Do I make anything from a night when a guest charges their car?

That depends on the price you charge for that electricity. If you charge the same price as for household electricity, you'll only keep a few euros on a charging night with, say, 18 kWh, while you do have more wear and tear and peak consumption. If you charge a higher, separately metered charging price, comparable to public charging points, that margin increases significantly without the rental price changing. Without separate metering, the charging session disappears into total consumption and you never know whether the night brought profit or loss.