For that size bill, an educated guess would be that the target temperature has been set at 55 degrees or higher with no interlink to an immersion for the last bit, which the system is struggling to hit so ends up running basically continuously at a very low COP.
Educated guess being that bill would mean it was pretty much running continuously 24/7.
Drop the target temp to 50 degrees (with weekly purge using the immersion heater) and I'd bet your bill would be more like the level you were hoping for.
Not that I'd be recommending that these systems should be replacing gas - their main use should be in off gas grid properties other than possibly those with solar pv installed that they want to use the excess generation from, and set the timer accordingly.
Educated guess being that bill would mean it was pretty much running continuously 24/7.
Drop the target temp to 50 degrees (with weekly purge using the immersion heater) and I'd bet your bill would be more like the level you were hoping for.
Not that I'd be recommending that these systems should be replacing gas - their main use should be in off gas grid properties other than possibly those with solar pv installed that they want to use the excess generation from, and set the timer accordingly.