That will depend on how big your yard is. Generally, a 50-100 lux level is considered enough. And, if the surface area of your yard is 50 sq. meter, the total number of lumens will be equal to 50x50 = 2500 or 50x100 = 5000 lumens. So, you could go with 50W LED flood light that can emit up to 6500 lumens.
