This ratio of average power to peak power is called the capacity factor or load factor.
It will vary by location, by day, and by technology.
It varies by location: some places are just sunnier than others. In the UK, 10% is a decent capacity factor; so an installation of 10 kWp will, averaged out over several years, yield 1 kW of power on average. In the tropics, the same technology might have a capacity factor of 20%, yielding 2 kW of power on average.
It varies by day: in winter, the capacity factor on a given day might be one tenth of what it is on a summer day, at a given location for a given technology.
It varies by technology: a PV installation that uses tracker motors so that the panels tilt during the day to follow the sun, will have a higher capacity factor than panels which are in one permanently fixed position, at any given location on any given day.
On units: Note that Wh/day is just another unit of power (it is energy per unit time). Wh/day is not a common unit of power, but you do sometimes see kWh/day. You can convert from kW to kWh/day easily: note that h/day is a constant (24 hours per day), so to get from kW to kWh/day, just multiply by 24. i.e. an average power of 1 kW is equivalent to an average power output of 24 kWh/day.
As LShaver wrote in a comment, tools like PVGis and PVWatts will help you calculate how the output of a given installation varies by location and over the year.