I think you're confusing your units for power vs energy. 3.5GWh (Gigawatt-hours) says nothing about the size of a power plant without also providing a time period that energy was generated in. You likely mean 3.5GW (Gigawatts), which is a measure of instantaneous power (in this case peak power). A 3.5GW plant generates 3.5GWh in 1 hour.