Five-minute Guide to… So­lar PV

So­lar PVS are still worth con­sid­er­ing — but you’ll need to hurry if you want to ben­e­fit from Fits pay­ments, says en­ergy ef­fi­ciency ex­pert Tim Pullen

Homebuilding & Renovating - - CONTENTS - Tim Home­build­ing Ren­o­vat­ing’s is & ex­pert in sus­tain­able build­ing and en­ergy ef­fi­ciency. He is the au­thor of Sim­ply Sus­tain­able Homes. Tim Pullen

So­lar PVS are still worth con­sid­er­ing — but you’ll need to hurry if you want to ben­e­fit from Fits pay­ments, says ex­pert Tim Pullen

It’s of­fi­cial — Feed-in Tar­iffs (Fits) will end in 2019. If you want to in­vest in re­new­able elec­tric­ity gen­er­a­tion, and hope to do so through us­ing the Fits scheme, then you have 14 months to do it — and for most home­own­ers, so­lar PV is the only vi­able op­tion.

What is So­lar PV?

There are three types of pho­to­voltaic (PV) cell: monocrys­talline, poly­crys­talline and amor­phous crys­tals. The monocrys­talline cell is made from a sin­gle sil­i­con crys­tal, poly­crys­talline is made from many crys­tals and amor­phous cells from thou­sands of bits of crys­tal: Monocrys­talline cells are the most ef­fi­cient and con­se­quently the most ex­pen­sive as a re­sult. Poly­crys­talline cells are a lit­tle less ef­fi­cient and less ex­pen­sive. Amor­phous cells, used in watches and cal­cu­la­tors, are very in­ef­fi­cient and very cheap and with ef­fi­ciency rat­ings

of around 7.5%, they of­fer no prac­ti­cal so­lu­tion at do­mes­tic or com­mer­cial scale. The best monocrys­talline cells, by com­par­i­son, are achiev­ing ef­fi­cien­cies of around 20%.

How Does it Work?

A through PV cell a sil­i­con gen­er­ates crys­tal. elec­tric­ity Be­cause by of al­low­ing its atomic light struc­ture, to pass an elec­tronic charge nat­u­rally moves around in­side the crys­tal in a ran­dom man­ner. Ar­senic and boron are added to the sil­i­con and th­ese im­pu­ri­ties cause neg­a­tive and pos­i­tive mol­e­cules to join and en­cour­age the flow of an elec­tric charge in a sin­gle di­rec­tion. Bom­bard­ing the mol­e­cules with sun­light causes elec­trons to be re­leased, in­creas­ing the elec­tric cur­rent which can then be har­vested. If you want to know more de­tails of how they work and how they are made, then Richard J Komp’s Prac­ti­cal Pho­to­voltaics is a good re­source. So­lar PV cells have been pro­duced on a large scale for over 40 years and have ben­e­fited from a sig­nif­i­cant amount of re­search and de­vel­op­ment, which con­tin­ues to­day.

What do you Need?

A typ­i­cal do­mes­tic PV system will be some­thing be­low 4kw max­i­mum ca­pac­ity, com­prise 12 to 15 pan­els (de­pend­ing on the po­ten­tial out­put of each panel) and cover an area of around 20m2. Con­fu­sion of­ten arises around this fig­ure of 4kw. The cor­rect no­ta­tion is 4kwp, the ‘p’ be­ing ‘peak’. This means that when the con­di­tions are per­fect (which may be mid­day on 21 June, with a cloud­less sky, for a system ori­en­tated due south) then the system will pro­duce 4kw of elec­tric­ity. In all other con­di­tions that system will pro­duce less. Be­lieve it or not, the amount of sun­light reach­ing the UK is broadly the same year-on-year. It has been recorded for many years and we there­fore know what a PV cell is likely to pro­duce in any year. We also know how that fig­ure varies across the coun­try. A 1kw system will pro­duce around 850kwh of elec­tric­ity in the most south­ern reaches of the UK, drop­ping to 750kwh in the north. For ease of cal­cu­la­tion, it is rea­son­able to as­sume 800kwh per year per 1kw, so a 4kwp ar­ray can be ex­pected to pro­duce around 3,200kwh per year.

For com­par­i­son, the ‘av­er­age’ UK house­hold will use about 6,000kwh per year.

