Power consumption of an inverter
From the efficiency rating of 98% you can calculate how much power it is consuming:
$$Eff=\frac{Pout}{Pin}$$

So if you have \$100W\$ going to your load, then you'll have
$$\frac{Pout}{Eff}=Pin \\$$ $$-$$ $$  \frac{100W}{0.98}=102W$$
And then
\$102W-100W=2W\$ consumed in the inverter.