A home battery carries a pool well and a sauna poorly. The pool pump and the pool heat pump are steady loads that run best on the midday sun. The sauna heater asks more power at once than most inverters give. A home battery for a house with a pool is sized on power first, with smart control that decides what runs when.
You have a pool, maybe a sauna or a hot tub, perhaps a pool house at the back of the garden. The solar panels produce plenty in summer, and you wonder if a battery can run it all. A Belgian owner put the question plainly on a solar forum: a pool of 8 by 4 metres, fully off-grid from April to September. How many panels, and a battery of how many kWh?
The trap is that kWh is the wrong first question. One sauna evening can ask more power than the battery can give, and one warm-up can set your peak for the whole month. The pool, meanwhile, can store more solar heat than any battery. Get the order wrong and the battery is empty before the sauna is warm.
In Sint-Laureins a large house was extended with a pool and a garage. The system has 30 kVA and 30 kWh, and smart controls make sure all the energy is used usefully.
How much power do a pool, a sauna and a hot tub ask?
Each wellness load asks something different: the pool asks energy over many hours, the sauna asks a lot of power at once. That difference decides how a battery can help.
| Load | Power at once | How long it runs | What it means for a battery |
|---|---|---|---|
| Sauna heater (home) | 6.6 kW for 4 to 8 m³, 8 kW for 6 to 12 m³, 10.5 kW for 10 to 18 m³ (Tylö) | Warm-up of 25 to 60 minutes at full power, then on and off (Küng) | Mainly power (kVA): too much for one small inverter |
| Steam cabin (home) | 8.5 to 12.5 kW (Küng) | About an hour for three sessions | Same as a sauna: power first |
| Infrared cabin | About 1.5 to 2 kWh per hour of use (Tylö) | Short sessions, no long warm-up | Easy for a home battery |
| Hot tub | 1,700 to 2,500 kWh a year for a well-insulated spa (Küng) | Keeps its water warm all year | Steady energy; steer the heating to the sun |
| Pool heat pump | About 1 kWh of electricity gives 4 to 5 kWh of heat (Daikin) | Many hours a day in spring and summer | Energy over hours: run it on the sun |
| Pool filtration pump | Small compared to heating | Hours every day in season | Easy; the one pool load worth keeping on back-up |
Tylö, a sauna maker, gives a rule of thumb: about 1 kW of heater per 1.3 m³ of sauna room. Küng, a Swiss sauna builder, puts an average home sauna for two to four people at about 9 kW. That is more than the whole evening use of many houses, drawn in one block.
A Belgian owner heats his pool and hot tub with heat pumps. On Tweakers he wrote that his house drew about 4 kW continuously, with peaks of 2 to 4 kW on top. His yearly use was 36,500 to 40,000 kWh.
Why is power (kW) the real question for a sauna?
Because a battery delivers energy only as fast as its inverter allows. The kWh on the label say how long; the kVA of the inverter say how much at once.
A Dutch owner asked it on Reddit. His battery is 20 kWh: can it run a 10 kW sauna heater for 2 hours when it is full? On paper, yes: 10 kW for 2 hours is 20 kWh. In practice, no. A 5 kVA inverter-charger such as the Victron MultiPlus-II 48/5000 delivers 4,000 W continuously at 25 °C, with a peak of 9,000 W. A peak lasts seconds; a sauna heater runs for the better part of an hour.
Sauna heater power next to what one 5 kVA inverter delivers continuously
So on a grid-connected house, the sauna simply runs on the grid, and the battery adds what it can. Victron calls that PowerAssist: the battery helps when the grid or a generator is not enough, and recharges when the load drops. Three inverters of the same model can form a three-phase set, and up to six can run in parallel for more power.
Larger home saunas often need a 400 V supply. Küng lists one 400 V/16 A line for its outdoor sauna and two for the XL version. Read whether you need a three-phase home battery before you plan the sauna on it.
Tip
The warm-up is the heavy part. Küng puts the preheating at 25 to 60 minutes. An 8 kW heater running flat out for that time uses about 3 to 8 kWh before anyone sits in it.
