An ROI example taken from Energy Savings Trust [EST] Ref1
Postcode SW1A 0AA 21p/kWh Faces South with optimum 30deg tilt.
Use EST calculator as fewer inputs to manipulate, it includes FIT/export/savings/S-values/degradation and after all EST is DECC funded.
As we use DECC proposed 21p/kWh then it must be new or retrofit =< 4kWp so as Ref1 use 2.9kWp
The EST calculator gives Total Income over 25 years is £14,548 (i.e. Gain). Please check!
it also suggests Total Costs of £11,700 and Ref1 suggests £11,500 so we'll use the mean 11600.
Ref2 and others define ROI as (Gain-Cost)/Cost and this is the same as (Gain/Cost) - 1
We need to multiply by 100 to get percent and divide by the Period to get % per year.
Simple Annualised ROI=((Gain/Costs)-1)*100%/Period so over Period=25 years this simplifies to...
ROI=((Gain/Costs)-1)*4 for above example this is ((14548/11600)-1)*4 = 1.017 %/year Note 1
Reasonable as the EST calculator and Ref1 gave Payback time 20 years so only 5 years are profitable.
If Mr Bloggs claims "ROI (return on investment) of 8-10% is still possible" we can now find out how much they charge to install including scaffolding. With Annualised ROI=10% then 10=((Gain/Cost)-1)*4
So Cost = Gain/3.5 = 14548/3.5 = £4156.57 ... Chinese gear can be very good. Note 2
Note 1: Check using Ref3 an ROI calculator. Original Investment=11600 Returned Value=14548 Go from 6/1/2000 to 6/1/205 press [Calculate] we get Simple Annualized ROI 1.0%
Note 2: Change Original Investment to 4156.57 press [Calculate] we get Simple Annualized ROI 10.0%
Ref1:
TinyURL.com - shorten that long URL into a tiny URL
Ref2:
Return On Investment (ROI) Definition
Ref3:
http://www.money-zine.com/Calculators/Investment-Calculators/Return-on-Investment-Calculator
Part 2 to follow - why a standard way of stating returns in a familiar form to the public is good news for the solar PV industry- the light at the end of the tunnel.