels with a battery system: 30 kVA of inverter power and 46.2 kWh of storage. The generator is still there, but it now serves only as back-up. The farm is fully off-grid and never needed a street connection. See the St-Georges-sur-Meuse farm reference.

Why is running a generator 24/7 so expensive?

A generator is sized for the biggest load on the site: the milking machine, the pump, the compressor, the workshop. Most of the day, and all night, the real load is a fraction of that. The engine keeps turning anyway, because something always needs power.

Take a typical 30 kVA diesel generator. An approximate consumption chart from a UK generator supplier (FW Power) gives about 2 litres per hour at a quarter load. At full load it gives about 7 litres per hour. Run it around the clock at a quarter load and you burn roughly 48 litres a day. That is about 17,500 litres a year, for 8,760 running hours. Those are illustrative numbers, not the St-Georges figures. Check your own fuel bills.

Four things make continuous running worse than the fuel bill alone suggests:

• Hours drive maintenance. Oil and filters are changed at the recommended operating hours (or annually, whichever comes first). A generator that never stops reaches its service interval far sooner.
• Low load is a poor fit for a generator. Wind & Sun, an off-grid specialist, advises a generator that runs at no less than 40% of its capacity under the expected loads. The exception is when the manufacturer says otherwise.
• Noise, fumes and theft never stop. A generator next to a house, a stable or a holiday lodge is heard all night. Fuel in a tank on a remote site is also a target for theft.
• One engine carries the whole site. Fuel has to be delivered and stored, and an empty tank or a breakdown stops everything until the engine runs again.

To be fair to the engine, a lightly loaded generator is not dramatically less efficient per litre on that chart. It gives about 3 kWh per litre at a quarter load against about 3.4 kWh at full load. The big saving does not come from running the engine better. It comes from not running it at all for most of the year.

How much can a battery cut generator running hours?

In a generator and battery system, the battery becomes the buffer between supply and demand. The inverter/charger forms the local grid from the battery, 24 hours a day. The generator no longer follows the load. It charges.

• Daytime: solar panels power the site directly and charge the battery with the surplus.
• Evening and night: the battery supplies the site. No engine is running.
• Dark periods: when the battery reaches a set state of charge, the control system starts the generator. It runs at a healthy load, powers the site and recharges the battery, then stops.
• Heavy loads: when a load exceeds what the generator can give, a Victron inverter/charger with PowerAssist adds power from the battery. So the generator does not need to be sized for the peak.

How far the hours drop depends on your consumption, your solar array and the Belgian winter. The best-documented example we have is Special Renting Van Zele in Zele. The company rents out (electric) lifting equipment and has no connection to the public grid at all. The grid study offered no connection to the classic distribution network. The site runs on 30 kWp of solar panels, a battery sized in the pre-study and an emergency generator.

The monitoring data show the pattern. The solar power easily meets demand for most of the year. Only in winter does the battery run low often enough to start the generator. From December the generator runs on a number of days, with January as the worst month. In the rest of the year it stays off. The newspaper article about Van Zele adds a nice detail. The generator sits in the basement on purpose, so that its heat is not lost but warms the floor. Want to know how Van Zele runs a building with no grid connection, with a generator that only helps out in winter? Read our article on a new building without a grid connection.

Belgium has dark weeks in December and January that no reasonable battery can bridge. That is why a year-round business site that is fully off-grid needs a generator in its design. We explain the winter numbers in detail in our guide to off-grid solar with a battery.

What are the alternatives to a generator that runs 24/7?

There is more than one way out of a generator that never stops. Here is an honest comparison. Costs depend so much on the site that we only quote figures we can back up.

If a heavy machine is the reason you would need a bigger connection, read our article on whether a battery can replace a medium-voltage cabin. Is the generator only there for power cuts at a connected business? Then it is an emergency power question: see backup power for business, or in French groupe no-break en entreprise. On a livestock farm, see how to keep barn ventilation running in a power cut.

Can you add a battery to a generator you already have?

Several of our generator projects started with a generator that was already on site. We do not throw it away. We give it a smaller, better job.

St-Georges-sur-Meuse: from 24-hour generator to back-up

A farm in Wallonia without a street connection, running fully on a generator. YouPower added solar panels and a battery system of 30 kVA and 46.2 kWh. The generator now serves only as back-up. No street connection was ever needed. See the St-Georges-sur-Meuse reference.

Wielsbeke: the generator was running anyway

A flax processing company with one heavy machine. Running it on the grid would have required a large medium-voltage cabin. Instead, a generator started and drove the flax machine and kept running while it worked, as is also the case at a concrete plant. Loading that generator a little more to charge a 30.8 kWh battery (3 x 8 kVA) proved the right choice. The battery supplies the rest of the day. See the Wielsbeke reference.

Maasmechelen: grid or generator, the customer chooses

The customer had a generator for emergencies. Our semi-off-grid system made it genuinely useful: the customer can choose between the grid and the generator as the charger for his own off-grid system. 24 kVA, 30.8 kWh. See the Maasmechelen reference.

Kasterlee and Maaseik: smaller sites, same logic

In Kasterlee, a stable and a cottage lay very far from the grid. The owner did not want to run a generator for every stay. So they chose an off-grid system: 5 kVA and 16 kWh, enough for the whole year. See the Kasterlee reference. In Maaseik a generator was already available, and a 9 kVA, 15.4 kWh home battery got an off-grid mode with a generator connection. See the Maaseik reference.

How to size a generator and battery system

Sizing starts from measurements, not from the generator nameplate. YouPower measures actual and peak consumption over a period and sizes everything from that: panels, inverters, battery, cables and the generator. Our off-grid systems page lists what our engineering office calculates.

