Solarpunk

well if you are extremely strict about everything having to be powered by on board solar, you could probably just use potential energy. Fly as high as possible and drop a guided projectile down that is as heavy and as aerodynamic as possible. like a light version of the rod from god.

Also we've seen how effective cheap drones can be in warfare. the same could be true for small solar drones, to drop various payload on your enemy. Yeah they are slow and have big ass wings. but still they are silent and cheap. If you design them well some of them could loiter "forever".

And also I would say solar powered artillery works. either using linear or rotary electromagnetic accelerators. Like a railgun or spinlaunch.

otherwise you could build gigantic transparent blimps covered in fresnel lenses that focus sunlight on an extremely small spot. normal sunlight has ~1kW/m². The Indirect Fire Protection Capability-High Energy Laser has 300kW, so you need ~300m² to match its power, so like 17.32x17.32m dimensions. The Hindenburg-class had like 10,000m². So you could have something in the ballpark of a 10MW laser. The destructiveness depends on how well you can focus it and how much enrrgy you loose via the air. And 2x longer blimps get 4x the death ray power. So you really benefit from scale. Or you just use a fleet of blimps all focusing on one point. You know, don't put all your eggs in one basket.

You could also use mylar if fresnel is too expensive.

But only works during the day lol.

like so:

mass produce these, outfit them with cheap drone hardware or something. unleash a swarm of them over your enemy burning them to a crisp.