Half Wave Unregulated Power Supply
If you use one diode, you cut the energy in half. Does that mean the other half is wasted? No, it's not used. This is confusing, because search on internet shows that diode 'wastes' power when it's not conducting.
Whether 'not using power' is equivalent to 'wasting power' depends on the situation.
For example, for grid power - when you are not using power, someone else can use it - so it's not wasted.
When you're running a power generator - you use only half of its available power, so in that case you would be wasting power.