In the past decade, new thermal modelling tools have been offered to system designers. These modelling tools have rarely been used for the cooled instruments in ground-based astronomy. In addition to an overwhelming increase of PC computer capabilities, these tools are now mature enough to drive the design of complex astronomical instruments that are cooled. This is the case for WIRCam, the new wide-field infrared camera installed on the CFHT in Hawaii on the Mauna Kea summit. This camera uses four 2K×2K Rockwell Hawaii-2RG infrared detectors and includes 2 optical barrels and 2 filter wheels. This camera is mounted at the prime focus of the 3.6m CFHT telescope. The mass to be cooled is close to 100 kg. The camera uses a Gifford Mac-Mahon closed-cycle cryo-cooler. The capabilities of the I-deas thermal module (TMG) is demonstrated for our particular application: predicted performances are presented and compared to real measurements after integration on the telescope in December 2004. In addition, we present thermal modelling of small Peltier cooled CCD packages, including the thermal model of the CCD220 Peltier package (fabricated by e2v technologies) and cold head. ESO and the OPTICON European network have funded e2v technologies to develop a compact packaged Peltier-cooled 8-output back illuminated L3Vision CCD. The device will achieve sub-electron read-noise at frame rates up to 1.5 kHz. The development, fully dedicated to the latest generation of adaptive optics wavefront sensors, has many unique features. Among them, the ultra-compactness offered by a Peltier package integrated in a small cold head including the detector drive electronics, is a way to achieve amazing performances for adaptive optics systems. All these models were carried out using a normal PC laptop.