Updating a table from a temporary table where clause error in MySql Community
SQL queries always explicitly reference their tables by name, apart from any references to the columns in the tables. Since your query doesn't include a reference to the 'a' table explicitly as a table, the engine has no idea what a is. I'm a SQL Server guy, but based on some quick Googling of "MySQL update join", looks like the statement should be:

UPDATE residential_summary
         INNER JOIN a ON residential_summary.meter = a.meter
SET residential_summary.load_factor = a.load_factor;


The UPDATE ... INNER JOIN ... tells the engine that this query will be working with the residential_summary and a tables. The INNER JOIN ties residential_summary and a together based on the values of meter in both tables, so that only residential_summary rows that have a matching row in a will be updated.