The modbus binding doesn’t support transforms so you will have to create a proxy Item and a rule to convert the value from I_AC_Power and post it to the Proxy Item.
Number I_AC_Power {modbus="slave3:0"}
Number I_AC_Power_Proxy "Wattage_PV [%d Watt]" (Solaredge,emoncms)
rule "Update I_AC_Power_Proxy"
when
Item I_AC_Power changed
then
I_AC_Power_Proxy.sendCommand((I_AC_Power.state as DecimalType) / 10)
end
Only put the Proxy on your sitemap. If you have rules that do something based on the value of I_AC_Power, have them use the Proxy instead.