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

pirotechnic 51699 33
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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]
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  • #1 15461270
    pirotechnic
    Level 9  
    Hello
    I would like to ask which batteries should be selected for the 4kW system.
    Annual electricity consumption at home approx. 3600kWh
    And the question is whether it is more profitable to store the produced energy and use it on your own when solar energy is not produced, or to resell it to the grid and not to play with the batteries.
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  • #2 15461294
    Zbigniew 400
    Level 38  
    Energy storage is expensive.
  • #3 15461300
    marcinbbb
    Level 26  
    First, consider whether you can afford such a number of batteries, later or after 2 years you will be able to afford their replacement, then add the cost of a 3kW full sine converter to the batteries. And then divide the value by the average quality on-line inverter, and I'll say that you will buy 5 of them. Write down how much I have overestimated the number of these inverters.

    I will just add that 1 12V and 200Ah gel battery will store 2.4kWh. And its cost is about PLN 500. Of course, I ignore any losses on the converter
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  • #4 15461331
    pirotechnic
    Level 9  
    I wrote the question a bit wrong, I mean how many of these batteries should be (how many Ah) and what are the best (gel, lithium-ion)
    And if by dividing the annual consumption (3600 kWh / 366 days a year = about 10kWh per day), how many percent would the batteries be discharged and what would their lifetime be then, because I know that their durability depends on the depth of the discharge. I do not want very precise data, but very ornational, or for example, for example, other similar installations.
  • #5 15461345
    Xantix
    Level 41  
    Assuming that in the most favorable period of the year (summer), the daily energy production of your installation will be 12kWh / day (which is quite a real value), you need 5 12V and 200Ah batteries (as provided by your colleague) to store this energy. So the cost of the batteries themselves is PLN 2,500. And you'll have to replace them approximately every two years (and maybe more often). By adding the price of the other devices, it is a big investment.
    that is, inferring:
    Zbigniew 400 wrote:
    Energy storage is expensive.


    Added after 2 [minutes]:

    pirotechnic wrote:
    and what are the best (gel, lithium-ion)

    Lithium Ion - they are very, very expensive (several times more expensive than gel). Rather unprofitable.
  • #6 15461382
    pirotechnic
    Level 9  
    Damn, I thought it was replaced after about 5 years. I have the installation itself with 85% co-financing, i.e. PLN 4,000 for 16 panels together with cabling, inverter and assembly.
    So it is more profitable to resell energy? How much are they now paying on average for selling 1 kWh?
    And from what I can see, one such battery (200Ah) costs PLN 1500, not 500.
  • #7 15461389
    vodiczka
    Level 43  
    Xantix wrote:
    Assuming that in the most favorable period of the year (summer), the daily energy production of your installation will be 12kWh / day (which is quite a real value), you need 5 12V and 200Ah batteries (as provided by your colleague) to store this energy.
    What will it do if there are three consecutive rainy days, not to mention a cloudy week in November :)
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  • #8 15461397
    Xantix
    Level 41  
    vodiczka wrote:
    What will it do if there are three consecutive rainy days, not to mention a cloudy week in November

    These were an estimate. And that's it.
  • #9 15461398
    vodiczka
    Level 43  
    pirotechnic wrote:
    So it is more profitable to sell energy to the collective? How much are they now paying on average for selling 1 kWh?
    Definitely sell, even if in your opinion they are paying too little.
    Xantix wrote:
    These were an estimate
    Estimates should also assume the least favorable conditions rather than the most favorable.
  • #10 15461412
    pirotechnic
    Level 9  
    So it would be necessary to use as many as 5 batteries? And this service life is so low, because on the websites of various manufacturers, they indicate the service life of up to 10 years.
  • #11 15461480
    Xantix
    Level 41  
    vodiczka wrote:
    Estimates should also assume the least favorable and not the most favorable conditions.

    Let's say the cost for the most unfavorable conditions the author would not be able to imagine ...
  • #12 15461534
    marcinbbb
    Level 26  
    pirotechnic wrote:
    And this service life is so low, because on the websites of various manufacturers, they indicate the service life of up to 10 years.

    And is it not "in the buffer work"?
  • #13 15461565
    pirotechnic
    Level 9  
    marcinbbb wrote:
    pirotechnic wrote:
    And this service life is so low, because on the websites of various manufacturers, they indicate the service life of up to 10 years.

    And is it not "in the buffer work"?


    http://sunsol.pl/produkty/akumulator_toyama_npg_200ah_cena
    as if it says here from 6 to 10 years, I also do not know what to think about it

    And is it specified by regulations somewhere, can I sell all the energy produced by the solar panels to the power plant, or only the electricity surplus?
  • #14 15461663
    vodiczka
    Level 43  
    pirotechnic wrote:
    So it would be necessary to use as many as 5 batteries?
    Minimum 5 - for the most favorable conditions. Practically 15-25.
    pirotechnic wrote:
    ..can I sell all the energy produced by the solar panels to the power plant or only the electricity surplus?
    The whole thing is not worth it because for 1kWh sold you will get less than you pay for the collected one. If you are acting as a prosumer, by definition you are selling the surplus.
  • #15 15461712
    hostii
    Level 25  
    If they do not change anything and you are balancing FORGET about batteries. Unless for some reason you are unable to report an on-grid installation. At the moment, batteries will come out MUCH more expensive than connecting to the network. I'm off-grid, but for me a lot of things were not normally bought, so for me it had some economic justification and there was no balancing. I already wanted to switch to on-grid, but the batteries will take a little longer and what they do with the law, I will not charge them money out of spite. And I will supply panels (I added 4 a week ago) and I already have 3.6kW in off.
  • #16 15461714
    pirotechnic
    Level 9  
    So it is most profitable to use solar energy during the day as much as you need and sell the rest, so in such a situation you could install a larger set of photovoltaic panels, I do not know if I understand it well, but I think it would be ok.
    Somewhere I read that now you can sell energy for 100% of the average amount per kWh, which is about 65gr, and not like it used to be 20gr. Is it true?
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  • #17 15461737
    hostii
    Level 25  
    pirotechnic wrote:
    100% of the average amount per kWh, which is about PLN 65, and not 20 PLN as it used to be. Is it true?


    uuu I can see that my friend has big shortcomings and he does not even look at what he pays for ... 1kWh costs 0.25gr and the rest is transmission and quality

    pirotechnic wrote:
    So it is most profitable to use solar energy during the day as much as you need and sell the rest, so in such a situation you could install a larger set of photovoltaic panels, I do not know if I understand it well, but I think it would be ok.


