Do I really need a winter bag?
First have a look at how cold your baby's room gets. If it regularly gets below 18 degrees then you definitely need a winter weight bag or your baby will likely wake a lot more often as they are cold.
If your baby's room is usually between 18 to 20 degrees you could use a standard or 3 seasons merino bag, such as the Woolbabe 3 seasons bag or 1.0 Tog cotton sleeping bag, such as the 1.0 Tog Ergopouch. However you will need to use additional clothing at 18 degrees and a heater if it gets any colder.
Using a winter weight sleeping bag often means you don't need to use a heater every night, which saves a fortune in power!
Also, have a think about your own bedding in winter, as this can give a good indication as to how warm or cold your house is. If you change to a warmer duvet, use an electric blanket or additional blankets, then you are likely to need a winter bag for your baby.