If your balcony collapses due to 800W worth of solar it’d also collapse due to a couple of planters or a fat friend coming over so I guess you should take such worries as an admission on their part and withhold rent until they prove that the balcony is safe to use.
If you are talking about Germany, that is no longer true - landlords can not deny installations without sound reason:
https://www.tagesschau.de/wirtschaft/verbraucher/balkonkraftwerke-mieter-gesetzesaenderung-100.html
The surrounding lawa were updated July this year, so it is a recent change.
If your balcony collapses due to 800W worth of solar it’d also collapse due to a couple of planters or a fat friend coming over so I guess you should take such worries as an admission on their part and withhold rent until they prove that the balcony is safe to use.