Almost all 1-can kits require 1kg of additional fermentables. Your options are:
- 1kg brewing sugar - probably the least-desirable option
- 1kg brew enhancer - mixture of dried malt extract and sugar
- 1kg dried malt extract (DME) / spraymalt
- tin of liquid malt extract (LME), these are generally 1.5kg
I've done various experiments over the years with all of these, generally I find the tin of LME to be the best but it's also the most expensive. Only on one occasion, brewing Coopers Stout, did I find it too rich. Generally I use brew enhancer with 1-can kits and it produces reasonable results.
You could add more than 1kg to beef up the ABV but you'd be altering the ratio of additions to the pre-hopped kit, so a better way to do it is to brew short, 18-20L rather than 23L - just add less water.
You can also significantly improve these kits by adding some hops, Goldings if you like British ales or Cascade or Citra if you like US-style hoppy beers. Chuck 30-50g into the FV 5 days before bottling / kegging.
