logo elektroda
logo elektroda
X
logo elektroda
Advanced Search
logo elektroda

Best Battery for 4kW Solar Panel System: Optimal Storage Capacity, Grid-Resale, 3600kWh Usage

pirotechnic 51702 33
Best answers

Is it more profitable to store energy from a 4 kW solar system in batteries for later self-use, or to sell the surplus to the grid, and what battery capacity/type would be needed?

It is generally more profitable to sell the surplus to the grid and skip batteries, because energy storage is expensive and an off-grid battery setup costs much more than an on-grid installation [#15461712][#15461663] If you still want storage, one estimate for the best summer conditions is at least 5 × 12V 200Ah batteries, but in practice 15–25 batteries would be needed for real-world conditions [#15461345][#15461663] The thread also says lithium-ion batteries are several times more expensive than gel batteries, so they are considered unprofitable here [#15461345] The battery costs were estimated to be roughly PLN 500 per 12V/200Ah gel battery (later corrected by the poster to about PLN 1500), and replacements may be needed about every two years in this kind of use [#15461300][#15461382] As a prosumer you can sell only the surplus, and the selling price per kWh is lower than what you pay for electricity, which is another reason the thread recommends grid connection instead of batteries [#15461663][#15461714]
Generated by the language model.
ADVERTISEMENT
Treść została przetłumaczona polish » english Zobacz oryginalną wersję tematu
Report a violation of the law
Reply | New topic
  • #31 20982490
    michalPV
    Level 31  
    @bigszuszu, just as I was starting my own topic, someone there advised me to go for a microinverter, so after my experience with off-grid I will write the same:
    Just do on grid with microinverters. Simple, cheap, hassle-free, maintenance-free.
  • ADVERTISEMENT
  • #32 20982523
    bigszuszu
    Level 3  
    Unfortunately, I can`t feed the electricity into the grid - so I only have off-grid.
  • ADVERTISEMENT
  • #33 20982534
    vodiczka
    Level 43  
    bigszuszu wrote:
    Unfortunately, I am unable to feed electricity into the grid
    since you have the option of conscription
    bigszuszu wrote:
    I would like to install an off-grip installation (with the possibility of drawing electricity from the grid on bad days)
    you also have the opportunity to donate.
  • #34 21496696
    bachin
    Level 18  
    Until today there is a promotion on LiFePO4 here Link . 32 pcs. 320Ah can be gulped down for 6wheels, which gives a slightly different perspective on the cost-effectiveness of off-grid and on-grid. I've done some preliminary counting and 4 sets of 16 pcs each should come out at zero in 3 years at the current annual cost of 4k. Maybe a long time, maybe not, especially as autumn-spring will still be able to reheat, meaning less gas.
    So that I recommend, because as they say he who does not risk, does not eat ham.
    In June we will see what came of it. :P .
Reply | New topic
Report a violation of the law

Topic summary

✨ The discussion addresses the selection of batteries for a 4kW solar panel system with an annual household consumption of approximately 3600 kWh (about 10 kWh per day). Key considerations include the optimal battery capacity, type (gel vs. lithium-ion), depth of discharge, lifespan, and economic viability of storing energy versus selling surplus to the grid. Gel batteries (12V, 200Ah, ~2.4 kWh capacity) are commonly referenced, with an estimated need for at least five such batteries to store daily production during favorable conditions, costing around PLN 1500-2500 each. Battery lifespan is debated, with practical replacement intervals closer to 2 years due to depth of discharge and efficiency losses, despite manufacturer claims of up to 10 years. Lithium-ion batteries, while offering longer life, are significantly more expensive and often deemed unprofitable for this scale. The economic analysis favors selling surplus energy to the grid rather than investing heavily in battery storage, especially given current feed-in tariffs (~0.25 PLN/kWh for energy, with transmission costs separate) and net metering policies that allow prosumers to sell only surplus electricity. Off-grid setups with batteries are considered more complex, costly, and less efficient, with recommendations leaning towards on-grid systems with microinverters for simplicity and cost-effectiveness. Additional technical points include the impracticality of using automotive alternators for battery charging and the importance of considering weather variability (cloudy/rainy days) in system design. A recent promotion on LiFePO4 batteries offers a potential shift in cost-effectiveness for off-grid systems, with preliminary calculations suggesting break-even in about three years for larger battery banks.
Generated by the language model.

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, post #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, post #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, post #15461926]

How do I size a battery bank?

  1. Sum daily loads (kWh).
  2. Decide autonomy days (1–3).
  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.
Generated by the language model.
ADVERTISEMENT