Why is the pool a better heat store than the battery?
Because water holds a lot of heat, and a heat pump turns one kWh of electricity into four or five kWh of heat. Your pool is a large thermal battery you already own.
Take the Belgian forum pool: 8 by 4 metres and 1.5 metres deep is 48 m³ of water. Warming it by one degree takes about 56 kWh of heat. With a heat pump that gives 4 to 5 kWh of heat per kWh, that is 11 to 14 kWh of electricity per degree.
Now compare it to the battery. A full 30 kWh battery, spent on the pool heat pump, lifts that pool by only 2 to 3 degrees. The same battery carries a large house through its evening and night. So the battery is for the house at night; the pool takes the surplus by day.
Owners who do this well follow one order. A Belgian owner described it on Reddit: first the battery fills to get through the night, then the pool takes its turn. He heats to 28 degrees as standard and to 30 on very sunny days. After a cloudy day, the water is still at 29, so no energy is needed.
Where the solar surplus goes in a house with a pool
We already apply that logic on other sites. In Wingene a boiler is heated through a resistor once the battery is full: water is used as a battery. On a farm, the same control steers pumps and boilers when there is surplus. On a dynamic contract, the battery itself can also follow quarter-hour prices, as explained in Victron Dynamic ESS on a home battery.
Watch out
A pool cover keeps the stored heat in. Without one, a warm pool loses heat quickly at night and the extra degrees are gone by morning.
How does a summer day run in a house with a pool, a sauna and a battery?
The sun carries the pool, the battery carries the evening, and the sauna is the one load the sun never sees. That is the pattern to plan for.
A sunny summer day in a house with a pool and a sauna (illustrative)
A sauna at 19:00 is the worst moment for the battery. The house is cooking, the sun is gone, and the heater wants its full power. Warming the sauna in the late afternoon, on the last sun, moves the heaviest block to where it hurts least.
In spring and autumn the picture changes. The pool asks more heat and the sun gives less. How big the battery should be for the whole house is in our article on sizing the battery for a large home.
What does a sauna or pool do to your capacity tariff?
One sauna warm-up a month is enough to set your monthly peak in Flanders. The capacity tariff counts your highest quarter-hour of the month, not how often you reach it.
The grid operator measures your offtake every 15 minutes. Everyone pays at least for 2.5 kW, and your bill uses the average of your last twelve monthly peaks. In 2026 one kW of peak costs on average about 53 euro a year, excluding VAT, according to the Vlaamse Nutsregulator.
An owner described his evenings on Reddit. Pool jet stream on, dinner on the stove: a peak of 4,000 to 5,000 W for half an hour. Add a dryer or an air conditioner and it reaches 7,000 to 8,000 W. A sauna heater on top of that pushes the peak higher still.
| What you do | Effect on the monthly peak |
|---|---|
| Sauna warm-up while cooking | Heater and kitchen add up in the same quarter-hours |
| Sauna warm-up in the late afternoon | The heater runs alone; solar can cover part of it |
| Battery with enough inverter power | The battery shaves the peak up to its own power |
| Pool heat pump steered to the midday sun | Little or no effect on the grid peak |
A battery shaves the peak only up to its inverter power. How much that saves for an all-electric house is in our article on the home battery and the capacity tariff.
What should keep running in a power cut: pool, sauna or neither?
Neither the sauna nor the pool heating belongs on the back-up. In a power cut, the battery keeps the house going: fridge, heating controls, lighting, network, and at most the filtration pump.
