Required power is the combined power of every device you selected, running at the same time. Recommended capacity covers the energy they will use over the time you set, plus a margin - this accounts for conversion losses and avoids draining the battery completely, which extends its lifespan.

Example: a 150 W fridge running 8 hours, plus a 60 W laptop running 4 hours, need 150 x 8 + 60 x 4 = 1440 Wh of energy. With a 0.9 efficiency coefficient and a 20% reserve, the recommended capacity works out to 1920 Wh.