The panel is supposedly a 2.5W panel and it's driving a 4.8V light bulb. The meter readings suggest I'm getting about 1.2W from the panel in this position. Which for late afternoon isn't too bad. I'm planning on repeating this test over the coming days in different light conditions to see how it compares. I'm also going to look at what's needed to connect it to the chip to get best results for trying to power that (It's likely to need both panels and I may need to look at bigger panels to give enough voltage).
For reference there's a Pi3 in it's case sitting on a similar panel (I bought two matching ones but I'm testing with one for now), and to the left is a CHIP and 5000mAh battery.