That 3-6% range is stored energy after the plant consumed some of the produced glucose to maintain itself and grow. Photovoltaics don't do that on their own, s

That 3-6% range is stored energy after the plant consumed some of the produced glucose to maintain itself and grow. Photovoltaics don't do that on their own, so you can't compare that to a solar cell's nameplate efficiency. Either you have to compare PV efficiency with gross glucose production or you need to compare the full PV lifecycle (production, installation, maintenance and recycling) with the harvestable sugar content.