Outdoor power supply consumes electricity very quickly

What if my power supply is 50% efficient?

If we assume, conservatively, that your power supply is 50% efficient, you'd still only be drawing 100W at the input side, not 300. Also of note is the fact that if your power supply were outputting a whole 300 watts, the input would be even more than 300W.

How much electricity does a 100W light bulb consume?

For e.g. a 100W light bulb consumes 100W of electricity per hour. If left 'on' for 10 hours, a 100W light bulb costs 1kW and 1 unit of electricity is burned. Various devices have a power rating. If the power rating is not directly given, voltage and current ratings are given.

How much power does an SMPS use?

The product of voltage and current gives us the power consumed by the device. For e.g. an SMPS generated a supply of 12V with constant current rating of 1.5A hour. Thus, an SMPS has power consumption of 12x1.5=18W-hour. Once you have the power ratings of all the devices in your household, estimate your usage in terms of hours per day or per month.

How much electricity does a light bulb use per month?

Once you have the power ratings of all the devices in your household, estimate your usage in terms of hours per day or per month. For, e.g. if you use a 100W light bulb for 5 hours a day, you use it 150 hours a month. Thus, a light bulb will consume 100X150 = 15000W or 15kW or 15 units of electricity per month.

What does the 300 watt rating on a power supply mean?

The 300 watt rating on the power supply just means that that's what it can output, not that it always outputs that. If you attached 5 more light bulbs, all in parallel, then it would produce the whole 300 watts. The only trick is that we have checked how much power the power supply is outputting.