I am quite late responding to this query as i have only just come across it and joined the forum.
Solar photo-voltaic systems are a really good way of generating your own electricity for your own use. the system module output is based on perfect conditions, i-e SOUTH FACING, FULL SUN, NO SHADING ISSUES, a module rated at 180kw will produce up to 180 kw in perfect conditions.
, i have had no dealings with the company you mentioned but am aware of a good company called solarsparks in the northeast. uk. but i fear some have been adding uninformed " expertise" on this subject. The new feed in tarriffs are only
eligeable to a system if it is installed by a company registered or about to be registered with the M C S (microgenerationcertification scheme) not all systems require battery back up these are generally for remote systems in farms outbuildings etc, domestic systems are tied to the grid so as to offer the customer electriciity when the solar cells are not generating i e at night. (no batteries required,) all generated electricity is eligeable for the tarriff even the electricity used by the consumer.
The p v system does not need direct sunlight, only daylight, west facing will mean simply most generation is afternoon early evening and there is more chance of low to medium shading issues. 20500 pounds is a lot of money and especially for 4.5kw. hybrid cells or not. i would look elsewhere. and i would also consider ground source heat pumps, and underfloor heating . best o luck.