Use value in table1 to pull through value from custom field in table 2 (VLOOKUP in Excel equivalent?

I'm trying to learn PowerBI and some of the things I took for granted in Excel seem to be extremely problematic in PowerBI! :(

I have two tables   UNITCOMP and LAMPS

UNITCOMP lists all of the streetlights in the Borough such as location, easting, northings, etc
LAMPS lists all the different lamp types eg 19W LED, Solar, 120W SON, 75W SOX
UNITCOMP and LAMPS are joined in a relationship based on a field called LAMPRATING

I have created a new calculated field in the LAMPS table called SUMMARY which ignores of the the W value and groups the light based on whether it is LED, SON, SOX, SOLAR etc.

What I want to do (and which was so easy in Excel but so difficult in PowerBI) is create a vlookup
In Excel it would be
=vlookup(UNITCOMP a1,LAMPS!C6:G22,2,FALSE)   *where the second row in lamps is the SUMMARY field

This would then return a value for every light showing what the summary value was for it.   I could then create a chart in the dashboard showing the lamp make-up in the Borough.

How the heck do you do this in PowerBI?!?