GAUGE is fine, just note that as data is consolidated you will end up with values between 0 and 1 representing the amount of time during the observation interval the heater has been on ... if you multiply the data with 100 you would get a percentage.
Do not do any of the XFF or LAST RRA bits, just your normal
"RRA:AVERAGE:0.5:1:288 " + // 5 min avg., last 24 hours
"RRA:AVERAGE:0.5:12:168 " + // 1 hour avg., last 7 days
"RRA:AVERAGE:0.5:48:315 " + // 4 hour avg., last 30 days
"RRA:AVERAGE:0.5:288:365" // 1 day avg., last 365 days
will do fine ... except for added detail you may want to add MIN and MAX variantes to the three top consolidation levels.