How do you check a semi-off-grid or off-grid quote?

You check a quote for a semi-off-grid system or an off-grid system by reading its basis before its price. A solid quote shows your measured use, a December balance and inverter power for your largest motor start. It says how the grid or generator is used, and who handles inspection and service. Only then compare prices.

Three quotes are on your desk, and they look like three different machines. One has a small battery and a big inverter. One has twice the battery and no generator. The third looks like a kit from a web shop, at a price that makes the other two look silly.

An island system has no grid to hide a design error. A connected site buys the missing kilowatt-hours without noticing. A site on its own finds out in the first December, with an empty battery every morning. Or at the first compressor start that trips the inverter. The fix is then a second battery or a generator nobody budgeted.

A plant nursery in Jabbeke shows what a good design can change. A new irrigation pump needed a heavier connection, and the proposal was a medium voltage connection at a high price. The nursery kept its 3x400 V 32 A connection. It now only charges a system of 45 kVA with 92.4 kWh of storage. Frequency drives let the heaviest pumps start smoothly.

Why do quotes for a semi-off-grid or off-grid system differ so much?

Because each installer starts from a different basis, and most quotes do not say which one. One sizes on your yearly total, another on a guess of your peaks, a third on a kit that happens to be in stock.

The price follows from those choices. A system sized on the yearly total looks cheap and runs short in winter. A system sized on the worst week without a generator looks safe and costs far more than needed. Neither quote is wrong on paper; they just answer different questions.

Read every quote in the same order. The price comes last, because it only means something once you know what it pays for.

What must a quote for an off-grid or semi-off-grid system contain?

A complete quote answers fourteen questions in writing. If a line is missing, ask for it before you compare. The table shows what each line should say, and the red flag when it is not there.

Want to know what each part of the system does before you read the quote? Our article on what is inside a professional off-grid system explains the inverter, the battery, the charge control and the generator in plain words.

Line 12 is where many quotes go quiet. Every off-grid installation needs an AREI inspection (the Belgian electrical safety code) before use. In Flanders, an island installation with an inverter of 800 VA or more is also notified to the Vlaamse Nutsregulator (VNR). A battery on a site that keeps its connection is reported to Fluvius. The details are in our article on whether an off-grid installation needs an inspection.

How do you check the sizing in an off-grid quote: winter, motor starts and the generator?

Check three numbers: the December balance, the largest motor start and the generator hours. A quote that is right on those three is rarely wrong on the rest.

Winter first. Solar panels in Belgium give about a quarter of their June yield in December. The quote should show how the battery and the generator cover that gap, month by month.

Then the motors. A motor draws several times its running current for a few seconds when it starts. The inverter must deliver that burst, not only the average load. A pump, a compressor or a cold room can trip an inverter that looks big enough on paper.

Inverter data sheets give two numbers, and a quote should give both. Take the Victron Quattro of 10 kVA. It delivers 8,000 W continuously at 25 degrees and 6,500 W at 40 degrees, but 20,000 W as a short peak. A frequency drive (a controller that lets a motor start gently) can lower the start current, as on the pumps in Jabbeke.

Then the generator. Ask how many hours it will run per year, and in which months. Ask whether it starts on its own. A Victron GX control can start and stop a generator on the battery level, the battery voltage or the load, with quiet hours at night.

For the full winter calculation, read how to size solar panels and a battery for a Belgian winter. Wondering whether the generator must run all night? Our article on whether an off-grid generator really has to run 24/7 explains how the battery and the generator share the work.

How does a semi-off-grid quote use the grid: back-up switch or charger?

A semi-off-grid system uses the grid in one of two ways, and the quote must say which one. Either the grid takes over through an automatic switch when the battery runs low. Or the grid only charges the battery, and the site always runs on the inverter.

Both set-ups run on our projects. In Gingelom, the solar panels kept dropping out; the street is now only used as a back-up when energy runs short. In Heusden-Zolder, the system switches to the grid automatically when the battery is empty and the sun does not shine.

The charger set-up suits weak or small connections. In Jabbeke, the 32 A connection charges a system that delivers a heavier 63 A. In Gedinne, a hunting lodge in the woods had only 180 V left on its 3x230 V connection. The system now delivers 3x400 V and uses the grid as a charger. In Maasmechelen, the customer chooses the street or an existing generator as the charger.

So ask three things. Which circuits run on the system, and which stay on the grid? What happens on the site during a power cut, and does a generator take over? And how long, if at all, does the power drop at the switchover? A Victron Quattro, for example, has two AC inputs with a built-in transfer switch and connects to the active source on its own.

Choosing between a manual and an automatic switch for a generator is a subject of its own: read when a battery makes the switchover instant.

What do cheap off-grid quotes leave out?

Mostly the parts that make a system work on its own: generator control, protection, commissioning and paperwork. The panels, the battery and the inverter are all there; the design around them is not.

Kits are the clearest case. A kit bundles panels, a battery and an inverter at one price. It rarely includes a load calculation, a generator that starts on its own or protection for island mode. The inspection and the service are usually yours to find.

Some kits even use an inverter built to follow the grid, not to replace it. On a French forum, the owner of a mountain farm without a grid received such a hybrid inverter from a kit seller. Another owner of a forest house wrote that only one of 26 installers came, and quoted grid self-consumption.

A home battery is a similar trap. Not every home battery can run a house on its own: our article on which home batteries work off-grid explains the difference.

Which red flags in an off-grid or semi-off-grid quote should make you stop?

Stop when a quote promises autonomy or payback without showing the numbers behind it. The cards below list the signs of a solid quote next to the red flags we see most.

The clause "or equivalent" deserves a question. It lets the installer swap the battery or the inverter for another make after you sign. Ask which alternatives are allowed, and whether the specifications stay the same.

How do you compare off-grid quotes side by side?

Put the quotes next to each other on the same lines, with one column per installer. Where a quote leaves a cell empty, that is your first question for that installer.

Then ask each installer the same six questions about the quote itself:

Is your project a business battery behind a normal connection, for peaks rather than independence? Then our guide on how to compare business battery quotes fits your situation better.

When is neither quote the right answer?

When the site does not need to be independent at all. Off-grid and semi-off-grid solve a specific problem: no connection, a connection that is too weak, or a grid upgrade that costs too much.

Is your connection strong enough, or can the grid operator extend it at a fair cost? Then a normal connection with solar panels may be simpler. Compare the options first: our article on whether a battery is cheaper than a grid extension shows how. When the grid operator asks for a study, read what a Fluvius grid study involves.

And if both quotes skip the measuring, the right answer is to measure first. At a garden contractor in Zele, we monitored the average and peak consumption for one month before sizing. Then we matched the number of solar panels to the real use.

Not sure whether off-grid pays off for your business at all? Read when off-grid pays off for a business.

What happens when you send us your off-grid or semi-off-grid plans?

We start from what must keep running, and from your data.

1. Step 1: You send us the facts. Your consumption or a list of the planned loads, and your largest motors and machines.
2. Step 2: We measure and calculate. Our engineering office calculates the panels, inverters, battery and, where needed, the generator for your site.
3. Step 3: Installation, and the inspection before use.
4. Step 4: You follow it in real time. You see production, storage and consumption in the app.
5. Step 5: The system grows with you. It can be extended with extra panels, storage or another energy source.

Our semi-off-grid systems keep the grid as a helper for SMEs, farms and homes. For sites without any connection, see our off-grid systems. We are based in Zele and work with Victron Energy equipment as a Victron Energy Certified Partner.