Calculating Payback Time for Solar Cell Panels: GCSE kWh Question

[Mukilab]

The question: Electricity supplied from the National Grid costs 8p per kWh.
A householder is considering installing solar cell panels. The household uses 4000 kWh of electricity each year. The cost of installing a solar cell panel is £6400 per kW
What is the payback time?

The answer is 40 years however I got 20 by doing the following.

I first worked out the total cost which I got as £320 = 4000kWh * £0.08 , this is per year

Then I just did £6400 / £320 to get 20 years.

How can you get 40? And where did I go wrong? I'd be very thankful for any help.

On another note... how do I use Latex in this forum? It just comes up with a box... do I have to do the latex tags myself?

 

[tiny-tim]

Welcome to PF!

Hi Mukilab! Welcome to PF!

Electricity supplied from the National Grid costs 8p per kWh.
A householder is considering installing solar cell panels. The household uses 4000 kWh of electricity each year. The cost of installing a solar cell panel is £6400 per kW
What is the payback time?

Always look carefully at the units in the question …

in this case, the cost is per kW, not per kWh. ​

... do I have to do the latex tags myself?

Yup!

 

[Mukilab]

Hi Mukilab! Welcome to PF!


Always look carefully at the units in the question …

in this case, the cost is per kW, not per kWh. ​

Yup!

I tried dividing the kWh by the number of hours in a year and it doesn't work.

 

[tiny-tim]

I tried dividing the kWh by the number of hours in a year and it doesn't work.

hmm … yes, it goes the wrong way, doesn't it?

The question seems to expect you to instal 2kW of panels, yet with about 9000 hours in a year, that seems totally unnecessary.

 

[Mukilab]

hmm … yes, it goes the wrong way, doesn't it?

The question seems to expect you to instal 2kW of panels, yet with about 9000 hours in a year, that seems totally unnecessary.

It's from an AQA GCSE science module paper... and it's not the first that has been dodgy. I'll wait for anyone else to see if they can figure out why it works.

 

[tiny-tim]

The only way I can see it working is if there's only about 6 hours of daylight a day.

Since that's only half the actual amount (of 12 hours), maybe there's some reason for taking half, eg that the sun spends most of the day at the wrong angle?

 

[Mukilab]

The only way I can see it working is if there's only about 6 hours of daylight a day.

Since that's only half the actual amount (of 12 hours), maybe there's some reason for taking half, eg that the sun spends most of the day at the wrong angle?

Sounds too complicated for GCSE :(

 

[Dadface]

Mukilab,how was the question worded exactly and have you left out any information?

 

[Mukilab]

Mukilab,how was the question worded exactly and have you left out any information?

no left out information at all.

Copied it exactly, word for word.

 

[Dadface]

no left out information at all.

Copied it exactly, word for word.

What year and what course was it from?

 

[Mukilab]

June 2008 Physics AQA GCSE Core Science Past Paper Module exam higher tier.

If you look at it, also please look at the dodgy question on meter readings.

 

[Dadface]

Back again.I did a search on the AQA website to look at past papers for 2008 but could not find the questions you referred to.

 

[Mukilab]

Back again.I did a search on the AQA website to look at past papers for 2008 but could not find the questions you referred to.

Here: http://store.aqa.org.uk/qual/gcse/qp-ms/AQA-PHY1AP-W-QP-JUN08.PDF

Question 8C

The dodgy meter one is 6C

 

[Dadface]

Question 8 is about drying?

 

[Mukilab]

8C of the higher paper.

That paper contains two parts: foundation and higher tier. Scroll down to page 14 or so to get higher.

 

[Mukilab]

yes page 14.

 

[Dadface]

Mukilab if you look at question 8a there are two bar graphs and the one on the left informs us that a 1kW panel produces 2000 kWh per year.That information needs to be carried across to 8c.

 

[Mukilab]

Mukilab if you look at question 8a there are two bar graphs and the one on the left informs us that a 1kW panel produces 2000 kWh per year.That information needs to be carried across to 8c.

Really? Usually it mentions that in the question e.g. 'using graph x as evidence'...

 

[Dadface]

You will often find that information provided in earlier parts of questions is needed for later parts of questions.Part c on its own does not give enough information.

 

[Mukilab]

You will often find that information provided in earlier parts of questions is needed for later parts of questions.Part c on its own does not give enough information.

thank you for the help