Game benefits greatly from playing in 60 FPS, so to optimize battery life I've tinkered with GPU clocks and landed on 1200 MHz as a safe and stable bet (since the initial phase of each wave is usually the most demanding).

For those looking to squeeze as much playtime from battery as you can: 40 FPS limit, motion blur and 700-800 MHz on the GPU clock (to warrant stable frametimes).

Regardless of GPU clock and power limits - the game can drop some frames in the wave opening phase later in the game.