    YES and when there is no sun, you will pay half the amount because it will balance and you will only pay for the transmission because only the kWh themselves balance
  • #18 15461752
    pirotechnic
    Level 9  
    hostii wrote:
    pirotechnic wrote:
    100% of the average amount per kWh, which is about PLN 65, and not 20 PLN as it used to be. Is it true?


    uuu I can see that my friend has big shortcomings and he does not even look at what he pays for ... 1kWh costs 0.25gr and the rest is transmission and quality

    pirotechnic wrote:
    So it is most profitable to use solar energy during the day as much as you need and sell the rest, so in such a situation you could install a larger set of photovoltaic panels, I do not know if I understand it well, but I think it would be ok.


    YES and when there is no sun, you will pay half the amount because it will balance and you will only pay for the transmission because only the kWh themselves balance


    Anyway, if I pay about PLN 4,000 for the entire 4kW installation, it probably pays off?
  • #19 15461760
    hostii
    Level 25  
    pirotechnic wrote:
    Anyway, if I pay about PLN 4,000 for the entire 4kW installation, it probably pays off?


    JUST GETTING and don't think for a second !!! it is definitely on-grid, a dream price, where is such a low price? I would buy it myself :) :D :D
  • #20 15461926
    Zdzisław7
    Level 21  
    Make yourself a gift for a few PLN and forget about the OFF installation, unless you live in the desert.
    New batteries worked for me for 1 year. 4 years as a UPS for the home. I'm doing electronic
    battery desulphurizer. and checked one. From 95Ah he has 1.5Ah left. Apart from the higher ones
    investment costs, the operation itself is more burdensome with low efficiency
    layout off.
  • #21 15462057
    pirotechnic
    Level 9  
    That would be it.
    Subject to be closed.
  • #22 15462079
    putas
    Renewable energy sources specialist  
    hostii wrote:
    uuu I can see that my friend has big shortcomings and he does not even look at what he pays for ... 1kWh costs 0.25gr and the rest is transmission and quality

    Even less, because about 17 cents
  • #23 15462535
    byCris
    Level 12  
    Hello

    I don't want to start a new topic, so let me ask here - if the topic author doesn't mind:

    1. Can an alternator from the Polonez (12V / approx. 14A - driven by an electric motor through a V-belt) be connected to the regulator (Tracer 3215RN) together with the panels for emergency charging of 24V batteries? (I think that such an hour charging is enough);
    - lighting installation outside the house on 24V (2x60Ah gel batteries) - in the current cloudiness, the batteries are undercharged - no 24V power supply means no lighting.
    I do not have a 24V-olt charger for gel tubes, hence the question, and disconnecting the battery and recharging each one in turn is not the happiest way.
  • #24 15462544
    Zbigniew 400
    Level 38  
    Why do you want to charge the batteries with a loss generation system? An engine drawing energy from the grid to drive a generator that charges the batteries is technical paranoia.
  • #25 15462551
    byCris
    Level 12  
    so I wrote: emergency recharge the battery and not all the time all the time.

    it is cloudy and there is no electricity in the battery - lighting installation a bit

    expanded and I have no way to power it otherwise.
  • #26 15462625
    Zbigniew 400
    Level 38  
    After all, there are chargers for charging batteries / lower losses /.
  • #27 15462719
    hostii
    Level 25  
    byCris wrote:
    Hello

    I don't want to start a new topic, so let me ask here - if the topic author doesn't mind:

    1. Can an alternator from the Polonez (12V / approx. 14A - driven by an electric motor through a V-belt) be connected to the regulator (Tracer 3215RN) together with the panels for emergency charging of 24V batteries? (I think that such an hour charging is enough);
    - lighting installation outside the house on 24V (2x60Ah gel batteries) - in the current cloudiness, the batteries are undercharged - no 24V power supply means no lighting.
    I do not have a 24V-olt charger for gel tubes, hence the question, and disconnecting the battery and recharging each one in turn is not the happiest way.


    Do not worry about anything, just switch to the network and let the panels keep the batteries in good condition. In this weather, you can skip the off-grid,
  • #28 15468902
    byCris
    Level 12  
    Let me not bore you with the question of whether I can ...


    After all, this is the technical possibility of connecting the allernator to the coby regulator

    recharge the batteries or it won't work, or worse, the regulator will fail

    damaged?
  • #29 15468918
    Zbigniew 400
    Level 38  
    Theoretically it is, but for what?
  • #30 20981428
    bigszuszu
    Level 3  

    Hello,

    I will say this - in today's galloping era of electricity prices, I would like to install an off-grid installation (with the option of drawing electricity from the grid on bad days). I won't say it, but I'm a total novice and I don't know where to start :) I only know that I want to put it on 4 200Ah (2.4kV) gel batteries - for starters :) and now my question :) what panels to choose, what voltage regulator and what inverter?
    Can someone help me build this device? :)
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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.
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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.
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