Imagine your photovoltaic system as a talented musician playing beautiful symphonies of electrons... but at 5:30 pm it gets tired and stops playing just when you get home and turn on the oven. Storage batteries are the pragmatic manager who takes those tracks produced during the day, puts them in a warehouse (the battery), and brings them back on stage when needed — without any drama. In this article, you'll find everything you need to know: how batteries work, which one to choose, how to size them, mistakes to avoid, costs, realistic ROI, safety, maintenance, and a few biting jokes to remind you that “leaving energy on the grid” is like paying for a lunch and leaving the plate on the table.
1. What does a storage battery really do?
The battery stores the energy produced by the photovoltaic system (or taken from the grid) when production exceeds consumption, and releases it when consumption is greater than production. In other words: the more self-produced energy you use at home, the less you have to buy from the grid. If your system is a shy genius, the battery makes it a useful genius 24/7.
Main functions:
- Increase self-consumption.
- Provide backup in case of blackout.
- Perform “peak shaving” (reduce withdrawals during peak periods).
- Enable time-of-use strategies (charge when the grid is cheap, discharge when it’s expensive).
2. Terms you need to know (without fear)
kWh (kilowatt-hour): amount of energy. E.g.: 1 kWh = using 1 kW for 1 hour.
kW: instantaneous power. E.g.: a 2 kW oven consumes 2 kW when on.
Nominal capacity: e.g. 10 kWh battery.
Depth of discharge (DoD): usable percentage without damaging the battery — if DoD = 90% and battery 10 kWh → usable 9 kWh.
Round-trip efficiency: energy loss between charge and discharge; if efficiency = 90% out of 10 kWh input, you recover 9 kWh.
Cycles: how many times the battery can be discharged/charged before losing significant capacity.
BMS (Battery Management System): the brain that protects the battery.
3. Types of batteries: chemistry is the new trend
Lithium (Li-ion, LFP — lithium iron phosphate): today the dominant choice for homes. Good energy density, long life, high efficiency. LFP is safer and has a longer life cycle.
Lead-acid (AGM, GEL): cheaper initially, but heavy, fewer cycles, maintenance and more limitations (not recommended for new installations).
Others (flow, etc.): more for industrial plants or experimental solutions.
For a home, LFP lithium is often the most balanced solution.
4. How to size a battery (step-by-step practical example)
Let’s do a concrete example. Suppose:
Annual household electricity requirement = 4,000 kWh/year.
Annual photovoltaic production = 3,500 kWh/year.
Current self-consumption without battery = 30% of production.
Calculation: 3,500 × 0.30 = 1,050 kWh self-produced used directly.
Goal with battery: bring self-consumption to 70%.
Calculation: 3,500 × 0.70 = 2,450 kWh.
Additional energy exploited thanks to the battery = 2,450 − 1,050 = 1,400 kWh.
Math done: 3,500 × 0.30 = 1,050. 3,500 × 0.70 = 2,450. 2,450 − 1,050 = 1,400.
If the average price of electricity you buy from the grid is €0.25 / kWh, you will save:
1,400 kWh × €0.25/kWh = €350 / year.
Calculation: 1400×0.25 = 350.00 €.
Now look at costs and payback. Suppose an 8 kWh battery (nominal capacity) with a net cost to the user of €6,000 (example price, varies a lot), and that this battery actually allows you the estimated savings.
Payback without incentives = €6,000 ÷ €350/year ≈ 17.14 years.
Calculation step: 6000 ÷ 350 = 17.142857... → rounded 17.14 years.
With incentives (e.g. 50% tax deduction — check current regulations), net cost = 6,000 × 0.5 = €3,000.
Calculation: 6000×0.5=3000.
Payback with incentive = €3,000 ÷ €350/year ≈ 8.57 years.
Calculation: 3000 ÷ 350 = 8.571428... → 8.57 years.
Practical conclusion: the convenience depends greatly on incentives, the self-consumption you can achieve, and the price of electricity. In many cases, with active incentives and good optimization, the payback becomes interesting; without incentives it can be long.
5. Electrical sizing: don’t confuse kWh with kW
- Choosing capacity (kWh) is different from sizing discharge power (kW).
- If you want to power the oven (2.5 kW) and the house in parallel, the battery must have adequate discharge power (e.g. 5 kW).
- Small batteries with high power are useful for peaks (peak shaving), large batteries with low power are for long autonomy.
- Always check continuous kW and peak kW that the battery inverter can deliver.
6. Integration with inverter and existing system
Two approaches:
AC-coupled: the battery connects to the AC grid via an inverter; easy to integrate on existing systems.
DC-coupled: the battery connects on the DC side (before the inverter), more efficient for new systems or to maximize minimal losses.
Choose based on your photovoltaic configuration, willingness to expand, and installer’s advice.
7. Cycles, lifespan, and warranties: what to expect
Modern batteries declare cycles (e.g. 6,000–8,000 cycles for some LFP). If you do 1 full cycle per day, 6,000 cycles = 16.4 years.
Calculation: 6000 ÷ 365 = 16.438356... → 16.44 years.
Commercial warranties often indicate minimum residual capacity after N years (e.g. 70% after 10 years).
The BMS manages charge/discharge and protects the battery: don’t underestimate it.
8. Safety, installation, and regulations
Lithium batteries require certified installers who comply with fire and spacing regulations.
Provide for ventilation, electrical protections, and outdoor areas/technical room integration according to regulations.
Emergency plan and user manual must be provided by the installer.
9. Maintenance and end of life
- Maintenance: minimal — cleaning, checking charge status, and BMS firmware updates.
- End of life: batteries must be delivered to WEEE collection centers; recycling of materials (lithium, nickel, cobalt) is growing but requires specialized management.
- Consider manufacturer/installer take-back programs.
10. Common mistakes and how to avoid them
- Buying the battery “because it costs less” without evaluating usable kWh, efficiency, and power.
- Ignoring the C-rate (charge/discharge speed) — leads to disappointment in case of peaks.
- Sizing only with the logic “if I have 10 kWh then I can run my house for three days” — think about how you actually use energy.
- Not considering round-trip losses (e.g. 90% efficiency → 10% lost).
- Skipping the check for incentives and regulatory requirements.
11. Smart strategies to get the most from your battery
- Optimize charging times (use daytime PV production; charge at night if the rate is low).
- Pair with monitoring and automation systems (smart load shifting): water heater that charges when there’s surplus, EV that recharges when it’s convenient.
- Use backup only when needed (not as a primary option every day if not necessary).
12. Is it worth it? It depends (but often yes, with brains)
Batteries transform your photovoltaic system from a “shy producer” to an “intelligent autopilot.” Economic convenience depends on PV production, consumption habits, energy prices, and incentives. Without dialogue with real numbers and serious quotes, you risk buying too much or too little. Morally: if you want energy security, higher self-consumption, and the zen satisfaction of consuming what you produce, batteries are a concrete and often advisable upgrade.
Quick checklist before investing
- Know annual PV production (kWh) and annual consumption (kWh).
- Determine current self-consumption (%) and realistic goal.
- Calculate required capacity in kWh and power in kW.
- Check total costs and available incentives.
- Request at least 2 technical quotes and a project.
- Check warranty, declared cycles, and guaranteed residual capacity.
- Make sure the installer is certified for local regulations.
