What is the total number of t (µm), when Technology is u c-si?
CREATE TABLE "comparison_with_other_technologies" (
  "technology" text,
  "pct" real,
  "v_oc_v" text,
  "i_sc_a" text,
  "w_m" text,
  "t_m" real
);
SELECT COUNT("t_m") FROM "comparison_with_other_technologies" WHERE "technology"='u c-si';