I'm building a lot of primitive shelves out of roof battens. Normally I'd use a handsaw like this for the job:
The battens are 2.5m x 50mm x 25mm.
What kind of handheld powertool would replace this handsaw optimally? It doesn't have to be extremely accurate. The shelves look something like this and we need to cut the battens on the go wherever necessary without much pre-planning: