Surplus solar: boiler or battery first for your business?

A boiler or buffer tank is the better first step when you use heat every day and your surplus comes in daytime peaks. Think of cleaning water, car washes, showers or process heat. A business battery is the better first step when you need power, lower peaks, back-up or electricity in the evening. Many sites end up with both, in a fixed order. The energy management software decides where each surplus kWh goes.

In June, the panels on the shed inject half the day for very little money. At the same time, the boiler heats the cleaning water on gas, or on night power. A battery quote lands on the desk. And someone at the co-op says: "Just put the surplus in the boiler, it costs a fraction."

Both answers can be right, and both can be expensive. A battery sized for summer surplus is often full by mid-morning and half empty all winter. A boiler that switches on by itself can set your monthly peak on a cloudy afternoon. Or it can sit at 40 °C for days, which is the temperature legionella likes. The real question is not "boiler or battery?" but "which job comes first on this site?"

A farm in Wingene shows how the two work together. The battery of 150.4 kWh delivers 3 x 400 V at 63 A, so no medium voltage was needed. When the battery is full, a resistor heats a boiler: water becomes the second battery. The smart control can also run pumps and boilers when there is overproduction. On a farm in Tielt, smart control sends surplus energy to the boilers too.

Why does surplus solar power so often end up in hot water on a business site?

Because many sites need heat every day, and water stores heat with simple parts: a tank, an element and a control. Every kWh that heats your water on site is a kWh you neither inject cheaply nor buy back later.

The International Renewable Energy Agency (IRENA) puts it plainly. When the final demand is heat, it is more efficient to store surplus renewable power as heat than as electricity. The Flemish government says the same for self-consumption. A heat pump boiler or an electric boiler with a storage tank keeps the water warm for many hours.

Power-to-heat is the general name for it: turning electricity into heat, here from your own panels. On a business site, it can be as small as one boiler or as large as a buffer tank of several thousand litres.

How much energy does a boiler or buffer tank really hold?

Heating one litre of water by one degree takes 1.163 Wh, so 1,000 litres heated from 15 to 60 °C hold about 52 kWh. A bigger tank or a larger temperature rise stores more.

The Flemish knowledge centre VITO gives a check from a real sector. In a self-service car wash, heating 30 litres of water from 20 to 60 °C takes about 1.4 kWh. That matches the physics.

For comparison: the battery in Wingene holds 150.4 kWh. The same amount of energy as heat sits in about 2,900 litres of water heated from 15 to 60 °C. But the two are not equal. The battery can give electricity back for any use; the tank can only give heat back. And a tank slowly loses heat, so it stores for hours or days, not weeks.

When is a boiler or buffer tank the better first step than a battery?

When your site uses heat every day, your surplus comes in daytime peaks, and that heat now comes from gas, oil or the grid. Then heat is the cheapest place to start, and a battery can follow later.

When must the battery come first, even if you use hot water?

When your problem is power rather than energy. A boiler absorbs surplus, but it cannot add a single kW to a connection that is too small.

The farm in Tielt is the clearest example. Its connection was too small to grow, and the next step would have been medium voltage. A battery system of 45 kVA and 123.2 kWh now gives the site more power, so the grid connection was not reinforced. The grid only charges the battery. The smart control to the boilers came on top of that.

The same logic applies to peaks. In Flanders, the capacity tariff is based on your highest quarter-hour peak of the month. A battery can deliver power during that quarter-hour; a boiler can only be switched off. Our article on peak shaving with a battery shows how that works in practice.

And a tank of hot water does nothing during a power cut. If cold rooms, ventilation or IT must keep running, a battery with an island function comes first.

Which businesses have heat loads that can take solar surplus?

Any site that needs hot water or low-temperature heat every day. Think of car washes, dairy farms, food businesses, campsites and hotels, and buildings with a heat pump and a buffer tank.

On a dairy farm, the order matters. ILVO notes that producing hot water for cleaning uses a lot of energy, also with a milking robot. Per litre of cooled milk, heat recovery can warm about 0.3 to 0.8 litres of water to 50 °C. That heat is free every milking. Surplus solar then tops the water up to the cleaning temperature.

