Internet-of-things (IoT) devices utilising energy harvesting often use solar cells as their harvester. The same solar cells can also be used to sense illuminance (in lux). Current and voltage sensing systems are researched. Also, the accuracy limits of the system are researched to see what accuracy is expected. Different design options, based on the maximum power point (MPP), open-circuit voltage, and short-circuit current are compared. A design based on floating-point (FP) logic and a look-up table (LUT) are made in Verilog. The final LUT design is synthesised using Cadence Genus and verified using an FPGA and Cadence Spectre circuit simulator. The design has an area of 1.46 mm² and has an average power consumption of 121.68 μW. It can sense illuminance with an average error of 7.0% and a maximum error of 15.9%, in a range of 188 - 6130 lx for an indoor solar cell, and with an average error of 4.7% and a maximum error of 13.5% in a range of 272 - 43400 lx for an outdoor solar cell.