How much lower depends on what time of year the home was completed- Usually can charge a much higher rent for new homes.

If you hire a PM, usually from what I'm hearing, Padsplit PM charges about 8% and some just charge a $800 per month flat fee because it's relatively easy I guess.

Or what I would charge for rent.

Charge the old tenant for cleaning and repairs, unpaid rent, and other expenses.

Plus they’d be collecting late fees I assume you’re not charging. 

Does that mean they’re paying 10c/kwh already and charging you 49c/kwh?

There is also a garage in the back we charged $100 per spot for. 

All of our services and charges are listed on a document on our website.

Unfortunately, she frequently turns it off to save on electricity, which exacerbates the issue.i have also sent her extra radiator heat so can just plug in , but she refuse to use because she has to pay high electric bill.She complained about the unit's insulation and requested that we install an additional baseboard heater( whats the use if she doesn't turn on the heat) .

If so, you may choose to simply add it to the lease as being damaged at move-in so the tenant doesn't get charged for it when they leave.