Really rough terms trying to allow real world application including heat losses etc, 210,000btu would be about 4 x 2400w elements.
Note: This is not using an energy calculator as such. LPG burners have more losses into the surrounding area etc. This is just a QldKev guesstimate.
Did you know 210,000btu is about 62,000w,
That is a lot of heat loss when on the QldKev scale you can get approx the same ramp rate from about 10,000w of elec elements.
But electricity Vs LPG is a whole debate in itself.