Dumnorix
A 2.5kw system at peak will be able to mostly offset a washer or dryer (the quick and dirty equation is watts/volts=amps though for AC power it’s not exact so 2500/230=10.9A which is slightly more than a standard white good). This will save you, based on 22p per kw 22×2.5=55p per hour. A rough estimation based on 6 hours of peak production average per day throughout the year would be .55×6(365)=£1205 saved per year assuming you use every kw.
When not using any power you’ll export about 2kw (accounting for any amount of ambient use) which is about 11p per hour you’ll earn from an export tariff, though you may get more depending on who you sell to. .11×6(365)=£240 but you’ll also offset your .5kw assuming 2.5kw at 6 hours per day so savings+earnings will be about £500-600 per year.
As you can see, it’s much better if you are going to use what you produce.
These are all estimates based on ideal situations. We had 26 hours of sunshine in December in Plymouth. In July last year we had 1.5x the usual amount of sunny, cloudless days, a factory we fitted 98kw on was producing near peak levels from about 0900-1800.
Have a look at the MCS website for approved installers in your area. I’d be surprised if many places turned you away for only wanting 2.5kw. Most money is made from the labour, and the install time isn’t that different from a 4kw as the infrastructure is the same other than physical cable sizes and the number of panels.
Hope this makes sense. I’m roasting as I type so I’ve been distracted a few times!