Electrical independence: the real numbers behind solar panels

Electrical independence: the calculation that debunks the solar myth

An average Spanish household consumes around 300 kWh a month, according to the figures cited at the start of this discussion. With electricity at €0.30/kWh, the bill comes to about 90 euros. If the price doubles, it goes to 180; if it quadruples, to 360. The question is inevitable: how many panels does it take to go off-grid? The first calculation doing the rounds —28 panels— is based on a unit error that needs to be dismantled before anyone spends their money.

The mistake of confusing watts with watt-hours

The first estimate put on the table claimed 28 solar panels were needed to cover 10 kWh a day. The reasoning was simple: if one panel delivers 350 W a day, you need many. The flaw is glaring. A 350 W panel's rated output is not 350 watts a day, but 350 watts for every hour of full sunlight. Confusing power (kW) with energy (kWh) is the most common mistake in this kind of calculation.

Once the error is corrected, the math changes. With 8 hours of average sunshine in Spain, a 350 W panel produces 2.8 kWh a day. To cover 10 kWh you would need four panels, or six if you factor in orientation losses. A more conservative calculation, based on real installation data, raises the figure to 14 panels of 350 W to guarantee 10 kWh a day in winter. The difference between four and fourteen is no detail: it is the difference between one budget and triple that.

How many hours of sunshine are actually usable?

This is where the battle of figures begins. One side of the analysis argues that average solar irradiation in Spain is around 3,000 hours a year, with Seville at 3,500 and A Coruña at 2,450. The other side counters that those are not peak sun hours: only a fraction of that time allows full output. In the south, winter does not even reach four peak hours and summer never exceeds seven. The real annual average, with optimal tilt and azimuth, comes to about 1,600 hours.

The practical consequence is that a 500 W panel, under ideal conditions, delivers an average of 2 kWh a day. And that is without counting clouds, shade or dirt, which take an extra 10% to 14%. Any calculation that ignores this seasonality is selling smoke, and winter will be there to remind you.

Batteries: the black hole of profitability

Storage is where the numbers break down. A 10 kWh lithium battery costs around 4,000 euros; a 19 kWh OPzS lead-acid one, about 4,200. The difference lies in the cycles: lithium promises 6,000 cycles at 95% discharge, while lead does not tolerate deep discharges without shortening its lifespan. Some argue that lead offers more short-term reserve for the same money; the technical answer is that lithium withstands more cycles and does not degrade in the same way.

Even so, payback is the weak point. A user with 4.4 kW installed reports 6,300 kWh produced in almost a year, at a cost of 3,300 euros in self-build mode. Another, with 16 panels of 450 W and a 10 kWh battery, gets 20 kWh on sunny winter days. The numbers add up for those who build the system themselves and consume during the day. For those relying on installers, the budget skyrockets: 9,000 euros in materials plus labour that can double the bill.

The deeper debate: profitability or survival?

The discussion drifts into more uncomfortable territory. Part of the analysis insists that the goal is not payback, but having power when the grid is down. The other side replies that batteries have a limited lifespan and that an off-grid system cannot sustain a Western lifestyle indefinitely. The question hanging in the air is how many years of blackouts a scenario can last before the batteries give up.

In between, mini wind turbines appear as a complement for areas with little sun and plenty of wind, though they require maintenance, height and rotor diameter that make the contraption more expensive. And the diesel or petrol generator remains plan C: noisy, but able to get you out of a tight spot.

The figure that unsettles is not how many panels are needed. It is that, with electricity at €0.60/kWh, the annual bill for an average household would exceed 2,000 euros. And even so, most solar payback calculations still fail to break even in under a decade.