• Peak power per phase. This sets the inverter/charger power, including motor start-up currents. PowerAssist lets the battery top up the generator for short peaks.
• Battery capacity. Sized on the consumption outside solar hours on a winter day, plus a short autonomy. A battery that bridges a full dark week is rarely sensible in Belgium; the generator covers those days.
• Generator power. Big enough for the charger power of the inverter/chargers plus the loads that run while it charges. Not so big that it idles: aim for at least about 40% of its capacity while it runs.
• Solar array. Sized on December, not June. The more winter yield, the fewer generator hours.
• Control. The start and stop thresholds (state of charge, load, battery voltage) and quiet hours decide how often and when the engine runs.

An illustrative example, rounded and not from a real project: a site uses 60 kWh a day in winter, most of it outside solar hours. A generator that delivers about 20 kW for charging and loads covers that day in about three hours. Without a battery it would run for 24. With solar on top, it only runs on the dark days, which is how the Van Zele pre-study arrives at about 5% of annual demand.

Which auto-start and transfer switch does a generator need?

A generator that has to be started by hand is not a back-up. It is a chore. For a hands-off system you need two things.

Automatic generator start

The generator needs a remote start input: a two-wire contact that tells it to start and run, and to stop when the contact opens. The engine's own controller handles cranking and its safety shutdowns. In a Victron system, the GX device (for example a Cerbo GX) closes that contact. It does so based on battery state of charge, battery voltage, battery current or AC load. It can also start the generator on an inverter overload warning. Quiet hours let you set stricter conditions at night, so the engine only starts in a quiet period when it is really needed.

Transfer switch (ATS)

In a classic set-up an automatic transfer switch (ATS) moves the load between the grid and the generator. In a Victron off-grid system the inverter/charger does that job itself. A MultiPlus-II has one AC input, which is enough when the generator is the only AC source. A Quattro has two AC inputs, for a grid connection and a generator, and selects between them automatically. That is how the customer in Maasmechelen can use either one as the charger. If you need uninterruptible power for sensitive equipment at a connected business, read backup power for business. For a company on the grid with a back-up generator, see manual or automatic transfer switch for your generator.

Should your off grid generator run on diesel, HVO, LPG or gas?

Once a battery has cut the running hours, the choice of fuel matters less than people expect. It still matters.

• Diesel: robust, widely available, strong for heavy start-up loads. The downsides are fuel storage, theft risk, local emissions and noise.
• HVO (hydrotreated vegetable oil): a paraffinic diesel to the EN 15940 standard. Caterpillar and Rolls-Royce (mtu) have approved it as a replacement for diesel in some of their generator set series. Check the approval for your specific engine before switching.
• LPG or propane: stores well and burns more cleanly. YouPower built a fully off-grid system in As with solar panels, a battery and a propane generator (24 kVA, 30.8 kWh). See the As reference.
• Natural gas: only where there is a gas connection. One set-up on our solutions page uses an emergency generator on natural gas.

Our advice: first cut the hours with a battery, then choose the fuel. Switching fuel on a generator that still runs 8,760 hours a year saves far less than making it run only on dark winter days.

Which rules apply to a generator in Belgium?

A generator that is your permanent power source is treated differently from an emergency generator. That matters.

• Flanders (VLAREM): generators fall under rubric 12.1 (electricity production) and, for fixed engines, rubric 31.1 (combustion engines). Emergency generators under 400 kVA that run less than 100 hours a calendar year are not classified, and the operator must record their running hours. A generator that powers a site every day does not fit that exemption. Check the classification for your site with your municipality or an environmental adviser.
• Fewer than 500 hours a year: VLAREM counts the power of such generators at only 50%. That applies when it adds up the installed power. Cutting the hours with a battery can therefore also affect how your installation is classified.
• Wallonia and Brussels: a fixed generator can fall under the environmental permit (permis d'environnement). Ask your commune which class applies.
• Electrical inspection and reporting: an off-grid installation is inspected under the AREI/RGIE like any other. In Flanders an island installation with an inverter from 800 VA must also be reported to the Flemish utility regulator. Our off-grid solar guide explains this.

What does a generator and battery system cost?

Every generator site is different: consumption, peak loads, roof space, the distance to the grid. That is why we do not publish price lists. What we can share is what our references say.

• Special Renting Van Zele: an expected payback within 6.5 years for the full off-grid installation. The higher the consumption, the more profitable the system. In the newspaper article the owner expects seven to eight years.
• The requirement at Van Zele was explicit: the off-grid system had to be economically responsible compared with a classic connection.
• Your own saving is mostly the fuel and maintenance of the hours the generator no longer runs. Your fuel bills of the last twelve months are the best starting point.

For a figure for your site, ask us for a calculation. We need your consumption, peak loads and generator details.

How do you turn a 24/7 generator into back-up?

Working with us starts with your own numbers. This is what you prepare, and this is how the project runs from there.

1. Step 1: Collect your data. Fuel bills, running hours, the generator type and power, and if possible a few days of load data.
2. Step 2: Measure. YouPower measures actual and peak consumption, so the design is based on your site and not on the generator nameplate.
3. Step 3: Design for December. We size the solar array, battery, inverter/chargers and the generator's role together, with the start and stop thresholds.
4. Step 4: Check the rules. VLAREM or the Walloon permit for the generator, the electrical inspection, and the report to the regulator where required.
5. Step 5: Install and commission. We install the system and connect the generator for automatic start.
6. Step 6: Monitor. Our monitoring software shows in real time how much solar energy is used directly. It also shows what goes into the battery and when the generator runs.