Apparently the HTC Desire and Nexus one both use the same 1400 mah battery.

source

While on the topic of batteries,instructions on how to calibrate Nexus one (I assume same for Desire) batteries:

Hi *******, I understand your concern regarding battery life on your Nexus One device. The following steps should significantly extend the battery life on your phone. Please connect the phone to the charger with the phone powered on, and allow the phone to charge until the notification LED is green, indicating the device is fully charged. Disconnect the phone from the charger, and power it off. Reconnect the phone to the charger with the phone powered off, and allow the phone to charge until the notification LED is green. Disconnect the phone from the charger and power it on. Once the phone is powered completely on, power it off again and reconnect it to the charger until the notification LED is green. Disconnect the phone, power it on, and use it. You need to use this sequence only once. If the issue of battery life on our phone persists, I recommend you contact our HTC accessory department directly.

source