How much so­lar PV do you need?

You’ve de­cided to join the so­lar revo­lu­tion and have a crack at turn­ing the sun’s en­ergy into elec­tric­ity to power your home or of­fice – great stuff! Now it’s time to fig­ure out – how big?

Element - - Contents - By James Rus­sell

The so­lar equa­tion is a slip­pery thing – be it on a global or lo­cal scale. Pro­jected global so­lar booms have been fore­cast be­fore, but this one looks like it’s for real – thanks mainly to the drop­ping price of so­lar panels over the past five years.

So, now you’ve com­mit­ted to get­ting so­lar panels on your home or busi­ness, some maths is re­quired to en­sure the time it takes to pay off your sys­tem is as short as pos­si­ble – en­sur­ing your in­vest­ment is a good one.

Some power com­pa­nies pay you for the ex­cess power you gen­er­ate, some don’t. Of those that do (see side­bar), rates can vary, and power com­pa­nies don’t par­tic­u­larly en­joy hand­ing you a cheque at the end of the month, rather than the other way around – although this is pos­si­ble.

The av­er­age home in New Zealand uses around 25kWh (kilo­watt hours) per day.

A 2Kw (kilo­watt) so­lar PV sys­tem will gen­er­ate 2 kilo­watt hours (kWh) of power for ev­ery hour of sun­light.

NIWA sta­tis­tics show that in summer, in Auck­land, the av­er­age hours of sun­shine a day is 6.5 (Oc­to­ber to March) and the av­er­age daily win­ter sun­shine hours is 4.6.

In the summer a 2Kw sys­tem could be gen­er­at­ing around 12 kWh per day. In the win­ter the sys­tem could be gen­er­at­ing around 9kWh. It’s rea­son­able to use five hours of sun­light per day as an av­er­age.

In a per­fect world, with panels aligned due north, on a 30 de­gree slope, you’d get 10kWh a day. It’s more likely, there­fore, to be 8kWh per day, or 2920kWh a year. At 0.27c per kWh (about what the power com­pa­nies charge you) this is $788 worth of power.

Now to cal­cu­late the re­turn on in­vest­ment (ROI) on a sys­tem. If you use all the power your ar­ray pro­duces in your home or busi­ness, you can di­vide the to­tal cost of your sys­tem by your sav­ings per year to get your ROI. In this ex­am­ple, a 2kW sys­tem cost­ing $7500 will take 9.5 years to pay off ($7500 di­vided by $788), with­out tak­ing into ac­count our ever-ris­ing power prices. For a sys­tem that pro­duces elec­tric­ity for at least 25 years, that’s not bad go­ing.

For a home which uses the New Zealand av­er­age of 25 kWh each day, it’s not hard to imag­ine us­ing up to half of that power dur­ing sun­light hours, par­tic­u­larly if peo­ple are in the build­ing dur­ing the day. If they aren’t, it’s pos­si­ble to put a bunch of ap­pli­ances on timers – towel rails, dish­washer, wash­ing ma­chine, dryer – even wa­ter heat­ing – while the sun is shin­ing.

Here’s where it gets in­ter­est­ing. Now triple that so­lar ar­ray to 6kW. You’re now pro­duc­ing 24kWh each day on av­er­age. You’re still us­ing some of that power, but the rest is be­ing ex­ported to the grid. Your power com­pany pays you well for some of it, but as soon as you get over a cer­tain thresh­old, the price you get for your panels’ elec­tric­ity is very small in­deed – as lit­tle as 3.5 cents per kWh. In this case your re­turn on in­vest­ment isn’t so rosy, and pay­back times may bal­loon out to 15 years or more. There­fore it’s im­por­tant not to go too big.

A large num­ber of homes are now wired up to the grid us­ing smart me­ters, which let you know your home’s elec­tric­ity us­age to the half hour, ex­tremely use­ful for choos­ing so­lar ar­ray size. You can see ex­actly how much power you use dur­ing day­light hours, and buy a so­lar ar­ray to suit. El­e­ment’s rec­om­men­da­tion is to buy an ar­ray that pro­duces 5kWh more than your use dur­ing sun­shine hours – those 5kWh hours can still be sold back to the re­tail­ers for a de­cent rate, while at the same time you’re cov­ered for any changes in your life­style.

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