I was in the same position last year before I had my tub installed. I live in a 2400 square foot house that was built in 2001 with only a 100amp service and all but 1 slot in the breaker box was vacant. I do have central A/C (50amp), pool pump and two children that consume much electricity(computers, iPads, video games and leaving just about every light on). I also have every modern appliance that a house has nowadays. The house also has natural gas as a utility which supplies my boiler, stove and dryer. The electrician that did the install of the tub said you will be fine. He installed a couple of tandem breakers to free up some space in my breaker box and for almost a year now with everything running never once tripped any of the breakers including the main 100 amp breaker. Like all others have said have a professional electrician see and evaluate your situation..I would get more then one professional opinion. I had 4 estimates on my job and one said to upgrade to 200amp and the others said you will be fine.