For farms in general, our article on a farm energy system compares a battery, an ice bank and a frequency drive. For growers, read whether a battery pays off in a greenhouse. Switching a workshop from gas to a heat pump is covered in what a heat pump does to your connection.

Resistance element or heat pump boiler: which one should take the surplus?

A heat pump boiler uses 3 to 4 times less electricity for the same hot water, according to the Flemish government. A resistance element follows a changing surplus more closely.

A solar router (a diverter) reads the meter and sends only the surplus to the element, in small steps. That way the boiler never pulls extra power from the grid for it. A heat pump boiler is better programmed to run on the sunny hours; the Flemish government mentions roughly 11:00 to 15:00.

How does the control decide between the battery, the boiler and the grid?

With a priority list: direct use first, then the battery up to a set level, then heat, and only then the grid. The list can change with the season, the price and the weather.

In Wingene the rule is simple: when the battery is full, a resistor heats the boiler. The system there is open source, with room for further programming. The order can also run the other way. On a sunny summer day, the control can heat the water first in the morning, when the battery will be full by noon anyway.

Prices can change the order as well. With a dynamic contract, heating water during negative-price hours can pay better than storing power. Our article on a dynamic energy contract with a battery explains those hours. Where the grid operator limits your injection, heat absorbs what would otherwise be curtailed; see injection limits for business solar.

Our article on what an energy management system does for a business explains how the control measures, decides and steers.

How do you keep a surplus-heated boiler safe from legionella?

By making sure the water regularly reaches 60 °C, even in weeks without surplus. A tank heated only by leftover solar power can sit at 30 to 45 °C for days, which is the range where legionella grows.

Energie+, the Walloon reference site for building energy, gives the numbers. Legionella grows between 25 and 45 °C and is not detected when the water stays at 58 °C or more. At 60 °C, 90% of the bacteria die within 5 minutes. Energie+ recommends 60 °C at the outlet of the water heater and 55 °C in the distribution network.

In Flanders, publicly accessible places must draw up a legionella risk analysis and a control plan, according to the Departement Zorg. That concerns campsites, hotels, sports centres and care settings, among others. There, the surplus control must never override the legionella plan.

A common setup uses two stages. A preheat tank takes the surplus at whatever temperature it reaches. The water then passes a second heater that always brings it to 60 °C.

Can a boiler that switches on by itself raise your capacity tariff?

Yes, if it keeps drawing power from the grid when a cloud passes or the machines start. In Flanders, one quarter-hour with a boiler, an oven and a compressor together can set the peak for the whole month.

That is why a simple timer or an on/off relay on "surplus" is risky on a business site. A proportional controller only uses what is really left over. An energy management system can also block grid-fed heating during your busiest hours. The Flemish capacity tariff counts the highest quarter-hour of each month. Our article on how the capacity tariff for businesses is calculated shows the effect on the bill.

When is power-to-heat not worth it for your company?

When there is little heat demand to fill, or when the surplus is small or rare.

A tank only works as a battery if you empty it. An office with a few taps, or a warehouse with no hot water, will have a full tank by Tuesday. After that, the surplus goes to the grid anyway.

Winter is the other limit. Many business sites have little or no surplus from November to February. A boiler stores for hours or days, not for months; neither it nor a battery carries summer sun into winter. And where hot water already comes from an efficient heat pump, a resistance element on surplus saves less than the numbers suggest.

Do your panels still pay off without an outlet for the surplus? Our article on whether business solar panels still pay off does the sum.

What happens when you ask us to look at your surplus and your heat demand?

We start by measuring what you inject, when, and what heat you use at the same moment.

1. Step 1: Analysis and sizing. If data is missing, we monitor your consumption for a short period first, so the sizing of panels and battery is right.
2. Step 2: Technical design. We decide what goes first: the battery, the heat, or both, and in which order.
3. Step 3: Installation and commissioning, with the control rules for your site.
4. Step 4: Monitoring and support. You follow consumption, yield and battery level in real time in our monitoring software.

Our battery systems for businesses range from small SME installations to modular systems up to 1 MW, with monitoring, control and optimisation included.