The Im­por­tance of In­vert­ers

An in­verter con­verts the vari­able di­rect cur­rent (DC) out­put of a PV ar­ray to a reg­u­lated al­ter­nat­ing cur­rent (AC) that can be used in the home or ex­ported to the grid. It pro­vides pro­tec­tion mea­sures to pre­vent ex­port to the grid if there is work be­ing done on it, and pro­tec­tion to the house. There are three types of in­verter avail­able for do­mes­tic in­stal­la­tions: String In­verter PV pan­els which are held in se­ries, or ‘strings’ (sim­i­lar to Christ­mas tree lights). A PV ar­ray may have a sin­gle string, or up to five strings, one for each row of pan­els. Th­ese are fine where there is no dan­ger of shad­ing or fail­ure of any sin­gle panel. If that hap­pens then the whole string will fail (sim­i­lar to a sin­gle lamp fail­ing on the Christ­mas tree lights). Mi­cro-in­verter Some­times called an edge-in­verter; as the name sug­gests, th­ese are very small and each panel will have its own mi­cro-in­verter. In this case, any prob­lem with a sin­gle panel will not af­fect the rest of the ar­ray. Bat­tery In­verter Th­ese are bi-di­rec­tional in that they act as both the in­verter and bat­tery charger. They are typ­i­cally con­nected to the grid, which means the bat­tery can be charged when the PV is not op­er­at­ing and al­lows PVpro­duced elec­tric­ity to be ex­ported to the grid when the bat­tery is fully charged. The type and qual­ity of the in­verter is at least as im­por­tant as the qual­ity of the PV ar­ray it­self. Most good qual­ity PV sys­tems will of­fer mi­cro-in­vert­ers now, and un­less the site is per­fect in terms of ori­en­ta­tion and shad­ing then th­ese are the ones to go for. If a bat­tery system is in­stalled then, by de­fault, it has to be a bat­tery in­verter. As far as as­sess­ing qual­ity, the nor­mal rules ap­ply. The cheap­est PV system is un­likely to come with the best qual­ity in­verter. The in­verter is the heart of the system; it con­trols the pan­els and will de­ter­mine how well, or poorly, it works. And it is still true that the in­verter is likely to fail be­fore the PV pan­els.

Is Plan­ning Con­sent Re­quired?

A change in leg­is­la­tion in 2008 al­lowed the in­stal­la­tion of PV pan­els to be deemed Per­mit­ted De­vel­op­ment, and there­fore plan­ning per­mis­sion is not needed, un­less the build­ing is listed, viewed by a listed build­ing or in a con­ser­va­tion area. If the house is in a con­ser­va­tion area it is worth ask­ing the lo­cal au­thor­ity, as per­mis­sion will some­times be granted.

What Tar­iffs are Avail­able?

There are ac­tu­ally two tar­iffs: The Feed-in Tar­iff, which is paid for each kwh of elec­tric­ity gen­er­ated, re­gard­less of what hap­pens to that elec­tric­ity — the cur­rent tar­iff is 3.93p/kwh (and will be next up­dated on 1 April 2018). The Ex­port Tar­iff, which is a min­i­mum fig­ure paid for each kwh of elec­tric­ity ex­ported to the grid; that cur­rently stands at 5.03p/kwh. The ex­act ex­port tar­iff fig­ure is dif­fi­cult to cal­cu­late, as it will be af­fected by the pur­chas­ing com­pany’s at­ti­tude to stand­ing charges, ad­min charges and the like.

How Much Do PV Sys­tems Cost?

The ray in­stal­la­tion) £3,000, (typ­i­cal price plus of for will a 3.8kwp in­stal­la­tion. a be do­mes­tic be­low arBud­get nected to £4,500 the grid. to That £5,000 3.8kwp for an system in­stalled is likely system, to pro­duce cona lit­tle over 3,000kwh of elec­tric­ity and gen­er­ate £119 in Fits in­come per year. It is rea­son­able to ex­pect to use 50% of the elec­tric­ity pro­duced in the house. If you are pay­ing 15p/kwh to buy elec­tric­ity, that 50% then has a value of £225 per year. The other 50% qual­i­fies for ex­port tar­iff and has a value of £67 per year. So in this case, the to­tal value of the system is some £411 per year. But if 100% of the elec­tric­ity is used in the home then the value could rise to £569 per year.

Are So­lar PV Sys­tems Worth the In­vest­ment?

In 2010 the Feed-in Tar­iff for PV was 44.19p/kwh — com­pared to the 3.93p/kwh be­ing paid now. A lot of peo­ple ar­gued that the rate in 2010 was too high and that it used up too much of the bud­get avail­able, at the ex­pense of other tech­nolo­gies. The re­al­ity is that it kick-started an oth­er­wise dor­mant tech­nol­ogy and there are now over one mil­lion PV in­stal­la­tions in the UK. The mas­sive drop in the cap­i­tal cost of PV sys­tems since 2010 means that it is still, just about, pos­si­ble to jus­tify the in­stal­la­tion in fi­nan­cial terms. Some would ar­gue that this is re­ally only true if 100% of the elec­tric­ity pro­duced is used in the home — in which case you’ll need a bat­tery.

“The mas­sive drop in the cap­i­tal cost of PV sys­tems since 2010 means that it is still, just about, pos­si­ble to jus­tify the in­stal­la­tion in fi­nan­cial terms”

A Pop­u­lar Choice with Self-builders and Ren­o­va­tors Self-builders Lind­say and Justin Stead ( op­po­site) and Andy Bayliss and Rachael Alexan­der ( above) all opted for so­lar PV sys­tems for their new homes. So­lar-pow­ered homes in the Ty So­lar eco-vil­lage ( left) in Wales are aimed at first-time buy­ers.

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