Just to clarify: There are 2 ways to use Siri. "Hey Siri" and just holding down the home/side button.
"Hey Siri", which is automatically activated when you say those words to your device can be used when not connected to power only on an iPhone 6s or later.
iPhone 6, 6 Plus and earlier require the phone be connected to power for "Hey Siri" to work automatically.
Holding down the home/side button to activate Siri works on any phone and does not require being connected to power.
Use Siri on your iPhone, iPad, or iPod touch - Apple Support
You can use Siri without pressing the Home or Side button. First, make sure that "Hey Siri" is on under Settings > Siri & Search > Listen for "Hey Siri." Then say "Hey Siri" and ask your question. For example, you can say "Hey Siri, how's the weather?"
If you have an iPhone 6s or later, iPad (6th generation), or iPad Pro*, you can use "Hey Siri" whenever you want. If you have an iPhone 6 or earlier, or another iPad or iPod touch model, you need to plug your device into power first.