Ejector, a 1kw PSU will run happily with loads of UP TO 1000w. It'll only draw as much power as the system requires so if your PC needs 300w to run then that is all that will be used.
Being in a pedantic mood this morning, PSU's are only about 80% efficient. the odd 75W or so are converted into heat, if they were 100% efficient they wouldn't need fan cooling.
Sorry.
