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Before you do that, make sure it's worth it. And do that by making sure you know how to calculate how much you're paying per kWh. Which means figuring out how much your flat monthly rate is, plus any riders your power company charges also monthly, plus state tax.

For me in Alabama, that's $14.50 per month flat charge, plus 75 cents for a natural disaster fee (total $15.25) plus a 4% state tax (total $15.86). That means before I installed my solar system, I knew that even in a totally optimum situation where I went a whole month without drawing any power from the grid I'd still pay $15.86. There's no getting around that without going off-grid. So count that as a portion of your bill your power wheel will never cut down on.

Then subtract that from your total bill to get your overall usage charge. This past bill was $54.06 (not bad for an all-electric two-story house in Alabama where we run the A/C in April), so my overall usage charge was $38.20. Divide that by the # of kWh you pulled from the grid that month (for me that was 285 kWh) and you get your true rate per kWh (for me that was 13.4 cents per kWh). For every kWh you save with your system you're saving that much in money.

My solar inverter recorded that I used a total of 1,093 kWh for that billing period. That means my solar system saved me 808 kWh (kWh consumed minus kWh bought from grid). Because I know from the step above that my true kWh rate was 13.4 cents/kWh, that means the solar system saved me $108.34 that month.

Be sure that math works for you before you spend money on a power wheel. But once you figure it out, also remember to factor inflation. Basically, the amount I'll save next year per kWh is more than I saved this year per kWh, then the year after that is more per kWh, then more after that, etc. For that I assume a 3% inflation rate. Once I realized that inflation is part of the equation, I knew that the cost of solar would pay for itself in decent time. Maybe the same will be for you with the power wheel.