There is no standard size. Take two numbers from your own meter. The first is what you use between sunset and sunrise on an ordinary day. The second is the highest power you need at once. The first gives the kWh, the second the kVA. For a large home battery in an all-electric house, our larger projects have 30.8 kWh, behind 5 kVA or 9 kVA.
Your house runs on electricity only: a heat pump, one or two electric cars, maybe a pool or a sauna. You want a battery that really carries the house, not a gadget. So the question is simple: how big? An owner with a heat pump said it plainly on the Victron community forum: his 32 kWh could have been 45 to 50 kWh.
But a bigger number is not always the better answer. A battery that is too big stays half empty in winter and half full in summer. A battery with too little power hands the evening peak back to the grid the moment the car and the heat pump run together. Both mistakes cost money every day, and the second is harder to fix later.
In Kampenhout a 30.8 kWh battery tower now powers the house. Own consumption clearly went up and injection into the grid keeps going down.
Why does a large home need two sizes: kWh and kVA?
Capacity (kWh) is the size of the tank; power (kVA) is the size of the tap. A large home needs both, and our projects show they are chosen separately.
| Project | Power | Capacity | Why this combination |
|---|---|---|---|
| Bonheiden | 5 kVA | 15.4 kWh | About 20 kWh use per day: the classic home battery |
| Kampenhout | 5 kVA | 30.8 kWh | More storage for more own use and less injection |
| Bierbeek | 5 kVA | 30.8 kWh | A large battery for a large use: no surplus wasted on sunny days |
| Hamme | 9 kVA, three-phase | 30.8 kWh | Nobody home by day, main use in the evening, the electric car charged from stored solar power |
| Kortenberg | 9 kVA, three-phase | 30.8 kWh | Full back-up in off-grid mode |
| Sint-Laureins | 30 kVA | 30 kWh | Large house with a pool and a garage: semi-off-grid |
The same 30.8 kWh can sit behind 5 kVA or 9 kVA. Capacity decides how many hours you run on your own power; power decides which appliances can run at the same moment. A 5 kVA inverter-charger such as the MultiPlus-II 48/5000 delivers 4,000 W continuously at 25 °C.
On a single-phase connection, 5 kVA of inverter power is the maximum, as in our project in Roosdaal. A home that needs more power at once goes three-phase. Read more in our article on a three-phase home battery for full back-up.
How do you size a home battery from your quarter-hour data?
Your digital meter already holds the answer. It measures your offtake and injection per quarter-hour, and those values show what a battery could have stored and used.
In Flanders you activate the reading of your quarter-hour values in Mijn Fluvius; history only starts from that moment. From 2026 Fluvius reads them by default. Take ordinary days from each season, not only the extremes.
From meter data to battery size
Why the smaller of the two? A battery only earns its place when it fills during the day and empties at night. Capacity above your daily surplus never fills; capacity above your night use never empties. For the power, Fluvius explains the conversion: a quarter-hour value in kWh times four gives the power in kW.
Fluvius also gives a rule of thumb: 1 to 1.5 kWh of battery per kWp of solar panels. It is a useful check on a quote, not a method. Your meter data is.
Tip
The meter only shows what goes in and out at the connection. A heat pump or car charger with its own measurement shows which load causes which peak.
What do the heat pump, the cars and the pool ask from the battery?
Each large load asks something different: energy at night, power at once, or simply a better time slot. Sizing starts by deciding which loads the battery must carry.
| Load | What it asks | Best source | What it means for sizing |
|---|---|---|---|
| Heat pump | Energy over many hours, most in winter; high power when it starts or defrosts | Sun by day, battery in the evening, grid on grey winter days | Mainly kWh in spring and autumn; enough kVA for the compressor |
| Electric car | A lot of energy in a short time; a three-phase 16 A charger draws about 11 kW | The sun at midday, or slow charging at quiet hours | Charging from the battery quickly empties it; decide how much of the car the battery serves |
| Pool pump and heating | Long running hours, mostly in summer | The sun, during the day | Little extra capacity if it runs on the sun; power if it runs at night |
| Sauna, oven, induction | Short, high power | Battery or grid, at that moment | Mainly kVA: the peak lasts one or two quarter-hours |
The heat pump is the load that makes large homes large. An owner in the Netherlands has a 45 kWh battery. On the Victron community forum he wrote that his use "easily reaches 40 kWh a day on cold days". Another owner saw his heat pump use 10 kWh in one day just to defrost. In winter a heat pump can empty any home battery. We explain how to run one on a battery in our article on the heat pump and home battery.
The car is the load you can steer most. Charged at midday on your own sun, it barely touches the battery. Charged every evening from the battery, it takes a large part of it. Read how to combine a charger and a battery in charging your car with a home battery.
The pool asks long running hours on the midday sun; the sauna asks a short, high power. Both are covered in a pool, sauna or pool house on a home battery.
An ordinary spring day in a large all-electric home (illustrative)
Why can a bigger battery not carry you through winter?
Because in winter the sun cannot fill it. A battery moves energy from day to night; it does not move energy from summer to winter.
