Quick Answer: Divide your battery's usable capacity by the off-peak window hours, then add about 10% for charging losses. A 10 kWh battery (9 kWh usable) with a 4-hour night tariff needs at least 2.5 kW of charging power (9 ÷ 4 × 1.1). A 5 kWh battery (4.5 kWh usable) fills in 2 hours at 2.5 kW. If your inverter's charging limit is lower than this number, the battery will not reach 100% before the window closes — and your arbitrage savings shrink.
Why the tariff window changes the sizing math
In the UK, Economy 7 and Octopus Go give 4–6 cheap night hours; in other EU markets, 5–8. A shorter window forces higher charging kW and a higher inverter charge rating.
Most buyers size a battery by kWh and forget to check the charger kW. They install a 10 kWh battery, set it to charge overnight, and discover at 7 AM that the battery only reached 60% because the inverter charges at 1.5 kW. That battery is now doing half the arbitrage job it was bought for.
The three-step calculation
Step 1 — Usable capacity. A 10 kWh LiFePO4 battery at 90% DoD delivers 9 kWh usable.
Step 2 — Divide by window hours. 9 kWh ÷ 4 hours = 2.25 kW minimum.
Step 3 — Add charging losses. Round-trip efficiency is ~94%, so charging losses are roughly 6–10%. Multiply by 1.1 to be safe: 2.25 × 1.1 = 2.5 kW .
That is the minimum continuous charging power your inverter must deliver to fill the battery inside a 4-hour window.
Worked examples
| Battery | Usable kWh | Window | Minimum charger kW |
|---|---|---|---|
| HZEB-LCT-5 | 4.5 | 4 h | 1.3 kW |
| HZEB-LCT-5 | 4.5 | 2 h | 2.5 kW |
| HZEB-LCT-10 | 9 | 4 h | 2.5 kW |
| HZEB-LCT-10 | 9 | 2 h | 5.0 kW |
| HZEB-LCT-16 | 14.4 | 6 h | 2.7 kW |
| HZEB-LCT-16 | 14.4 | 3 h | 5.3 kW |
Halving the window doubles the required charging power. A customer on a 2-hour cheap window needs a bigger inverter than a customer on a 6-hour window, even with the same battery.
Where the charging limit actually comes from
The battery does not set the charging rate — the inverter does. Three numbers matter:
1. Inverter charge rating. Most 5 kW hybrid inverters charge the battery at 80–100% of rated power: roughly 4–5 kW. Smaller 3.6 kW inverters often charge at only 2.5–3 kW.
2. The battery's max charge C-rate. A 0.5C-rated battery accepts 0.5 × capacity per hour. A 10 kWh battery at 0.5C accepts 5 kW — rarely the bottleneck for home systems. Cheap batteries rated 0.25C accept only 2.5 kW and can actually limit fast charging.
3. Solar + grid charging priority. During the off-peak window there is no solar, so the full charge rating is available for grid charging — but only if the inverter supports grid-charging mode. Some inverters ship with grid charging disabled by default.
Check your inverter's datasheet for "Max charge current" (A) and multiply by battery voltage. Example: 50 A × 51.2 V = 2.56 kW max charging.
What happens when the window is too short
Three real-world outcomes when charging power < required kW:
- Battery never fully charges. It reaches 80–90% each night. Over a week, that is 1–2 full cycles of lost arbitrage value.
- The battery starts discharging before it is full. Some EMS logic discharges at 5 AM regardless of SoC, capturing only partial savings.
- The customer adds a second battery later and the problem doubles. Two 10 kWh batteries need twice the charging power, but the inverter rating stays the same.
When to upsize the inverter
Upsize the inverter (or add a second charge path) when:
- The off-peak window is 3 hours or less
- The battery is 10 kWh or larger
- You plan to add a second battery within 2–3 years
- Your inverter's charge current × battery voltage is below the calculated minimum
A 3.6 kW inverter charging a 10 kWh battery in a 2-hour window simply cannot do the job. The buyer needs a 5–6 kW hybrid inverter or a dedicated fast-charge setting.
Checking the datasheet before you buy
When comparing quotes, ask for these numbers in writing:
- Max charge current of the hybrid inverter (A)
- Battery max charge rate (C-rate)
- Does the inverter support grid-charging mode with schedule?
- Is the grid charge limit adjustable (kW)?
Most suppliers answer the first two. The third and fourth determine whether your tariff window actually fills the battery.
A practical buying rule
For a 4-hour off-peak window, size charging power at about 28% of usable capacity per hour. For a 2-hour window, about 55% per hour. This rule covers the losses and the EMS overhead.
Ruibit configures Dawnice HZEB-LCT systems with grid-charge schedules matched to the customer's tariff window before delivery. If you send us your tariff times, we confirm the inverter charge setting fills the battery completely — no guessing after installation.
FAQs
Q: Is charging power the same as the inverter's rated power? No. A 5 kW hybrid inverter typically charges the battery at 4–5 kW, but some models limit charging to 60–80% of rated power. Check "max charge current" on the datasheet.
Q: Can a bigger battery charge faster? Not necessarily. Charging speed is set by the inverter and the battery's C-rate. A 20 kWh battery on a 2.5 kW charger takes twice as long as a 10 kWh battery on the same charger.
Q: What if my off-peak window is only 2 hours? You need roughly 0.55 × usable capacity in kW of charging power. For a 10 kWh battery (9 kWh usable), that means a 5 kW charge rating — typically a larger hybrid inverter.
Q: Does the battery charge slower in winter? Yes. Cold cells charge at reduced current below 5°C, and some BMS units limit charge rate until the pack warms. In cold climates, add 20–30% margin to your kW calculation.
Q: Can I charge from solar during the day and top up at night? Yes, and this is the best setup. Solar fills most of the battery during the day; the off-peak window tops up the rest. The required night charging power drops, and a smaller inverter can still capture the tariff benefit.
