PowerDown charges a flat 15% of what you save on your cloud bill. If you do not save, you do not pay.
PowerDown saves by replacing On-Demand server instances with cost effective Spot instances and by powering down idle servers. PowerDown counts the compute portion of your cloud bill for these savings — network and storage components are not included.
When a resource is turned off by PowerDown, the savings are counted until the resource is next powered up. When an On-Demand server is replaced with a Spot instance, PowerDown calculates the difference between the two as savings. From those savings to your cloud account, 15% is charged by PowerDown and will be invoiced at the end of the month.
For resources that are powered down for long periods, PowerDown will stop counting the savings after 2 weeks. Thereafter, you will not incur any further charge from PowerDown for those resources. So you can safely use PowerDown to turn off any resource for a long duration.
For definitive information about pricing, please go to PowerDown Pricing.