Using the rated wattage of a device might not provide an accurate picture of the energy used. A wattage meter is better. There are a lot of them on the market. Here's one on Amazon that I've been using:
Poniie PN1500 Portable Micro Electricity Usage Monitor Electrical Power Consumption Watt Meter. It costs about $18.
My light panels are rated at 240 watts, but at full power they only draw 233 watts. I also haven't yet run them at full power, which would be 466 watts for the two panels. They're set to a combined 350 watts now. One panel is a bit brighter than the other. Using a power meter to set the power level is also much more accurate than the markings on the dimmer switch.