Even if you purchase multiple products you can only get a maximum of $1,500 over the 2-year period (2009 & 2010). Basically you can spend up to $5,000 during this 2 year period on a single or multiple products, and get 30% or $1,500 (30% of $5,000 = $1,500) back as a tax credit. If you get the entire $1,500 credit in 2009, then you can't get anything additional in 2010.
The $1,500 maximum does not apply to geothermal heat pumps, solar water heaters, solar panels, fuel cells, and wind generators. These are all eligible for a 30% tax credit with no upper limit. However, the tax credit for fuel cells is limited to $500 per .5 kW of power capacity. The tax credit with no upper limit is completely separate from the one limited to $1,500 - so you can get both. For example, you can get $1,500 back for new windows, and $3,000 back for a new geothermal heat pump - for a total tax credit of $4,500.
If this was helpful please press the Accept button. Positive feedback is also appreciated.