I doubt anyone will ticket you over being 10 kilos extra...
On top of that you could probably pick any other part to make lighter and come out cheaper than buying fancy solar panels
It is not about + or - 10 kg. But things definitely add up. 10kg here - 10kg there.
Solar panels is something that is under the control of whoever buys the vehicle. You have almost no influence of the vast majority of what makes up the van.
You also risk insurance issues when your car is over weight.
Believe me - weight is definitely an issue.
The weight is usually determined by 4 scales (1 for each wheel).
Then they look at your papers which also includes information about max load at the front + back.
If you are over weight you have to get rid of the additional weight and if you can't they will seize the car. You get fined. It does happen.
It happened to me twice in Germany, once in Norway and once in France.
I read an article about it in Czech a while ago, so I don't have a link at hand, but you should be able to find it easily.