That's a practical question that doesn't have a single, universal answer, as it depends heavily on the trash can's material and construction. For a standard, mid-sized plastic outdoor bin you might buy at a home improvement store, the lid isn't designed to hold significant weight. It's primarily a cover, not a shelf. You could likely place a few lightweight items like a potted plant or a bag of pool chemicals on it temporarily, but we're talking maybe 5-10 pounds (2-4.5 kg) maximum before you risk cracking the plastic or damaging the hinges.
Heavy-duty plastic or metal cans (like commercial ones) will have more robust lids. A galvanized steel lid might support 25-50 pounds (11-23 kg) if the weight is evenly distributed, but it's still not advised. The real point of failure is often the hinge pins or the plastic rim the lid attaches to, not the lid panel itself.
Manufacturers rarely publish a specific "lid weight capacity" because it's not an intended function. The key takeaway: treat the lid as a cover, not a load-bearing surface. Placing excessive weight can void warranties, cause breakage, and create safety hazards. For regular heavy items, consider a bin with a flat, reinforced top or a separate storage solution.