If I install solar panels, can I sell unused electricity back to the grid

Yes, you can. The government in Spain got rid of the "sun tax" in 2018 when you had to pay the grid an admin fee for generating your own electricity.. You can sell it directly or sell it so you can use it in the future and they generally pay 6 to 8 cents per kWh. 

If you need it in the future, they sell it back to you at 18 cents per kWh. So if you overproduce one month by 500 kWh but then need to buy back 100 kWh in a month where it was cold and cloudy, you would benefit by about 12 euro. 

But to answer your question, yes.

You might find the following link informative:

https://www.ecovidahomes.com/blog/can-you-sell-electricity-back-to-the-grid-in-spain/#:~:text=How%20suppliers%20compensate%20homeowners%20who,need%20to%20run%20their%20home.

John. Thanks, while your information is disappointing, its critical to have this piece of information. The property is not always used and I was hoping to a) run cameras and a broad band route when not there using solar and B) perhaps generating some money by selling the unused solar and reducing the costs of the property. 

The best I think now is to look at a cheap solar build that would deliver a above? 

Colm

To make even more use of solar panels, you can buy "Power Stations" that you can plug solar panels into, and they will store your excess electricity during the day, ready for you to use when you require it. Then there must be a way to divert it back to the grid for when you are not at your home.

I have the following here in the UK when at this time of the year, I can power my powerful, PC, the widescreen monitor, the internet, my kettle, my toaster, my slow cooker, all my hot water, and even my air fryer. Max output is 2000 Wh so it wouldnt power an electyric oven though.

Imagine what it could achieve with the endless sun in Spain?

Thanks that was helpful, I will think about it. Colm