Your refund is irrelevant, what counts is your tax liability, what the government keeps over the year. For example, if you have $5000 of withholding and get a $1000 refund, your tax liability was $4000. The energy credit can't be more than your tax liability.
The credit is 30% of the cost, assuming you meet the efficiency requirements, up to certain maximums.
For central A/C, or efficient gas or oil furnace, the credit is 30% up to a maximum of $600 for each component (a traditional system might include both A/C and furnace, they are listed separately and separately eligible. For a heat pump system or biomass system, the credit is 30% with a maximum of $2000.
Read the instructions and make sure the system you want to buy meets the requirements. "Energy Star" is not enough, the rules are more specific.
https://www.irs.gov/forms-pubs/about-form-5695
