Hi,
Efficiency = ( Power out / Power in ) * 100%
99% efficiency would mean, with 100W power input, you get 99W power output. This, I feel, is not possible practically. Maybe upto 95%, higher becomes very difficult, since every component has some resistance, and wastes energy through that.
However, simple efficient circuits are those that reach 80-85% efficiency. Above that, the system would be highly efficient.
You can find the efficiency, like:
If you have an inverter, measure the output voltage and current flowing to load, measure the input voltage and current flow. Find Pout and Pin and find Eff = (Pou/Pin)*100%
Hope this helps.
Tahmid.