well what I use are CTs mine are particularly 100 amp CTs so accuracy is about +/-12 watts you be using lower amperage I would assume you just use a smaller CT as mine is 10bit (1024 steps) if it 5 amp CT is about 1/2 watt accuracy.. you also could use higher bit 12 bit or higher to improve accuracy. if you want you could use iotawatt it measure 14 circuits at once . it upload will upload to a website either emoncms or influxdb with some movement towards MQTT currently I believe .
still looking for a way to transform the the total of RRD graph in open wrt-- where is the rrdopts.conf ( if it exists)or other mention it would seam not to be a hard thing to do but I can not find any info on how to to do it. cati and nagois seam to to tell you how but nothing I can find with openwrt
looking through your source code wondering how workable it is to add by example transform_rpn to the total ie min,avg,max
_tif( _args, "CDEF:%s_avg=%s_avg_raw,%s", inst, inst, source.transform_rpn )
and add to Total source.transform_rpn
if source.total then
_tif( _args,
"CDEF:%s_avg_sample=%s_avg,UN,0,%s_avg,%s,IF,sample_len,*",
source.sname, source.sname, source.sname, source.transform_rpn
or what ever to it as I not 100% clear on your on how it works