Rent a generator or buy a battery for a temporary power shortage in your business?

Rent a generator when the shortage is short, one-off or lasts day and night. Buy a battery when it comes as peaks above your connection, returns every season or lasts months until a reinforcement. A business battery charges in quiet hours and adds power only above your grid limit.

The season starts in three weeks, the connection is full, the heavier one comes next year. A 100 kVA generator quote looks cheap at a few hundred euros a week.

Special Renting Van Zele in Zele rents out electric lifting gear. Solar panels (30 kWp) and a battery power its warehouse, offices and machines off-grid. A generator covers about 5% of its yearly energy need, mostly in winter.

When does a company run short of power for weeks or months?

A business needs temporary extra power when the site asks more than the connection delivers, for a limited time. The grid works; it is simply too small for now.

• A peak season. Harvest, a cold store in summer, the holiday run-up in food production: more machines at once for a few weeks.
• A new machine or line before the reinforcement. Our article on a heavier connection refused by grid congestion explains how long that can take.
• Waiting for a grid reinforcement. In April 2026 Fluvius announced a solution for nearly 800 companies seeking a grid connection. Relaxed rules for its flexible connection (Fall-Back Flex) will eventually let 760 applications connect in congestion zones. Our article on the cost of waiting for a grid reinforcement prices that wait. In Wallonia, ORES offers flexible access above 250 kVA, as our article on connecting a business in Wallonia explains.
• Building work on your own site. An extension needs site power before the new connection is live: read what to do when a site connection comes too late.
• An event on your grounds. Open days, a company party or a market need power for a day or a weekend. Our article on a battery for a festival or event sizes that case.

A planned interruption by the grid operator is different: no grid at all for a few hours, which our article on planned power interruptions covers. Need power during a grid failure? Then read our article on emergency power for businesses first.

What is the real generator rental cost for a business?

The rent is the smallest part. The list price counts 8 running hours a day; a production shift of 16 hours doubles that. Someone refuels the generator, it stands in your yard for weeks, and next year you pay again.

The items come from the public price lists of two Belgian rental firms, Huurland and Boels (checked in September 2026). Boels rents this size only to business customers and shows no price online, but lists the same items.

Ask every rental firm for a price with your real running hours, the transport, the insurance and a fuel estimate. That is the generator hire cost per month to compare.

How much diesel does a rented generator burn per kWh?

Roughly 3 to 3.5 kWh per litre of diesel at a good load, in the datasheets of rental generators. At low load it gets worse.

A 60 kVA generator delivers about 48 kW (generators are rated at a power factor of 0.8), so 36 kW at 75% load. At that load Huurland lists 13 litres an hour and Boels 10.5 litres: about 2.8 and 3.4 kWh per litre. The 100 kVA set at full load (80 kW) and 23 litres an hour gives about 3.5 kWh per litre.

Energy monitoring at Glastonbury Festival found that a generator works best between about 40 and 80% of its power. From about 30% load the diesel per kWh rises; below 10% it can waste 60% or more of its diesel per kWh. A generator sized "to be safe" for the peak, then run all day on a smaller load, makes the most expensive kWh.

Why can a rented generator not simply top up your grid connection, and a battery can?

In practice a rented generator does not share the load with the grid: it feeds its own group of machines and carries that whole group. A battery adds only the excess.

A generator in parallel with the grid is a generation unit under Synergrid C10/11, the Belgian rule for anything that produces power on the grid. That needs protection settings and the approval of the grid operator, and C10/11 allows a back-up generator at most 5 minutes in parallel per switchover. A rented set on a trailer is not built for that.

So the electrician moves a group of machines, say the new line or the cold store, from the grid to the generator. It then delivers all the power of that group, every hour it runs, even when your connection had room. You wanted 30 kW extra for three hours; you get 45 kW for 16 hours.

A battery's inverter runs in parallel with the grid, as an approved generation unit you report to your grid operator. A controller keeps what you take from the grid under a limit you set, and the battery adds only what goes above it. This is peak shaving, which also lowers the monthly peak in the Flemish capacity tariff.

Two numbers size the battery. The largest excess above your limit, in kW, sets the inverter power: a 30 kW excess needs at least that much. The energy above the limit per day, in kWh, sets the capacity, plus a reserve of 20 to 30%.

The battery must be full again before the next peak, charged in the room under your limit in the evening, the night or the weekend. Solar panels on the roof help during the day.

How do a generator and a battery compare over a ten-week season?

In this illustrative example a rented generator for a new line delivers about 720 kWh a day. A battery that only covers the excess delivers about 90 kWh.

A production site can take about 200 kW from its connection and uses about 90 kW at night and 150 kW by day. From 8:00 to 11:00, ovens, cooling and compressors together push it to 185 kW. For a ten-week season, five days a week, a new 45 kW line runs from 6:00 to 22:00.

• A rented 100 kVA generator for the new line. It carries 45 kW for 16 hours: 720 kWh a day. At 2.8 to 3.5 kWh per litre that is about 205 to 260 litres a day. Over 50 working days that is roughly 10,000 to 13,000 litres. It runs about 800 hours, twice the 400 hours in a ten-week day-rate price. The rent alone, at 1,158 euros per four weeks, starts around 2,900 euros excluding VAT, before extra hours, transport, insurance and accessories.
• A battery that covers the excess. It delivers 90 kWh in the morning peak and recharges at night in the 110 kW of room. With a 25% reserve that is about 115 kWh of usable capacity and an inverter of about 40 kW. The line stays on the grid, and no machine changes over.

Should you rent a generator, rent a battery or buy a battery?

Decide on how long the shortage lasts, whether it returns, and whether it is a peak or a constant load.

The Dutch firm B-Charge, for example, rents mobile batteries of 90, 240 and 500 kWh for grid congestion, peak shaving and back-up. For a one-off shortage of a few months, a battery instead of generator rental can be the right step. The measured data then shows how big your own battery should be.

In Maasmechelen, a customer's existing emergency generator and the street connection are the two chargers of his battery system. It has 24 kVA of charging power and 30.8 kWh of battery. The generator runs short, at a good load, and the battery delivers the power.

When the shortage is over, the rented generator goes back to the depot and the diesel is gone. A battery keeps cutting the monthly peak, stores solar power you would otherwise inject and backs up critical machines in a power cut. So a battery that bridges a wait for a reinforcement is rarely a lost investment.

Which rules apply to a rented generator that stays on your site for weeks?

A rented generator that runs at your site every day counts as a fixed installation for the environmental rules, even on a trailer. The exemptions for building sites and emergency generators do not cover production use.

Flanders (VLAREM). That is how the Departement Omgeving sees a motor used regularly at one location, because the nuisance is the same. Only generators for the actual building or demolition works and emergency generators under 100 running hours a year are exempt. A production generator may therefore fall under heading 12.1 (electricity production); the Departement Omgeving page has the details.

Wallonia. A generator is a combustion installation. From 0.1 MW of thermal input it is a class 3 installation that needs a declaration to the municipality. The Walloon list of classified installations makes no exception for emergency use.

Electrical inspection (AREI). Mobile and temporary installations are inspected and get an inspection visit every year, according to the AREI. The fuel tank can need its own classification. Our article on permits and inspection for a business generator goes through every rule.

A battery. A battery in parallel with the grid is a generation unit under C10/11 that you report to your grid operator. Placement and fire safety are in our article on business battery safety.