How can I use a screensaver without draining battery on iPad?

"Screensaver" software was something used years ago to prevent screen to cathode-ray and plasma screen displays.

Modern LCD displays are considerably less susceptible to damage from persistent display of a static image for long periods. These displays are typically automatically turned-off or the display backlight dimmed when the associated device is inactive. As such, screensaver software/images offer no benefit, contributing little more than unnecessary battery utilisation.

The most significant power-draw by a display is its backlight. When dimmed, or entirely turned-off, the display draws little or no power.

Run it while the iPad is getting power from the wall?

If you are running a screen saver for entertainment, the screen backlight is going to be on, and you are going to be draining power for that. If you are on battery power, there is no place for that to come but from the battery.