I've rebuilt a home and currently in the process of sizing a mini split for heating and cooling. I contacted a few companies through Mitsubish and Fujitsu's recommended installers and have settled on a Fujitsu installer. He visited the house recently and suggested a 18k unit to be safe after I had thought 15k may be enough. I was hoping for the highest SEER rated mini-split that was properly sized as to not have issues with cycling. I'll try to give enough information below about the construction methods.

Residential one story home located in Los Angeles, CA Overall dimensions are roughly 32'x36', fairly rectangular with a couple bump outs. 4' Side porch covers two-thirds of south facing wall. Front Porch overhang is 6' on the east facing side Rear Deck has 3' overhang on west facing side. Home is mainly post and beam construction with 2x4 stud walls in between.

EXTERIOR WALL have all been sheathed with plywood, seams taped, 1" rigid foam, 3/4" vertical furring strips and then covered in Hardie lapsiding. (I realize this rainscreen method is overkill for drier environments.)

ROOF was sheathed in 1/2" plywood, seams taped, R17 - 3" rigid foam, furring strips for a ventilation gap then standing seam metal roof, charcoal grey with SRI 30. There are soffit and ridge vents to help keep the roof cooler in the summer.

FLOORPLAN is fairly open with kitchen in the center room and living room connected. No walls separating them. The main space is about 450sqft. Bedrooms are located on opposite ends of the home. Doors will likely be 5' wide sliding barn doors to help ventilation between main space and bedroom while they are open.

CATHEDRAL CEILING with 2x8 rafters. House interior is currently unfinished but I plan to fill with Roxul Comforbatt R23. I realize I could fit a bit more insulation in but have heard issues with condensation when your ratio of insulation is higher on the inside of the roof.
Peak ceiling height is 12" along the ridge, then 8" near outside walls. Roof is simple gabled with no hips or valley. No attic.

INTERIOR WALLS Unfinished but planning Roxul Comfortbatt R15 inside wall cavity, covering with drywall, then possibly finishing with plaster or wood

GLAZING 200sq ft of doors and windows are aluminum framed, double pane with U-0.45 Part of the total square footage is a 15' wide x 80" tall sliding door that is west facing and thermally broken.

SKYLIGHTS- 24sq ft WASCO 49% VLT / 0.54 SHGC / U-0.16 = R-6.25 The 4'x 4' skylight in the main space facing south will likely overheat the house in the summer so I plan to experiment with shading options once I gauge the amount of heat it brings in.

CRAWLSPACE - There is a complete story underneath the house that is unfinished and used as a workspace. Currently no insulation.

I've lived in the house for 4 years to monitor temps, wind, weather conditions, etc. The outside shell just got rebuilt before Christmas so I'll be monitoring how the building methods have shifted the interior conditions. With 35 degree nights recently and no interior insulation, I've been quite pleased. I've been able to keep the interior at high 60s with a portable 12k heat pump and small electric space heater. My goals for this single Fujitsu mini split are to keep the home comfortable in winter 35 degree nights but also cool it off on 105 degree summer days. The unit is planned to be installed central in the house, 10' up on a wall.

Any advice or information for deciding on which BTU unit to purchase? Fujitsu ASU 15RLS3 is 25 SEER - 18K heat - 15k cold

An 18K is is likely to be insanely oversized for the cooling and heating loads of a ~1000 square foot house in L.A. Even a 1-ton would likely cover it, and a 3/4 tonner isn't out of the question! They put out more than the nominal "rated" cooling, and you have very low latent loads.

CA Title 24 requires that a Manual-J heating & cooling load calculation be done, though that is (strangely) often ignored. Unless you have a LOT of west facing window your peak loads are going to be nowhere near 15,000 BTU/hr, let alone the 18,400 BTU/hr maximum cooling capacity of the -15RLS3

http://www.admorhvac.com/admor-files/submittals/ASU15RLS3.pdf

As sucky are rules of thumb are, most tight code-min new homes will have a cooling load to conditioned space area on the order of 12,000 BTU/hr for every 1200-1500 square feet of conditioned space. The rare exceptions are those with expansive "sunset view" windows. This graphic was compiled from Manual-Js performed on a range of existing house by a contractor in Georgia that covered a number of houses from FL to TX (where the latent loads are MUCH higher than yours):

http://www.greenbuildingadvisor.com/sites/default/files/images/Bailes%20graph%20for%20Manual%20J%20blog.preview.png

With a tighter better new house it's likely that a 3/4 tonner would have you covered. The 9RLS3 is good for 12,000 BTU/hr @ 95F outdoors, 80F in.

http://www.admorhvac.com/admor-files/submittals/ASU9RLS3.pdf

Hire a P.E. or RESNET rater to run an AGGRESSIVE Manual-J on the place, or you'll almost certainly be oversizing it!

BTW: With as little as a inch of foam between the roof deck & shingles you can go unvented with full-depth batts.

Thanks for all the info Dana. I had a feeling you would say the unit specified might be oversized.

The 15' x 80" sliding door is a west facing "sunset view."

It sounds like you're saying I can go full depth in the rafters and achieve R-30 inside and R-17 outside. R-47 total should help.

Just got on the resnet site and contacted 3 Home Energy Auditors in my area. Will see what they say.

The big west facing slider might add quite a bit, but could also be glazed with a heat-rejecting soft-coat low-E glass to minimize the gains, but any exterior shading factors such as trees & shrubs, deep overhangs etc have to be evaluated correctly too. Discuss those details with whomever is doing the Manual-J, and be sure reiterate that all assumptions need to be aggressive.

It's even written in the Manual to be aggressive, taking every possible factor that reduces load into account, but that is widely ignored by HVAC contractors.