The most common mistake in battery sizing is matching it to the solar array's power. The correct starting point is the household's evening and night consumption, because that is what the battery replaces. Here is the calculation.
1. Where does the household consume?
Take a family using 4,500 kWh per year. That averages 12.3 kWh per day. Roughly 40% of daily consumption falls in daytime, when the panels are producing, and 60% in the evening, night and early morning. That is about 7 kWh for the battery to supply. In the heating season, with a heat pump or an electric car, the numbers rise considerably.
2. Three common sizes
- 5 kWh: for smaller households using 3,000 to 4,000 kWh per year, or tight budgets. It covers evening base load (fridge, lighting, TV, computer) but not the whole night.
- 10 kWh: the optimum for most family houses. It covers a large part of the daily evening and night demand, and self-consumption can rise from 35 to 40% towards 70%.
- 15 kWh: for higher-consumption households (6,000+ kWh per year), those with a heat pump or electric car, or where backup power matters.
In our experience the benefit grows at a diminishing rate with capacity: the second 5 kWh brings less than the first, because daytime production in summer is finite and in winter the battery simply cannot fill up.
3. Power matters, not just capacity
The battery's rated power (kW) determines how many loads it can serve at once. A heat pump, induction hob or boiler can draw several kW together. If output power is low, the grid takes over the load at the evening peak and stored energy goes unused. For backup operation, the combined power of the inverter and battery is especially important.
4. LFP and cycle life
For residential storage, LFP (lithium iron phosphate) is now the established technology. It is safer, thermally more stable and long-lived: manufacturers typically commit to 6,000 or more cycles and a 10-year warranty, with 90 to 100% usable capacity (DoD). At one cycle a day that means a lifetime beyond 15 years. Check the warranty terms: many manufacturers also set an annual energy throughput (kWh) cap.
5. Self-consumption or backup?
Two different goals:
- Raising self-consumption: the aim is a daily cycle, using the daytime surplus in the evening. Capacity matters here, and the battery is discharged every day.
- Backup power: the aim is for critical loads (fridge, lighting, internet, heating controls) to keep running in an outage. What matters is the reserve level, meaning a charge held back, and the output power. This reduces the daily usable capacity.
The two can be combined, but the proportions should be chosen consciously.
6. What does it earn?
A 10 kWh battery with 250 to 300 cycles a year can shift 2,500 to 3,000 kWh into the evening hours. If the difference between purchase and feed-in price is HUF 20 to 25 per kWh, that is HUF 50,000 to 75,000 saved per year. On its own that means a long payback, so the grant, the backup function and future price changes together make the decision. Ask your installer for an honest calculation using your own consumption data.
Quick recommendation
| Annual consumption | Suggested size |
|---|---|
| 3,000 to 4,000 kWh | 5 kWh |
| 4,000 to 6,000 kWh | 10 kWh |
| 6,000+ kWh, heat pump or EV | 15 kWh |
With a modular system the starting size can be expanded later, so it is fine to start smaller. If your bill and inverter app data are available, we will give you a precise recommendation.
