Which home pumps must keep running in a power cut?

Keep every pump running that protects the house or keeps it usable. That means the sump pump, the sewage lift pump, the well pump and the aeration of a treatment unit. The rainwater pump and the heating pumps come next. A house backup battery on a separate back-up board carries them all automatically, as long as it can deliver their start-up current.

Your cellar or garage sits below the street. At the bottom of the ramp is a drain, and in a pit in the cellar a sump pump pushes the rainwater out. "What if the power goes out during a thunderstorm?" a Belgian owner asked on a building forum. His fear: "the cellar will one day be seriously under water". Or you live in the countryside on your own well, with a treatment unit in the garden. An owner on an American forum wanted his well and septic pumps on back-up. A neighbour had lost power "for over a week in the winter".

What most advice leaves out is that the risk is not the average power cut, but the one during a storm. Heavy rain, wind and lightning take out power lines, and fill your pump pits at the same time. A pump that stops for two hours on a dry day is an annoyance. During a cloudburst it can mean a flooded cellar, drains backing up, and a claim to your insurer.

In a house in Sint-Niklaas the whole home runs on full back-up. When the street loses power, the house keeps running on its solar panels and battery.

Which pumps in a house stop when the power goes out?

All of them, unless they are on a back-up supply. What differs is how fast the stop turns into damage.

Heating is a subject on its own: read whether a home battery can carry the heat pump in winter.

Why is a sump pump in the cellar the most urgent one?

Because a power cut and a flood often have the same cause: the storm. And water in a cellar or garage does damage in hours, not days.

A sump pump sits in a pit in the cellar, the garage or the crawl space. A float switches it on when the water rises. Often it also empties the drain at the bottom of a steep garage ramp. When the power drops, the float still rises, but nothing happens.

Forum owners describe three ways out:

• A larger pit. One owner dug a pit of 2,000 litres under the cellar. With the pump hanging higher, about 1,500 litres of buffer stays free for a power cut. It buys time, not safety.
• A second pump on its own battery. Its float sits above the main pump, so it only runs when the main pump fails or cannot keep up. Pump makers sell such battery units for single-phase 230 V pumps. It protects one pit, for as long as its battery lasts.
• A generator. It works when you are at home and someone starts it. At night or on holiday, the cellar fills anyway, unless the start is automatic.

A home battery with a back-up circuit does what none of these do. It takes over automatically, day and night, for every pump on the back-up board. More on that below.

Sewage lift pump: what happens to your toilets without power?

A house whose drains lie below the sewer needs a lift pump to push the waste water up. Without power, the pit fills and then the water has nowhere to go.

Lift pumps are common in cellars with a toilet or shower. You also find them in houses lower than the street or far from a sewer. The pit holds a reserve. How long it lasts depends on its volume and on how much water the family uses. Showers, the dishwasher and the washing machine fill it much faster than a toilet alone. Many lift stations can get a flood alarm as an accessory, so you hear it before it overflows.

Rainwater pump with automatic switch-over: why does it stop in a power cut?

Because the switch-over to tap water is done by valves in the pump unit, and they run on mains power too. When the power fails, the toilets and the washing machine get no water, whether the tank is full or empty.

In Flanders, the rainwater regulation says collected rainwater must be used as much as possible for toilet flushing, cleaning, the washing machine and outdoor use. So many houses have a tank with a pump. The automatic switch-over solves one problem: an empty tank in a dry summer. Solenoid valves then feed tap water into the same pipes, and the unit itself runs on 230 V. It does not solve a power cut, because both the pump and the switch-over valve stop.

Two ways keep water flowing. A manual bypass valve lets tap water reach the rainwater pipes without the pump, if the plumbing allows it. Or you put the rainwater pump on the back-up board. Its running power is modest, but check its start-up current.

Well pump or borehole pump: how do you keep water in the taps?

If your well is your only water supply, the well pump is as critical as the fridge. And it is often the hardest load in the house to start.

A borehole pump hangs deep in the well and pushes water up to a pressure tank. In a power cut, the taps run until the pressure tank is empty. Then nothing comes out, and a toilet flush becomes a bucket job.

"Well pumps easily draw more than 1 kW," an experienced member of a Dutch forum warned an owner planning a battery system. A pump maker's catalogue puts the start-up current of a motor at 4 to 8 times its rated current. A back-up system that runs the pump easily can still fail to start it.

Septic tank, IBA or micro-station: what happens without power?

A classic septic tank or a reed bed works without electricity; a treatment unit with aeration does not. Its blower pushes air to the bacteria day and night.

Houses that cannot connect to a sewer treat their waste water on site. In Flanders that is an IBA (individual treatment unit), in Wallonia an individual treatment system. "IBAs need a power supply," the Flemish water company Farys writes. Many of these units use a compressor that blows fine air bubbles into the water. Without that air, the bacteria that clean the water slowly die, and the unit lets out poorly treated water.

Manufacturers therefore tell owners never to switch off the circuit of the unit, not even on holiday. After a long stop, the bacteria must first grow back before the treatment is normal again. The power use is small: the manuals quoted by a French energy site give 0.46 to 1.9 kWh per day, depending on the model. So a treatment unit costs a back-up battery little energy, but it must never be forgotten on the back-up board.

Which pumps go on the back-up board, and how big must the back-up be?

Put every pump from the "high" rows on the back-up board, plus the fridge, lighting, the internet and the heating controls. Then size the inverter for the largest start-up current, not only for the kWh.

We can connect your main systems and equipment separately to the battery, on a back-up board. Or the whole house runs on the battery, as in Wetteren, where everything keeps working on three-phase when the grid fails. Three-phase pumps need a three-phase back-up: read whether you need a three-phase home battery for full back-up.

For the start-up current, the inverter matters more than the battery. A Victron MultiPlus-II 48/5000, for example, gives 4,000 W continuous and 9,000 W as peak power. Two or three pumps that start at the same moment can need more than one inverter can give. A good design staggers them or adds an inverter.

Home battery, battery back-up pump or generator: what fits a large home?

A back-up pump protects one pit, a generator needs someone to start it and fuel, and a home battery takes over every critical circuit automatically. Many large homes combine a battery and a generator.

In a hunting lodge in the woods at Gedinne, the grid was weak and outages were a risk. We installed a semi-off-grid battery system of 46.2 kWh on 3 x 400 V, with a generator coupled in case of a power cut.

How do we put the pumps of a large home on back-up?

We start from your pumps and your worst storm, not from a battery size. Our team has three certified battery specialists.

1. Step 1: We list every pump, its type plate and how critical it is: sump, lift, well, treatment unit, rainwater, heating.
2. Step 2: We measure your consumption and decide: a separate back-up board, or the whole house on the battery.
3. Step 3: We size the inverter for the start-up currents and the battery for the hours of back-up you want. Where cuts can last days, we add a generator input.
4. Step 4: We install and commission the system, and follow it with you. In the free VRM app and portal you see the battery at work, also during a power cut.

Choosing the place for a large battery? Read whether a large home battery can stand outside. Or see what our home batteries do in a power cut.