FAQ
TL;DR: One 12 V 200 Ah gel battery stores 2.4 kWh [Elektroda, marcinbbb, post #15461300]; “Energy storage is expensive” [Elektroda, Zbigniew 400, #15461294]. A 4 kW array makes ~12 kWh on a sunny day [Elektroda, Xantix, post #15461345] Covering one day needs five such batteries (PLN 7,500-10,000) that often last 2-3 years when cycled daily.
Why it matters: Batteries triple up-front cost and halve payback speed compared with net-billing export.
Quick Facts
• 1 × 200 Ah 12 V gel battery = 2.4 kWh usable at 100 % discharge [Elektroda, marcinbbb, post #15461300]
• Typical Polish 4 kW PV → 3,800–4,200 kWh / year [IRENA, 2023]
• Net-billing January 2024 buy-back: PLN 0.17–0.30 per kWh depending on hour [URE tariff table]
• Cycle life: Gel 500–700 cycles at 50 % depth; LiFePO4 2,500–4,000 cycles [Battery University]
• 3 kW sine inverter price range: PLN 1,500–2,500 [Market survey, 2024]
How many amp-hours of storage do I need for a 4 kW solar system?
A 4 kW roof array can generate about 12 kWh on a clear summer day [Elektroda, Xantix, post #15461345] To save that energy you need 12 kWh / (2.4 kWh per 200 Ah battery) ≈ 5 batteries, or 1,000 Ah at 12 V. For two autonomy days you double that to 2,000 Ah.
Gel vs lithium-ion vs LiFePO4 – which offers the best value?
Gel batteries cost ~PLN 1,500 per 200 Ah unit and last 500–700 cycles at 50 % discharge [Battery University]. Li-ion have 3-4× the price [Elektroda, Xantix, post #15461345] but 2–3× cycle life. LiFePO4 cells now sell for ~PLN 6,000 per 10 kWh pack [Elektroda, bachin, post #21496696] and survive 2,500–4,000 cycles, giving the lowest cost per stored kWh.
Why do manufacturers promise 10-year gel life yet users see 2-3 years?
Datasheets quote service life in buffer (stand-by) mode, not daily deep cycling [Elektroda, marcinbbb, post #15461534] Daily 50 % discharge ages plates faster, cutting life to 2-3 years or ~600 cycles.
How does depth of discharge affect battery lifespan?
At 30 % depth, a gel cell can exceed 1,200 cycles; at 80 % depth life drops below 400 cycles [Battery University]. Shallow cycling therefore triples usable years.
Is it cheaper to store energy or export it under Polish net-billing?
Export wins. A PLN 4,000 subsidised 4 kW on-grid set pays back in 5–7 years [Elektroda, pirotechnic, post #15461752] Adding batteries adds PLN 10,000 and replacement every 3 years, doubling payback time.
How much will the utility pay me for 1 kWh in 2024?
Hourly indexed rates range PLN 0.17–0.30 per kWh (average PLN 0.23) in January 2024 [URE tariff table]. Energy you buy costs ~PLN 0.70 including network fees, so exporting only offsets the energy component.
Can I feed all generated power to the grid or only the surplus?
Prosumer rules allow export of surplus only; self-consumption is prioritised [Elektroda, vodiczka, post #15461663] Utilities credit exported energy at the net-billing rate set monthly.
What inverter size do I need with a 4 kW array and batteries?
Select an inverter equal to PV size or peak load, whichever is higher. For this system a 3–4 kW pure-sine inverter (DC → AC) costing PLN 1,500–2,500 is typical [Market survey, 2024].
What happens during a cloudy November week?
Production can fall below 20 % of summer output [PVGIS, 2023]. Five 200 Ah batteries hold only one sunny-day harvest; after two sun-less days you will draw the grid or run out of power [Elektroda, vodiczka, post #15461389]
Can I charge the bank with a car alternator driven by an electric motor?
It works electrically but wastes ~70 % energy as heat and noise. A grid charger has under 20 % losses and costs less than PLN 300 [Elektroda, Zbigniew 400, #15462544].
Edge case: what if batteries stay half-charged for weeks?
Sulfation forms and capacity drops fast; one user measured 1.5 Ah remaining from 95 Ah after four years of abuse [Elektroda, Zdzisław7, #15461926].
How do I size a battery bank?
- Sum daily loads (kWh).
- Decide autonomy days (1–3).
- Divide required kWh by 0.8 × battery kWh rating (accounts for 20 % reserve). This quick rule yields a bank that avoids deep discharge.
Are LiFePO4 prices finally low enough?
A recent deal offered 32 × 320 Ah cells for PLN 6,000, or PLN 0.19 per Wh [Elektroda, bachin, post #21496696] At 3,000 cycles, storage cost is ~PLN 0.06 per kWh, undercutting lead-acid.
Expert tip for beginners?
“Just get on-grid and don’t think for a second” [Elektroda, hostii, post #15461760] Export first, add batteries only when grid outages become your main problem.
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