Additionally, you'd have to subtract the monthly power bill from those returns. I.e. you can't say "I would have doubled my $20k return by investing it for 10 y

Additionally, you'd have to subtract the monthly power bill from those returns. I.e. you can't say "I would have doubled my $20k return by investing it for 10 years", because you can't defer your monthly power bill for 10 years and pay it in a lump sum after you doubled your money in the S&P. Paying $100+/month for power would substantially impede the interest compounding effect.