spanz

I'm just wondering if anyone could help me. We have a spa tub. Usually at winter time we turn the temperature up to 40 C. We live in Australia temp usually around 20 C during the day but gets cooler at night. Anyway my electricity bill is always sky high due to the spa. My question is if we only use it once in every 2-3 weeks would that cost more or less in electricity if i dont heat it all the time just turn it up a day before we use it and than down again after use? Its generally sits on around 30 C if i dont heat it. Thanks in advance.