My solar panels have generated 1500 kWh of energy since they were installed 6 months ago.

Now, Einstein tells us that energy has mass. How heavy is 1500 kWh? We can use E = mc^{2} to find out.

Here, E is 1500 kWh, the speed of light c is 186000 miles/second. Unfortunately, we can’t just divide 1500 by the square of 186000. Sure, that would give the mass, but not in any unit I’m familiar with – when you tell me how much something weighs, I want to hear it in pounds, or kilograms, or ounces… even centiweight would do, if I have Google handy.

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