Pocket digester, solar panels and a battery: who runs when?

The digester runs steadily and covers the base load of the farm. Solar panels add power by day. A battery stores the surplus of both and releases it at the peaks, so almost nothing goes to the grid at a low price. A semi-off-grid system puts that battery and the control next to your existing connection, and uses the grid only as a back-up charger. Before you buy a battery, first send surplus power into heat where the farm can use it.

You installed a pocket digester to turn manure into power and heat. It runs day and night, and you planned it around the needs of the farm. Then solar panels came onto the barn roof. Now, on a sunny day in June, both produce at the same time. Power leaves the farm at a poor price, and the grid operator limits what you may inject. At six in the evening the sun is gone. The milking robot, the manure pumps and the cooling tank all run, and you still buy power.

Farmers describe the mismatch in their own words. A dairy farmer on a British farming forum asked about 120 kW of solar. The answer: "Suspect it will show a huge shortfall in winter & a huge surplus in summer". On a Flemish solar forum, an off-grid owner summed up the winter: "dec en januari zijn een flop". Only a generator or a CHP saves you then, he added. That is exactly the gap a digester fills. The problem is that solar and digester then compete in summer.

Here is the hidden cost. Inagro, the West Flemish agricultural research centre, lists what decides the return of a pocket digester. The main factors are how much power you use yourself, solar panels, a medium-voltage cabin and the use of the heat. Every kWh of digester power that leaves the farm at midday earns a fraction of what it saves when you use it. The obvious fix, a bigger connection to inject more, sells your power at the lowest price of the day.

We have not built a system around a digester yet. But we did solve the same puzzle on dairy farms with a steady source next to solar panels. In Londerzeel and Lokeren, dairy farms combined solar panels, a wind turbine and a battery to avoid an expensive medium-voltage cabin. The wind turbine plays the role a digester plays: power at night and in winter. In Tielt, a semi-off-grid farm uses smart control to send surplus power to boilers. It expects to take only about 5% of its yearly use from the grid.

Why do a pocket digester and solar panels get in each other's way?

Because one produces all day at a steady level, and the other produces a lot at midday and little in winter. Add them up on a summer day and you have too much power at noon. Look at a winter evening and you are short again.

A pocket digester is a small biogas installation on the farm itself, fed mainly with its own manure. In Flanders, small-scale digestion means an installed power of up to 200 kWe, and Varkensloket calls installations up to 10 kW micro-digesters. The biogas usually goes into a combined heat and power engine (CHP, in Dutch WKK) that makes electricity and heat at the same time.

The Flemish Department of Agriculture and Fisheries (with ILVO) gives the order of magnitude. A 10 kW micro-digester can produce up to 60,000 kWh net per year, with the manure of about 80 to 100 cows. One cubic metre of biogas gives about 2 kWh of electricity and 3 kWh of heat. The same publication calls continuous production "a clear advantage" over solar panels and wind turbines.

Enerpedia, the Flemish farm energy platform, puts a number on it. For small digesters without certificate support, it calls the installation profitable when 84% of the electricity it produces is used at the same moment. The milking system and solar panels are the deciding factors. So solar panels can lower the value of your digester, unless something moves the surplus to later in the day.

What does each source do best on a farm?

Each source has one job it does better than the others. The mistake is to let all of them do everything at once.

The digester is the source you control least in the short term. Inagro stresses that the process needs a steady feed, several times a day, and a residence time of at least 30 days for mesophilic digestion. You do not switch it off because the sun comes out. That is why the battery and the heat buffer have to adapt to the digester, not the other way round.

A wind turbine gives a similar night and winter profile, and many farms weigh it against a digester. Whether a small turbine pays off next to solar and a battery is covered in our article on a small wind turbine on a farm.

Who runs when? The control order on a farm with a digester, solar and a battery

A simple priority order decides it: digester first, solar second, then heat, then the battery, and the grid last. The control follows that order every few seconds, not once a day.

In practice this means four situations:

• Summer noon: the digester runs, the panels produce, the farm uses little. Surplus heats water first, then charges the battery. Only when both are full does power go to the grid, or the panels are limited.
• Evening milking: the panels stop, the digester keeps running, the robot and pumps start. The battery covers the part above what the digester gives.
• Winter night: almost no solar. The digester carries the base load; the battery was topped up by the digester during quiet hours and covers the peaks.
• Digester maintenance or a feed problem: the battery and the panels take over what they can, the grid fills the rest.

Can the digester shift a little too? Ask your supplier whether your installation can store biogas, and for how many hours. If the engine can run harder at the evening peak, the battery can be smaller.

