Sizing my solar system - no consistency in quotes

Hi guys
I have received a number of solar quotes and all of them have recommended a different inverter size, from 5.1kw to 10kw. I supplied my last 4 Utilities bills - so how do I really know what I need?
They have all worked out a different average daily use and used a different method to work out the inverter size.
For 2018, I used 28.22 kw a day on average (Summer 33kw/d, Autumn 23kw/d, Winter 37kw/d Spring 20kw/d). In 2017, I consumed 25.34 kw a day, with similar season numbers except for Winter when we used 26kw/d).
Typically, what inverter size should be recommended?

Hi Chris,

Which state are you located and do you have any shading issues (especially from trees or buildings in winter). As a really rough estimate I would be looking at a min 6kW inverter and 7 to 8kW of solar (assuming you have 3 phase power supply?)

Hey Svarky
I live in Adelaide. See picture of roof orientation - facing north. I believe azimuth is 45 degrees.
Shading can come from antenna (morning) with late afternoon shade from chimneys and neighbours tree on NW side hitting NW facing back roof (currently tree shade gets just past the black pool solar matting).
Roof valley pitch is 6 degrees, whilst NE side next to neighbours is 35 degrees pitch - as other roof pitches.
I do have a pool+chlorinator (2.4kwh), and 3 phase reverse cycle (59,000 BTU/h).
Looking at Solar Choice website, electricity consumption pattern is probably ‘double hump’ - family with 3 grown up children (uni/work).

I could also move antenna if needed and put panels on NW roof - thoughts?

Many thanks


Hi Chris,

Yes I would definitely go with panels on the NW roof as well. It would flatten out the generation curve and make a big difference in summer to cover the afternoon A/C loads. Since your consumption is high during winter I would go as big as you can afford to account for partly cloudy and overcast days.

Ideally if money wasn’t as issue you would want to go 10kW. In winter you are looking around 3 PSH (peak sun hours) average so even with a 10kW array this will generate an average of 25kWh (including losses) and up to 45kWh on a clear winter day.
In summer however a 10kW array would pump out 60+ kWh on average so you would get more credits to cover excess winter consumption.

Ok thanks. Budget will probably get in the way, but will look to make it as large as possible with Tier 1 panels+inverter.

Also, someone was asking if the ratio of 4kwh:1kw is pretty reliable for sizing solar systems? I think this is mentioned in the Guide.