Solar panels: why producing 8,000 kWh won't cut your bill

Solar panels: producing 8,000 kWh is not the same as saving 8,000 kWh

According to an analysis circulating in the thread, an average Spanish family consumes around 8,000 kWh a year. A twelve-panel installation produces, on paper, those same 8,000 kWh annually. The conclusion the salesman sells is immediate: a zero bill. The problem is that electricity has to be consumed at the very moment it is generated, and that detail —the only one that matters— rarely comes up in a call from a hidden number.

The basic mistake is confusing power with energy. kW measures what is consumed or generated at a given moment; kWh, what accumulates over a period. A typical panel delivers around 0.4 kW, so twelve add up to roughly 5 kW of instantaneous power. With that, over a year and during daylight hours, those 8,000 kWh are produced. But between 9:00 and 19:00 the house is empty and consumption is minimal. The surplus is fed into the grid and given away.

The kWh you produce by day is not the kWh you use at night

The timing mismatch is the heart of the matter. Household consumption peaks are concentrated between 6:30 and 8:30 and after 19:30, precisely when there is no production. If the system is sized to cover annual consumption rather than instantaneous consumption, the bill barely moves and the utility keeps the injected energy to resell it to another customer. A tidy business. For them.

Hence several analyses agree on a figure that is uncomfortable for the seller: a home with normal habits needs no more than four panels. Anything installed above what is consumed in real time is wasted production. Oversizing responds not to technical criteria but to two commercial goals: inflating the quote and increasing the volume fed into the grid.

How much do utilities pay for surplus energy fed into the grid?

Very little. Around 5 or 6 cents per kWh injected, a rate that has not moved despite the brutal rise in electricity prices in recent years. The compensation is applied as a discount on the next bill and anything left over is lost. In other words, the surplus is not paid in money: it is traded, with a cap.

The alternative most often mentioned by those who have run the numbers is changing habits before buying panels. Scheduling the washing machine, dishwasher, water heater, air conditioning or electric car charging during the middle hours of the day, and signing up for a time-of-use tariff for what is consumed without sun. The system works when production and consumption coincide; when they don't, the energy is given away.

Batteries: storage still doesn't pay off

Storing surplus energy would solve the mismatch, but the cost rules it out. Batteries are expensive, require maintenance and have a limited lifespan. Some claim that a 7.6 kWh system with eight LiFePO4 batteries, a 3,000 W inverter and six panels can be put together for around 2,000 euros including VAT, though that figure corresponds to a DIY build, not a turnkey installation. Others point to repurposed truck batteries as a survival solution, not a profitable one.

The consensus, with nuances, is that batteries only make sense in off-grid homes with modest consumption. For a grid-connected house, the math rarely adds up.

The subsidy trap and the antiestéticar of changing rules

When there is a subsidy, the math changes. Municipal, regional and European grants can reduce the initial investment until it becomes attractive. The problem is the fine print: conditions change, rules are modified, and those who jumped on board under one regulatory framework find a different one a few years later. Distrust of the regulatory framework is, in many cases, greater than distrust of the salesman himself.

Some go further and argue that the only sensible installation is one that is not reported to any authority nor contracted with any utility: zero export, pure self-consumption and nothing else. It is the route that avoids paperwork, inspections and regulatory surprises, though it forgoes any compensation.

The case that does add up: 7,200 euros and 11-euro bills

It's not all smoke. A 4 kW installation set up in March 2022 for 7,200 euros generated 4.48 MWh until the end of the year, of which 2.04 MWh were self-consumed and 2.44 MWh were exported. The average bill came to around 11 euros a month, with a peak of 14.98, compared with the 150-200 euros previously paid depending on the season. The payback period, in that case, is around three years.

The difference from the rip-off lies not in the technology, but in sizing and habits. Those who produce what they consume, win. Those who produce to give away, foot the bill.

How many salespeople are willing to explain that difference before signing?