Take two sites: one hydro, one solar photo-voltaic; both are domestic scale installations: the hydro is 0.75 kW peak, the solar 4.0 kW peak; both are located in the same part of the UK, 50 km apart; both have been monitored throughout 2014 using Wattson technology which made it possible to extract the same data from each installation.
Question: which did better in 2014?
Answer: it depends how you look at it:
1. In terms of kWh yield: the solar installation generated 4,441 kWh against the hydro's 3,649* kWh, the patterns of yield for the two sites being as follows:
2. In terms of how much of the generated energy was used on site - hydro: 2,920 kWh, solar: 2,014 kWh. These figures as proportions of total energy generated are as follows:
- solar export deemed at 50% of total generated, hydro at 75%
- using the RPI uprated solar tariffs effective from 1 April 2014 and giving figures for two 'eligibility dates' for solar:
- eligible in FIT year 1/2 (2010/11): FIT rate 48.07 p/kWh; Export rate 3.39 p/kWh
- eligible in FIT year 4 (2013/14): FIT rate 15.3 p/kWh; Export rate 4.77 p/kWh
- using RPI uprated hydro tariffs effective 1 April 2014 for an installation with an 'eligibility date' in July 2013: FIT rate 22.23 p/kWh; Export rate 4.77 p/kWh
- the solar was worth £2,210 (FIT year 1/2) or £785 (FIT year 4)
- the hydro was worth £942*
4. In terms of the saving on expenditure arising from energy not purchased from the grid because renewable energy was used 'in house', with the following assumption:
- price per unit, inc VAT @ 5%, 15.13 p / kWh (SWALEC direct debit and paperless billing)
- the hydro was worth £442*
Putting all this together, the solar appears to pay better if you had installed it in 2010/11 at the beneficial FIT rates then available: £2,515 against the hydro's worth of £1,384.
But if you missed the generous early FIT rates and installed your solar in 2013, then the income in 2014 was £1,090, against the same £1,384 for the hydro.
This analysis would not be complete without mentioning the capital expenditure involved for the different schemes which, in the case of solar, depended on when it was done. The cost of installing a 4kWp solar in FIT year 1 (2010) was about twice what it was in FIT year 4 (2013). Since my hydro was installed in 2013 and cost about the same as a 4 kWp solar installed in the same year (see previous post), the fairest comparison is between the revenue figures for a 2013 installation date: hydro £1,384, solar £1,090.
So the answer to the question posed involves a lot of variables and many complexities, but the bottom line is that my 0.75 kWp hydro compares favourably with a 4 kWp solar PV.
Hydros might be more noisy and they might be less "fit and forget" than solar, but finances aside, to me at least, they're much more fun.
*regular readers may notice these figures are different to figures given elsewhere in this blog (3,649 kWh vs 3,672 kWh, £942 vs £996, and £442 vs £430). The explanation is twofold: 'the year' under consideration is different: the 2013/14 'water year' vs the 2014 calendar year; and the utility tariff I have used is different: standard unit rate vs Economy 7 rate.