Select range of non-identical date from different table
There is no connection between these two tables, so a join won't really get what you need. If everything goes into a single result set, you need to UNION them together. Note that when doing a union, you get one set of columns, and you can't mix datatypes within a column (no concern here since all your data are FLOATs). Here's an example starting point:

SELECT
  'temp' AS sourceTable,
  id,
  ts,
  temp AS measure1,
  humidity AS measure2,
  NULL AS measure3
FROM 
  temp_data
WHERE 
  ts &gt;= @StartDate AND
  ts &lt;= @EndDate
UNION ALL
SELECT
  'power' AS SourceTable,
  id,
  ts,
  lux AS Measure1,
  kw AS Measure2,
  volts AS Measure3
FROM 
  power_data
WHERE
  ts &gt;= @StartDate AND
  ts &lt;= @EndDate