18 LEDs use much more current than the single LED in a solar garden light, and AA battery cells provide much more current than the voltage-boosted AAA cell in a solar garden light.
I have a solar garden light string sold last year at Dollarama (but not sold this year) that uses one AA Ni-MH cell, a voltage boosting circuit with high output current, a pretty large solar panel and about 12 LEDs.
This year Costco sells a box of LED fence post lights that have 12 LEDs and a pretty large solar panel. I can't remember how many AA Ni-MH battery cells are in one and it is a nightmare to take apart and put back together.