CREATE TABLE table_19920 ("Series Ep." TEXT, "Episode" FLOAT, "Segment A" TEXT, "Segment B" TEXT, "Segment C" TEXT, "Segment D" TEXT)

Name the number of sement d for solar water heaters
SELECT COUNT("Segment D") FROM table_19920 WHERE "Segment B" = 'Solar Water Heaters'
SELECT COUNT("Segment D") FROM "table_19920" WHERE "Segment B" = 'Solar Water Heaters'