The House Has a 22 kW EV Charger. The Battery Does Not Need to Be 22 kW.
A three-phase Home ESS should be sized from the loads that actually overlap, not by adding every appliance nameplate rating. For a home with an EV charger and heat pump, the design should establish the grid-import limit, phase configuration, controllable EV charging power, heat-pump demand, battery/inverter power, usable battery energy, solar contribution, and backup scope. EV charging is usually the most flexible large load; heating and essential loads usually receive higher priority.
This worked example uses illustrative numbers rather than representing a specific Ruibit/Dawnice customer project.
The objective is to see how the design decisions interact.
The House
Assume a European-style three-phase home with:
Grid supply:
3 × 25 A
Rooftop PV:
12 kWp
EV charger:
11 kW, three-phase
Heat pump:
3.5 kW typical electrical input
Other household loads:
1.5 kW typical evening load
Proposed battery:
20 kWh class
Home ESS inverter:
three-phase
The theoretical three-phase grid capacity at 400 V is approximately:
√3 × 400 × 25 ≈ 17.3 kW
That number immediately matters.
If the EV charger consumes 11 kW while the heat pump and household simultaneously require another 5 kW, the house is already close to the connection limit before another major appliance starts.
The battery can help.
But load control may be cheaper than designing the battery to solve every possible coincidence.
18:10 — The Car Plugs In
The homeowner arrives home.
At that moment:
EV charger requests: 11.0 kW
Heat pump: 3.5 kW
Household loads: 1.5 kW
Total:
16.0 kW
The battery does not need to supply 16 kW.
The grid is still available.
Suppose the energy-management strategy limits grid import to:
12 kW
Required battery contribution becomes:
16 − 12 = 4 kW
A suitable Home ESS could discharge approximately 4 kW to hold the grid near the selected limit.
But there is another option.
Reduce EV charging from 11 kW to 7 kW.
Now:
7 + 3.5 + 1.5 = 12 kW
Battery discharge required:
0 kW
This is why I would integrate EV load management before simply buying a larger battery inverter.
The EV may remain connected for eight hours.
It usually does not need maximum charging power every minute.
19:30 — Cooking Changes the Equation
Now add:
Induction cooking: 3 kW
If the EV is still charging at 7 kW:
EV: 7.0 kW
Heat pump: 3.5 kW
Cooking: 3.0 kW
Other loads: 1.5 kW
Total:
15 kW
The EMS has choices.
It could discharge the battery by 3 kW.
Or reduce EV charging by another 3 kW.
Or combine both.
For this house, I would probably give load priority roughly as:
Essential household loads
→ Heat pump
→ Cooking
→ EV charging
The EV becomes the adjustable load.
That prevents the Home ESS from wasting expensive battery cycles simply because the car could not wait twenty minutes at full charging power.
Now Size the Battery Inverter
The battery still needs meaningful power capability.
Suppose the design objectives are:
limit normal grid import
increase solar self-consumption
shift evening energy
provide selected backup
After modelling the load profile, assume we determine that 8–10 kW of three-phase battery-inverter power handles the useful battery duties without trying to independently supply every connected appliance.
That does not mean any 10 kW inverter is suitable.
I would verify:
three-phase output architecture
per-phase limitations
battery voltage window
battery charge/discharge current
PV input limits
grid requirements
backup/EPS behavior
BMS communication
and how the EMS coordinates the EV charger.
For a Ruibit/Dawnice Home ESS quotation, the battery, inverter, meter/CT and controllable loads should therefore be treated as one operating system.
How Much Battery Energy?
Now move from kW to kWh.
Assume the battery's main daily job is to cover evening household and heating demand after solar production falls.
From 17:00 to 23:00, suppose the home consumes:
Heat pump: 9 kWh
Other household loads: 6 kWh
EV energy supplied from battery: 2 kWh
Total battery-supported demand:
17 kWh
A 20 kWh-class battery now looks reasonable.
But I would not assume all 20 kWh reaches the AC loads.
Usable SoC range, inverter losses, battery limits and backup reserve reduce the energy available for everyday dispatch.
If the homeowner reserves 20% for outages, the EMS should not spend that same energy on EV charging just because the evening tariff is expensive.
Midday Solar Changes Tomorrow's EV Energy
The next day, the 12 kWp PV array produces strong midday surplus.
Instead of exporting everything, the battery charges.
If the EV is parked at home, another option appears:
PV → EV directly
This can be more efficient than:
PV → battery → EV later
when the vehicle's schedule allows it.
So the control hierarchy should consider:
House loads first
Heat-pump demand
Battery charging / reserve target
Flexible EV charging
Grid export
The exact priority depends on tariffs and homeowner objectives.
There is no universal sequence.
What Happens During a Grid Outage?
This is where I would deliberately change the rules.
The homeowner may want:
refrigeration
lighting
internet
heat pump
selected sockets
but not:
11 kW EV charging
electric oven
other discretionary high-power loads
That means the backup load might be only:
4–6 kW
rather than the 15–20 kW the house can occasionally demand during normal operation.
This distinction can dramatically reduce the inverter and battery capacity required for useful resilience.
If EV charging during outages is genuinely required, it should be specified explicitly and the system sized accordingly.
Do not let "whole-home backup" accidentally become "every load operates normally forever."
The Final Design Is a Control Strategy
Our preliminary system might therefore look like:
| Design Element | Worked Example |
|---|---|
| Grid | Three-phase, ~17.3 kW theoretical connection capacity |
| PV | 12 kWp |
| EV charger | 11 kW, dynamically controlled |
| Heat pump | ~3.5 kW typical input |
| Home ESS inverter | 8–10 kW class, three-phase |
| Battery | ~20 kWh class |
| Grid-import target | 12 kW example |
| Backup | Selected essential loads + heat pump |
| EV during outage | Disabled |
| Control priority | Essential loads / heating before EV |
These are worked assumptions, not a universal product recommendation.
A different household may need 15 kWh.
Another may need 30 kWh.
A larger heat pump, 22 kW charger, three EVs, electric resistance heating, different grid connection, or different tariff can change the design completely.
The important lesson is the sizing sequence:
Grid constraint → load coincidence → flexible loads → inverter kW → battery kWh → backup boundary → control strategy
not:
11 kW EV charger + 8 kW heat pump = buy a 19 kW battery inverter.
In a three-phase home, the best battery design often comes from deciding which loads do not need to run at full power at the same time .
That is especially true for EV charging.
Use the Home ESS for energy that needs to move through time. Use load management for power that can simply wait.
FAQs
1. Does an 11kW EV charger require an 11kW home battery inverter?
Not necessarily. EV charging is usually a flexible load. Dynamic load management can reduce charging power when the heat pump, cooking equipment, or other household loads are operating, reducing the battery power required.
2. How should a three-phase Home ESS be sized for an EV charger and heat pump?
Start with the grid connection limit and simultaneous household loads , then identify which loads can be controlled. Size inverter kW for the useful peak battery contribution and battery kWh for the required energy shifting and backup duration.
3. Should an EV charger operate from the home battery during a grid outage?
Usually it is more practical to prioritize essential household loads and heating while disabling or limiting EV charging. If EV charging during outages is required, the battery and inverter must be specifically sized for that additional load.
4. Why is dynamic EV charging useful with a Home ESS?
Dynamic charging can reduce EV power when household demand rises and increase it when capacity becomes available. This can prevent grid-limit violations and avoid unnecessary battery discharge or inverter oversizing.
