A higher wattage heater or two might help. Yes it uses more power when its on but it will be on less often and might cut cost, but I doubt it.
If you have to have lights on for plants, upgrade your lights to a multi bulb t5ho setup. Look for one that has a cooling fan on the hood, that means it will generate some heat, with one of these on it will not only be good for your plants, but it will also heat the water up while its on. My mom ran a helios 4x55 cf over head light on a marine tank and was able to turn the heater off during the day.