What 10 kWp of solar panels in Flanders yields per day, month by month
In June, 10 kWp gives about 42 kWh a day: more than a large home can use or store. In December it gives about 11 kWh. A heat pump house that needs 40 kWh on a cold day then gets a quarter of it from the roof. No battery size changes that.
So size for the months when the battery fills and empties every day: spring, summer and autumn. In winter a large battery still earns its place in two ways. It carries the house in a power cut, and it can cover the evening peak. With a dynamic contract, it can also charge at cheap hours.
How big should the battery be for back-up in a power cut?
Back-up is sized in hours, not in percentages. Decide which circuits must keep running, and for how long; then reserve that energy in the battery.
A battery that is used every day for your own solar power is never full at the moment the grid fails. That is why the back-up reserve comes on top of the daily part. Keep the heat pump off the back-up in winter, or the reserve has to be very large.
| Back-up choice | Power (kVA) needed for | Capacity (kWh) needed for |
|---|---|---|
| Essentials only (fridge, lighting, network, heating controls) | Small loads at once | A few kWh per evening and night |
| Whole house, heat pump excluded | Cooking and household appliances at once | Your normal evening and night use |
| Whole house including heat pump in winter | Compressor start and defrost on top of the house | Can reach the full daily use of a cold day |
In Wetteren the whole house runs on the battery in a power cut, on 9 kVA three-phase and 15.4 kWh. In Kortenberg the same full back-up has 30.8 kWh. How to choose the circuits is in our guide to a home battery with backup power.
Does a larger battery lower your capacity tariff more?
Not by itself: the capacity tariff is about power, so the kVA and the control matter more than the kWh. This applies in Flanders.
The grid operator measures your offtake every 15 minutes. Your monthly peak is the highest quarter-hour average of the month. Everyone pays at least for 2.5 kW, and the average monthly peak of a Flemish household is 4.24 kW. Your bill uses the average of your last twelve monthly peaks.
According to Fluvius, electric cars and large heat generators such as heat pumps are the heaviest users. When the car charges and the heat pump starts, the battery can only shave the peak up to its own power. A 30 kWh battery behind 5 kVA shaves no more than a 15 kWh battery behind 5 kVA. It only shaves longer.
Too big or too small: what goes wrong?
An oversized battery costs money without working; an undersized one hands your evenings and peaks back to the grid. The symptoms show up in your monitoring within weeks.
| Sign in the monitoring | What it means | What helps |
|---|---|---|
| The battery is rarely full, even in summer | Capacity above your daily surplus | More panels, or no extra modules |
| The battery is rarely empty in the morning | Capacity above your night use | Shift loads to the evening, or stop adding modules |
| Empty by 22:00 on ordinary days | Capacity below your night use | Add modules, if the system allows it |
| Grid offtake while the battery still has energy | Power (kVA) below your loads at that moment | Spread the loads, or add inverter power |
Capacity is the easier one to correct: the Battery-Box we use is modular and expandable. Power is harder, because it means more inverter capacity and sometimes a three-phase set-up. So get the kVA right on day one, and let the kWh grow with your data.
When is a large home better off going semi-off-grid?
When the house needs more power than a home battery set-up normally gives, or when the connection itself is the limit. Then the question is no longer "how big" but "which system".
In Sint-Laureins a large house was extended with a pool and a garage, and there were two connections. The system has 30 kVA and 30 kWh, with smart control. The battery only charges from the grid when the sun does not shine and it runs empty. The goal remains to keep going even without the grid.
That is the step from a home battery to a semi-off-grid system. If your site has two connections, read whether one battery can combine them.
Not sure which size fits your house? Send us your quarter-hour data and your plans for car, heat pump and pool.
How do we size a large home battery?
We measure first, then choose the power, then the capacity. Our team has three certified battery specialists, and we control the whole cycle from the first measurements.
- Step 1: We read your quarter-hour data and ask what is coming: a second car, a heat pump, a pool or a home office.
- Step 2: We choose the power: single-phase up to 5 kVA or three-phase, and which loads the battery must carry at once.
- Step 3: We choose the capacity from your ordinary days, and add the back-up reserve for the circuits you choose.
- Step 4: We install, commission and follow the system with you. You see every kWh in the free VRM app and portal, and add modules when your data says so.
Where will a large battery stand? Read whether a large home battery can stand outside. Or see what our home batteries can do for a large home.
Frequently asked questions
Is 30 kWh too big for a home?
Not for a large all-electric home with enough solar panels and a high evening use. It is too big when your ordinary days never fill or never empty it. Your quarter-hour data shows which case you are in.
Can I start smaller and add capacity later?
Yes, with a modular battery you add modules later. Plan the inverter power and the space for the final size from the start.
How many solar panels do I need to fill a large battery?
Fluvius uses 1 to 1.5 kWh of battery per kWp as a rule of thumb. What counts is your real daily surplus: the injection on ordinary days in your meter data.
Do I need three-phase for a large battery?
Not for the capacity, but often for the power. On single phase 5 kVA is the maximum; a house with a heat pump, cars and a pool usually needs more at once.
Want to know which size fits your house?
Send us your quarter-hour data from Mijn Fluvius, or a few monthly peaks. Tell us what you plan: a second car, a heat pump, a pool. One of our battery specialists will tell you which power and which capacity fit, and what can grow later.