The first green lease was signed by a commercial tenant Tuesday in what Mayor Michael Bloomberg hopes will become blueprint for future leases and help cut the city's emissions.

Law firm WilmerHale signed what the mayor called an "energy-aligned green lease" for floors 41 to 45 of 7 World Trade Center, which will include floor-to-ceiling windows, efficient air conditioners and will use rain water collected from the roof to conserve water.

With the new green leases, the developer and the landlord will share utility costs on new buildings. If a developer spends $500 on energy-efficient lights, for example, the landlord would pay the full cost and deduct a portion each month from the tenant until it is paid off. Once that happens, the rent drops.

When the tenant moves out, the landlord can then charge the next tenant higher rent if it’s offset by a lower energy bill.

"Instead of lease negotiations being a zero-sum game the landlord and tenant in this case will both share the savings from energy efficiency improvement," Bloomberg said.

The mayor said the lease is part of PlaNYC’s long-term plan to reduce the city’s emissions by 30 percent by 2030. The city leases 12.5 million square feet a year, and large buildings are responsible for 45 percent of the city’s carbon emissions, according to the mayor.

Larry Silverstein is the developer of the building, which is at 90 percent capacity.