In case you have a feed-in with kW you could create a specific time series with kW/kWp or with kW/kW_max or kW/kW_something.
For the ep_costs you have to define accordingly EUR/kWp or EUR/kW_max etc.
As a result for the invest variable you will get the installed capacity in kWp or kW_max or kW_something.
All variants will work but fancy units may confuse people so I would use common/standardised units.
In an older version of solph the maximum was set to one by default, but we fixed it. If you think that values greater than one are cut of by the solver, this might be a bug and should be reported. In that case you should create a minimal example that shows the bug.