Welcome!
First things first, details. What's the square footage? Insulation? Windows? Gas? Electric?
The best thing to do is to insulate everything and get the house air tight. Then live there awhile to see what your power bills are going to be. Then you will know about how many panels you will need.
Also is the house angled correctly for roof mount?
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שְׁמַע יִשְׂרָאֵל ה' אֱלֹהֵינוּ ה' אֶחָד
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