Solar panels: doubts over a 3,500W off-grid kit in Spain
A forum user asks whether a 3,500W off-grid solar system needs panels covering that full capacity. Several users reply that with fewer panels, charging is far too slow.
English · Original discussion in Spanish · Published
Solar panels: doubts over a 3,500W off-grid kit in Spain
The question seemed simple: if you install an off-grid solar system with 3,500-watt batteries, do you need panels covering that same capacity, or is less enough? The answer from experience is uncomfortable: with fewer panels the batteries do charge, yes, but so slowly that in practice they are useless. A calculation doing the rounds in the discussion makes it clear: with a single 350W panel, it would take ten hours of peak sunlight to fill a 3,500Wh battery, something that never happens in reality, not even in summer. And that is where the problem begins.
Why online configurators reject less than 10 square metres
The automatic configurators of online shops refuse to install less than 10 square metres of panels for a 3,500W system. It is not a commercial whim: with less surface area, the battery never fills in a daily cycle. The maths is simple. A 350W panel delivers, in real conditions, between 70% and 80% of its rated power. That means ten 350W panels, at best, deliver around 2,800W of real output during the central hours of the day. If consumption exceeds that figure, the battery never fully charges.
The problem gets worse when the orientation is not optimal or there is shading. One case cited in the discussion: 3,240W of panels (six 540W units) rarely exceed 2,500W of production. South-facing orientation at a 15-degree tilt is not enough. Spring improves things, but winter is relentless. The conclusion is that sizing is not done with the panel's rated power, but with the real expected output in the worst month of the year.
The myth of the panel that charges "even without sun"
One participant asks whether, with fewer panels, the batteries still charge, just more slowly. The answer is yes, but with caveats that dismantle the myth. On cloudy days, output drops to 25% of rated power at best. And if a panel is partially shaded, production plummets: a shadow over a single panel can cut the output of the entire string. Some argue that on very cloudy days it is better to cover the panels to avoid unnecessary wear.
The experience of one participant with six 400W panels and two 4.6 kWh batteries is revealing: they reach 100% every day. But another with 3,240W of panels barely exceeds 2,500W of production. The difference lies in orientation and the absence of shading. For an apartment with an enclosed terrace and buildings around it, the verdict is clear: not viable.
Lithium, lead and the real cost of independence
Lead batteries only allow you to use 40% of their capacity if you do not want to ruin their lifespan. LiFePo4 lithium batteries allow deeper discharges and need less maintenance, but they cost around 1,000 euros for 4 kWh of capacity. One participant who built his own 4 kWh LiFePo4 battery runs a fridge, cooker and water heater on just three panels (two 300W and one 400W) and pays 10 euros a month on his bill. But he warns: "If cloudy days come, I have to charge from Iberdrola... but hey, it's rare".
The payback calculation for a consumption of 70 kWh a month (about 20 euros on the bill) is bleak: a 3,500W kit with lithium batteries can cost more than 3,000 euros. It never pays for itself. The conclusion that prevails is that off-grid solar makes sense for those with no other option, or for those who value independence over financial return.
The generator as an inevitable crutch
The recommendation that keeps coming up is clear: for a main home, an off-grid system needs a backup generator or a massively oversized panel array. A practical case: four 450W panels, two facing east and two facing west, with batteries and a generator used one or two hours a day for the washing machine and the water treatment system. Total cost: 2,500 euros installed plus 700 for the generator. For lighting and television, enough. Cooking and fridge, on gas.
The problem is that many of these kits are sold as complete solutions when in reality they are for weekend use. The open question is how many charge and discharge cycles the batteries can withstand in real use, with cloudy days and unexpected consumption. Nobody gives a definitive figure.
The question seemed simple: if you install an off-grid solar system with 3,500-watt batteries, do you need panels covering that same capacity, or is less enough? The answer from experience is uncomfortable: with fewer panels the batteries do charge, yes, but so slowly that in practice they are useless. A calculation doing the rounds in the discussion makes it clear: with a single 350W panel, it would take ten hours of peak sunlight to fill a 3,500Wh battery, something that never happens in reality, not even in summer. And that is where the problem begins.
