Can you use a solar powered backup generator as a energy credit deduction?

Yes.  According to the IRS instructions for Form 5695: 

Qualified solar electric property costs are costs for property that uses solar energy to generate electricity for use in your home located in the United States.

No distinction is made between primary system and backup system.  You may be able to take a credit of 30% of your costs of qualified solar electric property, solar water heating property, small wind energy property, geothermal heat pump property, and fuel cell property.

To enter the information for the credit in TurboTax, please follow these steps:

1. Click on Federal Taxes > Deductions & Credits.
2. In the Your Home section, click on the Start/Revisit box next to Home Energy Credits.  (See Screenshot #1, below.)
3. On the Energy-Saving Home Improvements screen, click Yes and then Continue. (Screenshot #2)
4. Continue through the screens until your come to Enter Any Additional Energy-Efficient Improvements.  (Screenshot #3)
   
5. Entering the information on the solar generator.  Click Continue when done.
   
6. TurboTax will then calculate any credit due.  

For additional information see the IRS instructions for Form 5695.