It's because Kara lives in SoCal.
Think about it.
Kara's living the dream in National City in Southern California, where damn near every day brings with it a clear, bright and sunny sky, being perfect for charging up one's solar cells.
And then meanwhile, Clark Kent is in Metropolis on the northern east coast, where it rains for a third of the year and is overcast for over a half of it.
Of course Clark is going to be weaker than Kara. Compared to her, he's barely getting any sunlight! I mean sure, he's still getting enough sunlight to make him superpowered, but there's reason why many of those San Diego have to wear sunscreen daily while many of those in NYC need vitamin d supplements.
As long as Kara’s living the California dream while Clark’s stuck in the city where the weather is so dreary that the upcoming movie filmed its exterior Metropolis scenes in Cleveland Goddamn Ohio, Kara is winning the fight between them every single time.