Why online configurators reject less than 10 square metres
The automatic configurators of online shops refuse to install less than 10 square metres of panels for a 3,500W system. It is not a commercial whim: with less surface area, the battery never fills in a daily cycle. The maths is simple. A 350W panel delivers, in real conditions, between 70% and 80% of its rated power. That means ten 350W panels, at best, deliver around 2,800W of real output during the central hours of the day. If consumption exceeds that figure, the battery never fully charges.
The problem gets worse when the orientation is not optimal or there is shading. One case cited in the discussion: 3,240W of panels (six 540W units) rarely exceed 2,500W of production. South-facing orientation at a 15-degree tilt is not enough. Spring improves things, but winter is relentless. The conclusion is that sizing is not done with the panel's rated power, but with the real expected output in the worst month of the year.
The myth of the panel that charges "even without sun"
One participant asks whether, with fewer panels, the batteries still charge, just more slowly. The answer is yes, but with caveats that dismantle the myth. On cloudy days, output drops to 25% of rated power at best. And if a panel is partially shaded, production plummets: a shadow over a single panel can cut the output of the entire string. Some argue that on very cloudy days it is better to cover the panels to avoid unnecessary wear.
The experience of one participant with six 400W panels and two 4.6 kWh batteries is revealing: they reach 100% every day. But another with 3,240W of panels barely exceeds 2,500W of production. The difference lies in orientation and the absence of shading. For an apartment with an enclosed terrace and buildings around it, the verdict is clear: not viable.
Lithium, lead and the real cost of independence
Lead batteries only allow you to use 40% of their capacity if you do not want to ruin their lifespan. LiFePo4 lithium batteries allow deeper discharges and need less maintenance, but they cost around 1,000 euros for 4 kWh of capacity. One participant who built his own 4 kWh LiFePo4 battery runs a fridge, cooker and water heater on just three panels (two 300W and one 400W) and pays 10 euros a month on his bill. But he warns: "If cloudy days come, I have to charge from Iberdrola... but hey, it's rare".
The payback calculation for a consumption of 70 kWh a month (about 20 euros on the bill) is bleak: a 3,500W kit with lithium batteries can cost more than 3,000 euros. It never pays for itself. The conclusion that prevails is that off-grid solar makes sense for those with no other option, or for those who value independence over financial return.
The generator as an inevitable crutch
The recommendation that keeps coming up is clear: for a main home, an off-grid system needs a backup generator or a massively oversized panel array. A practical case: four 450W panels, two facing east and two facing west, with batteries and a generator used one or two hours a day for the washing machine and the water treatment system. Total cost: 2,500 euros installed plus 700 for the generator. For lighting and television, enough. Cooking and fridge, on gas.
The problem is that many of these kits are sold as complete solutions when in reality they are for weekend use. The open question is how many charge and discharge cycles the batteries can withstand in real use, with cloudy days and unexpected consumption. Nobody gives a definitive figure.
Also available in: Deutsch
Summary of a discussion on Burbuja.info - Foro de economía, actualidad y política., translated from Spanish and reviewed before publication. Read the full discussion (53 replies).
More summaries
-
Forum user runs electric car and solar panels at home
Are solar panels worth it for charging an electric car? Real data: 757 kWh/year, 4,450 km, and the scheduling and cost problems.
-
KFC sells 22 chicken wings for 9 euros, less than 50 cents each
22 KFC chicken wings for 9 euros cost less than 50 cents each. Details and terms of the limited-time offer in Spain.
-
Sans Segarra: 600 million views and his 'supraconsciousness' theory
Sans Segarra, an 83-year-old retired surgeon, fills theatres with his theory of supraconsciousness: 600 million views and hundreds of thousands of books sold.
-
Dehumidifiers: why a higher price doesn't mean better performance
Forum experiences: one user claims a sub-€100 LG outperforms a Delonghi costing twice as much. Minimum operating temperature and noise are the key factors.
-
Thermal underwear instead of heating: thin layers to beat the cold at home
Three thin layers keep you warmer than one thick one: thermal clothing from Decathlon and Lidl and the layering system for living without heating in winter.
-
Car insurance for young drivers: €1,200 a year and the Mutua trick
A 20-year-old pays €1,200 a year for third-party cover with theft. Mutua Madrileña is cheap but won't issue a claims history certificate. The tricks to pay less.