Our home battery systems can connect your main systems separately to the battery. When the grid fails, the inverter takes over within 20 milliseconds. Which circuits you put behind it decides how long the reserve lasts. A sauna heater would empty it in a few hours and exceed the inverter power.
| Load | On the back-up? | Why |
|---|---|---|
| Pool filtration pump | Optional | Small load; useful in a long outage in summer |
| Pool heat pump | No | Heating can wait; the water keeps its heat for hours |
| Sauna or steam heater | No | Too much power at once for the back-up |
| Hot tub heater | No | Keeps warm for a while; restart after the outage |
| Pool house lighting, alarm, network | Yes, if needed | Small loads, part of the house security |
The Victron MultiPlus-II has a second AC output that is live only when the grid or a generator is present. It is meant for loads that should not drain the battery, such as a water heater. A pool heater or a hot tub on that output switches off by itself in a power cut. A Victron user on Reddit does it by hand: in an outage he stops the spa pool and the EV charging.
How a back-up board is wired is in our article on the full no-break house. Which pumps must keep running is in pumps during a power cut. How to choose the circuits is in our guide to a home battery with backup power.
A pool house or wellness building: from the house or on its own?
A pool house is an outbuilding with heavy loads. It needs a feed from the main board sized for the heat pump, the sauna and the pumps, or its own connection.
Most pool houses are fed from the house board, with a sub-board in the pool house. The cable and the protection must carry the sauna and the heat pump together. Put the solar panels of the pool house roof on the same system as the house battery, or they inject while the battery waits.
Sometimes the site has two connections, as in Sint-Laureins. Then the question is whether one battery can combine them. For panels on a barn, stable or pool house, read whether to connect them to the house or build a separate system.
Our verdict
Feed the pool house from the main system when you can. One battery, one control and one monitoring for the whole site keep the surplus where it is useful.
When does a house with a pool and sauna go semi-off-grid?
When the loads together ask more power than a home battery set-up normally gives, or when the grid connection is the limit. Then you design a system, not a battery.
In Sint-Laureins the house has 30 kVA of power and 30 kWh of storage. The battery only charges from the grid when the sun does not shine and it runs empty. That can happen at market price, but the goal stays the same: keep going even without the grid.
That is the step from a home battery to a semi-off-grid system. A heat pump for the house adds its own winter load: read how a heat pump runs on a home battery.
Pool, sauna or pool house in the plans? Send us your quarter-hour data and your list of loads.
How do we design a battery system for a house with a pool and sauna?
We start from the loads and the moment they run, then choose the power, then the storage. Our team has three certified battery experts, and we control the whole cycle from the first measurements.
- Step 1: We list every wellness load with its power: sauna or steam heater, pool heat pump, filtration pump, hot tub.
- Step 2: We read your quarter-hour data, or measure first, to see when the peaks fall.
- Step 3: We choose the inverter power and the phases, and decide which loads the battery carries and which stay on the grid.
- Step 4: We set the control order: house, battery, pool, then the grid. The back-up board gets only the circuits you choose.
- Step 5: We follow the system with you. You see every kWh in the free VRM app and portal, and adjust the pool settings to the season.
In Kortenberg the house runs on three-phase batteries with full back-up in off-grid mode: 9 kVA and 30.8 kWh. See what our home batteries for large homes can do.
Frequently asked questions
Can a home battery heat my pool?
It can, but it is rarely the best use. The pool heat pump runs best directly on the midday sun, once the battery is full. The battery is more useful for the house in the evening.
Can I use the sauna during a power cut?
Usually not. A home sauna heater draws 6.6 kW or more, above what a typical back-up inverter delivers continuously. Keep the sauna off the back-up board.
Do I need three-phase for a sauna?
Often, for the larger heaters. Küng lists a 400 V/16 A line for its outdoor sauna and two for the XL version. Check the connection data of your heater before you plan it with a battery.
Is an infrared cabin easier on a battery?
Yes. Tylö puts its infrared room at about 1.5 to 2 kWh per hour of use, without a long warm-up at full power. That fits a home battery much better than a sauna heater.
Does a pool cover matter for the battery?
Indirectly. A cover keeps the heat you stored with solar power in the water, so the heat pump needs fewer hours and less surplus.
Planning a pool, sauna or pool house?
Send us your quarter-hour data from Mijn Fluvius, or tell us which loads are coming: sauna, pool heat pump, hot tub, pool house. One of our battery experts will tell you which power fits, what the battery should carry and what the control should steer.