That would involve a lot of calculation and where one lives is a factor. Many months of the year I break even or make a slight profit. Over the course of a year my average is probably about $40 per month. I haven't added it up for the past year, but that's in the ball park. I have about 4100 square feet under roof including a full basement which I use. The system cost around $32,000. I did get a $10,000 tax credit and another $1000 rebate. I used $$ from my IRA to map the balance, so there is no interest on the package. It's all paid for. The IRA $$ was going to need to be withdrawn due to my age. The estimated return on investment is around 7% depending on the price of power which is ever increasing. At 25 years the panels are projected to be more than 80% efficient. I won't get into whether or not solar power is a good investment for the country, but it is working for me. You might say I chose to pay up front for my power for the rest of my life. As the price of electricity goes up, it will be even better.