Like @BridgeBrew, I use a 30 litre Cygnet but with one of @foxbat’s power controllers. It works well for me enabling me to maintain a rolling boil whatever volume of wort is in the boiler. Given TETB’s comments, I think I ‘ll fit a fan to increase airflow over the heat sink. Having said that, diluting wort post-boil is a common commercial brewing practice, and I frequently brew at higher gravity then “liquor back” to the desired OG with boiled water during transfer to the fermenter. This enables me to make batches up to 35 litres with a boiler which has an effective starting boil capacity around 27 litres.