The Battery Does Not Simply Charge by Day and Discharge at Night
In a typical grid-connected solar-plus-battery home, solar can supply household loads first, surplus PV can charge the battery, and additional surplus can be exported. When solar production falls below household demand, the battery can discharge until it reaches its configured reserve or another operating limit; the grid then supplies the balance. The exact sequence depends on inverter/EMS settings, battery limits, tariffs, backup reserve, and the actual load profile.
A 24-hour example makes this easier to understand than an energy-flow diagram alone.
The figures below are illustrative assumptions , not measured Ruibit/Dawnice project data.
06:30 — The House Wakes Before the Solar Array
Assume the house is consuming:
House load: 1.4 kW
PV output: 0.2 kW
The solar array supplies part of the load, leaving:
1.4 − 0.2 = 1.2 kW
If the battery has sufficient SoC and its operating strategy permits discharge, it can supply approximately that remaining power.
But suppose the battery has already reached its configured backup reserve.
Then the grid supplies the shortfall instead.
This illustrates an important point: having energy in the battery does not mean all of it is available for daily self-consumption.
A homeowner may deliberately keep part of the battery unused for outages.
09:00 — Solar Catches the Household Load
PV production rises to:
2.5 kW
The house is using:
2.0 kW
There is now approximately:
0.5 kW surplus
If the battery can accept charge, that surplus can flow into storage.
The simplified path becomes:
PV → House
and:
Surplus PV → Battery
At this point the home may need almost no grid import.
But the battery's kWh rating does not tell us how quickly it can charge. The inverter charge limit, battery current limit, BMS, SoC, and temperature can all constrain charging power.
12:30 — Solar Production Is Much Higher Than the Load
Now assume:
PV = 6.0 kW
House load = 1.5 kW
Potential surplus:
6.0 − 1.5 = 4.5 kW
Suppose the battery can currently accept only:
3.0 kW
The remaining:
4.5 − 3.0 = 1.5 kW
needs another destination.
Depending on the installation and control settings, it may be exported to the grid, used by a flexible household load, or curtailed.
So this statement is too simple:
A 10 kWh battery stores excess solar.
The more useful question is:
Can the battery accept the available solar power when that surplus actually occurs?
A battery with plenty of empty kWh can still miss solar energy if its permitted charging kW is too low.
14:30 — The Battery Is Full
Suppose the battery reaches its configured upper SoC.
PV production remains:
5.0 kW
House demand is:
1.8 kW
The battery no longer needs charging.
Approximately 3.2 kW of surplus PV remains.
If grid export is permitted, that energy may be exported.
This is where tariff structure matters.
A homeowner receiving attractive export compensation may operate the system differently from someone whose exported electricity has little value.
The inverter/EMS strategy should therefore be based on the actual tariff rather than a generic rule that maximizing battery charging is always best.
17:30 — Solar Falls, but Household Demand Rises
Now the pattern reverses.
Assume:
PV = 0.8 kW
House load = 3.5 kW
The shortfall is:
3.5 − 0.8 = 2.7 kW
If battery SoC and discharge limits allow it, the battery can supply approximately 2.7 kW and reduce or eliminate grid import.
This is the part of the day when solar self-consumption becomes easy to see.
Energy generated around midday has effectively been moved into the evening.
But the battery must satisfy both sides of that transaction:
enough kWh to store the energy
and
enough kW to supply the evening load .
20:30 — A Large Load Starts
The house is drawing 2 kW when an appliance adds another 4 kW.
Total demand becomes:
6 kW
Suppose the battery inverter can deliver only:
5 kW
Even with plenty of stored energy, the battery cannot independently supply the full 6 kW load at that moment.
The grid may provide the remaining:
1 kW
This is why:
battery kWh ≠ inverter kW
A system can have enough energy for several hours while still needing grid support during a short high-power event.
23:00 — The Battery Reaches Its Reserve
Assume the homeowner configured:
20% backup reserve
Once the battery reaches that boundary, normal self-consumption discharge may stop.
The grid then supplies overnight loads.
That 20% should not be counted twice in a sizing calculation.
If it is reserved for outage protection, it is not simultaneously available for ordinary evening energy shifting.
02:00 — Should the Grid Recharge the Battery?
Not every solar battery waits for sunrise.
Suppose the homeowner has a cheap tariff from:
02:00–04:00
The EMS may intentionally charge from the grid during that period.
If the battery needs 8 kWh before the cheap window closes, the first-pass charging requirement is:
8 kWh ÷ 2 h = 4 kW
before accounting for charging losses and system limits.
Now the installer must check whether the battery and inverter can actually sustain the required charging power.
Cold battery temperature can reduce that capability further.
This is why tariff strategy, charging power, and battery operating conditions belong in the same design review.
What I Would Check Before Quoting the System
For a Ruibit home-storage proposal using a compatible Dawnice battery and inverter, I would want:
| Input | Why It Matters |
|---|---|
| 24-hour load profile | Shows when energy is needed |
| PV production profile | Shows when surplus occurs |
| Battery usable kWh | Determines energy-shifting capacity |
| Inverter kW | Limits instantaneous power |
| Charge/discharge limits | Defines actual battery capability |
| Export tariff/rules | Changes the value of surplus PV |
| Backup reserve | Reduces energy available for daily cycling |
| TOU tariff | May justify grid charging |
This is more useful than selecting a battery from annual electricity consumption alone.
Follow the Energy, Then Size the Battery
Over one day, a Home ESS may move through several operating states:
PV → House
PV → Battery
PV → Grid
Battery → House
Grid → House
and, where the operating strategy allows:
Grid → Battery
The order is not universal. It depends on the system settings and the homeowner's objective.
The best way to size a solar-plus-battery system is to compare when the house needs energy with when solar produces it, then check whether the battery has enough kWh to shift that energy and enough kW to move it at the required time.
FAQs
1. Does solar power charge the battery before supplying the house?
Usually not. In many grid-connected systems, solar serves household loads first , and surplus PV then charges the battery. The exact priority depends on inverter and EMS settings.
2. What happens when the home battery is full?
Additional solar may be exported to the grid, used by flexible loads, or curtailed , depending on system settings and local export rules.
3. Why can a home still import grid power when the battery has energy?
The battery may have reached its backup reserve , or household demand may exceed the inverter's available discharge power.
4. Can a solar home battery charge from the grid?
Yes, where the system and local rules allow it. Grid charging can be useful during low-cost time-of-use periods , especially when the battery needs energy before the next solar-production window.
5. What determines the right home battery size?
Use the 24-hour load profile, solar-production profile, usable battery kWh, inverter kW, charge/discharge limits, tariff, and backup reserve rather than annual electricity consumption alone.
