The general consensus it to have the A/C on as much as possible. Apart from the obvious benefits in the summer months, A/C also dehumidies the air which helps with de misting during the cold, wet weather.

Obviously you don't want to run the A/C on Max cold when the outside temperature in as low as it is now and there's no need to. Just set the heat to whatever's comfortable with the A/C on until the car has warmed up, then turn the A/C off leaving the standard heater on. If you experience the glass misting up, switch it back on for a while.

Modern A/C systems are a lot more efficient than they used to be and will automatically switch the compressor in/out as and when needed so don't run permanently (called "duty cycle") so if you are accelerating or have a larger than normal engine load, it will turn the compressor off aiding engine power and therefore fuel consumption.

Get the A/C serviced every year when you have the car serviced and don't forget to have the cabin (pollen) filter changed and an anti bacterial treatment to help stop any nasty smells from the build up of condensation that all A/C equipped cars get and you won't go far wrong.

Me and the Mrs have had A/C or climate in 7 cars since 1999. We use it when needed in summer (and in winter to demist) but we always run it for a least 10 minutes every 2 weeks as per the advice in the manuals. In that time only had one car system cleaned and it made a right mess of the system, garage sorted it FOC but never again, car stank for weeks after their balls up. Other than that the only issue was a failed condensor replaced under warranty and a regas under warranty. I take the filter out every few months and knock of any debris and replace as per the service shedule.

Use it sensibly and it will be fine.

In my last two cars I have run the ac all the time. Hardly any difference in fuel consumption from what I can tell andno loss of power. Plus if you run it all the time it stops the system from drying out.

Based simply on my observations I think that with A/C on my cars have done about 6% less mpg. The wifes car with climate appears to be affected less simply because it only cools when it needs to. The Puma we had was terrible on the motorway with the A/C on, forever surging as the compressor cycled. Tried another, exactly the same. You had to increase the pressure on the accelerator when it happened o kepp your speed up, obviously this used more fuel. never notive#ced this on any other car although the Micra did have a very uneven idle as the A/C cycled on and off.