How to calculate voltage-power to lower voltage-power?

I have a solar panel 18V 2W. How do I calculate how much power that is at 5V?

• Depends on the efficiency of your 18V to 5V converter. But it'll be something around 2 Watts minus inefficiencies of your converter. Also, don't forget an 2 Watt solar panel is only 2 Watts in the best case, usually it will be significantly less. – Puffafish Jul 2 '19 at 12:17
• Just remember that you can't get a power output that is higher than the input when you are stepping the voltage up or down. Any voltage converter cannot have an efficiency of more than 100% – Harry Svensson Jul 2 '19 at 12:23

You will have 2W less losses at 5V, which means more current.

2W / 5V = 0.4A max assuming no losses...

• Thank you for your fast reply – Henrik Lauridsen Jul 2 '19 at 12:54