jager heater ratings
watt litres
25 (20-25)
50 (25-60)
75 (60-100)
100 (100-150)
125 (150-200)
150 (200-300)
200 (300-400)
250 (400-600)
300 (600-1000)
i have a 50, 150 & 300 watt heater. I was using the 150 for a 300L tank in my garage which has 10mm poly insulation & it heated it really well. the garage itself is not insulated & gets very cold at night.
In your situation, i would go for the 250w heater. these last so long that buying 2 to safeguard for failure is not necessary IMO. I estimate my 50 watt heater is around 7 years old & it keeps the tank at perfect temp & if you were worried about them staying on apparently they have a fuse type mechanism that shuts them off if they jam on. they have a 3 year warranty, i had 5 jebo heaters & all failed after around two years of use.