That control layer is what makes the difference. How an energy management system reads the meter and steers loads is explained in our article on energy management for a business. On the Tielt farm, smart control sends surplus to boilers that heat extra when there is too much energy.

What does the grid allow when a digester, solar panels and a battery share one connection?

The grid sees every source as a production unit, and the connection limits what they may inject together. Adding solar panels next to a digester can push the farm past the injection the grid operator allows.

The Synergrid C10/11 rules (edition 2.4, 15 October 2025) apply to every production unit that runs parallel with the grid. The energy source does not matter: gas, biofuel, sun or batteries. Nor does the technology: rotating machine or inverter. A battery that can run parallel with the grid counts as a full production unit. The same document says CHP modules may need extra measures when heat and power are rigidly coupled. Examples are a heat buffer, or a brake chopper that burns off excess electricity.

For you this means three things. First, a new battery or extra panels must be reported to the grid operator like any other production unit. Second, the total injection of digester, panels and battery must fit your connection. Third, a battery that keeps the surplus on the farm reduces the injection problem at the source.

How injection limits for business solar work is explained in our article on injection limits for business solar panels. What to do when you want more panels than your connection allows is covered in our article on more solar panels than your connection allows. The peak you take from the grid also counts. In Flanders the capacity tariff bills your highest quarter-hour, as explained in our article on how to calculate the capacity tariff.

Where does the battery go: behind the grid connection or in a semi-off-grid island?

There are two ways to connect it. They change what you may inject, what the certificates count and what keeps running in a power cut. Which one fits depends on how your digester is connected today.

Our semi-off-grid system works as the second set-up. The public grid acts as a generator, called upon when there is little sun or the battery is low. A transfer switch separates the two systems at high speed. That is how the dairy farms in Londerzeel and Lokeren avoided a medium-voltage cabin while the farm kept growing. In Londerzeel the owners report a saving of at least 60,000 euro, and panels and batteries can grow with the farm.

Where the digester engine sits in such a design is a question for the study. It can stay on the grid side with its own meter and certificates. Or it can work on the island side, like a generator that charges the battery. Each choice has consequences for the support you receive. Above 10 kW, a biogas CHP in Flanders receives heat-power certificates; up to 10 kW there is a micro-CHP premium instead. Never change the connection of a CHP with certificates before checking what happens to them. Our article on green power certificates and a battery explains what to ask.

Is the running load of your farm well above the 63 A of a standard connection? Then a business battery behind the connection is the other route.

Does the digester keep the farm running in a power cut?

Not by itself, in most cases. A CHP that runs parallel with the grid must disconnect when the grid fails, just like solar inverters. Otherwise it would feed a grid that line workers think is dead.

Under C10/11, the grid decoupling protection disconnects a production unit when it detects an unplanned island. So on the day the grid fails, the farm with a digester, panels and a grid-connected battery can still stand dark. For ventilation in a barn full of animals, that is serious, as our article on ventilation in a barn during a power cut explains.

A system built as an island is different. The battery inverter then forms its own local grid, separated from the public grid by the transfer switch, and the connected loads keep running. Whether the CHP can join that island depends on the engine and its control; that is a question for its supplier. How to organise back-up for a business in general is covered in our article on back-up power for a business. How a generator and a battery share the work is in our article on a generator with a battery.

When is a battery not worth it next to a digester?

When the farm can use the surplus as heat, or when the digester already covers almost all of your use at the right moment. Then a battery adds cost without much gain.

Farmers on forums are blunt about it. One dairy farmer with 100 kW of panels wrote: "From experience I wouldn't put batteries in again". He pointed to easier wins: variable-speed drives, heat recovery and efficient compressors. Others preferred an ice bank for the milk tank. That is fair advice for a farm without a digester and without a connection problem. But when the digester and the panels together overflow the connection, the battery keeps the digester power on the farm.

First steps such as an ice bank or frequency drives are in our article on the energy system of a livestock farm. For greenhouses with a gas CHP, lighting and dynamic prices, the logic is different: see our article on a battery in a greenhouse or nursery.

What do we need to look at your farm?

Four things: your use, your production, your heat demand and your connection. With those we can tell you whether a battery, a boiler, more control or a semi-off-grid system makes the most difference.

1. Step 1: Send us a year of quarter-hour data from your digital meter, with your connection size.
2. Step 2: Add the production log of the digester CHP (power per hour or per day, downtime) and the power of your solar panels.
3. Step 3: Tell us where the heat goes today: milking parlour water, the digester itself, the house, calf drinking water.
4. Step 4: List the heavy loads and when they run: milking robot, cooling tank, manure pumps, feed mixer, ventilation.
5. Step 5: We compare your options: more control, heat first, a battery, or a semi-off-grid system. The last one lets the farm grow on its current connection.