We are targeting using 18.800KwH/year for a Norcal home, south facing, assuming 5.5 hrs/day sunlight, "extremely complex roof".

My calculations show that I need a 13Kw system with a 125% offset. Tesla's calculations show I need a 18.35Kw system with 103% offset. Who is right?

Detail:

I'm seriously looking into the V3 Tesla roof and have iterated a few times with Tesla on the system size. Their estimate ranged widely from ~10Kw to ~18Kw causing me to questioning the credibility of their numbers.

We are moving into a new home in Northern California. We have 18 months of prior owner's usage. In addition we have 2 electric cars where they didn't have any.

We estimate needing 18,800Kwh/year (1150 Kw/mo x 12 mo + 2 electric cars (at 5000Kwh/year)). We will also get 1 Powerwall.

The home is a two story, south facing, largely away from tall trees. I've been using 5.5 hrs/day sunlight in the calculation. Tesla deem my roof extremely complex.

My estimated system size is 13,000Kw and 1 powerwall. My math looks like this:

(13Kw x 5.5 x 365) x .75 = 19,573Kwh/year (Note: the 25% reduction is something someone suggested - I don't have a clear reason other than a built in offset of 25%)

Tesla is suggesting a 18.35Kw + 1 Powerwall system. In their estimation, this will yield 19,150 Kwh/year with 102% offset. I don't understand how they get this large of a number.

What is the right calculation method?