A solar powered fan for your attic is a good choice if switching completely to solar power is not feasible. The fan will have a temperature sensitive gauge and switch itself on when necessary. This will remove heat and lower the cost of cooling. It's also solar powered, so you don't have to pay to power it.
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You should consider switching to solar power if your home receives 5 hours of sun per day and you experience high energy rates. In this case, your investment in purchase and installation will be well worth it.