It doesn't seem particularly high to me. 45kg used in 6 weeks is 14kWh/day or 5400kWh/year. To go through the same amount of gas in 4 weeks is 21kWh/day or 8000kWh/year.
Edit: Actually, considering you're only using it for hot water, 14kWh gives you 230L/day of 55°C water (assuming incoming water temperature of 12°C and 80% energy efficiency). That does seem a lot.