If the temperature during the day stays 99 degrees and only falls to 90 at night, newly hatched chicks don't need any heat.
However, while summer days have been consistently hot enough for chicks, night time temps often fall into the 70's and even the 60's. This is too cool for newly hatched chicks and they'll pile up, trying keep warm, and the risk of smothering is grave.
It doesn't take a lot of wattage to raise the summer time temps, even at night. The whole brooder box doesn't need to be heated, just a sufficient warm spot circle, where the chicks can come and go as they decide. Hope that helps. There is a Learning Center button at the top of this page. Lots of good reading in those various pages. Click this: https://www.backyardchickens.com/atype/1/Learning_Center