Solar Panels

scarletjim

Reed_Richards said:

scarletjim said:

If we had a system that generated 6000 kWh/yr, and a battery to store it, would we be able to basically reduce our grid electricity usage to zero (or presumably zero plus standing charge)?

No.  The easiest way to understand why not is to ask yourself "Why is it cold in winter?". The answer is because we have much shorter and often cloudier days so we don't get anything like the amount of solar radiation that we get in summer.  This translates into much less solar power in winter.  We also tend to consume more electricity in winter, although in your case it may not be that much more.  Of course if you had a 6000 kWhr battery you could do it but that would be the size of a house and more expensive than a house.    

Aha i see, thanks for explaining. In fact, as you quite rightly intimate, we use slightly less electricity in the winter because we don't have the air-con on, but not much less so of course your point still stands.

So if I understand you correctly, the battery normally stores electricity generated on a single day for use that night (or perhaps for 2 nights in some circumstances, but certainly not enough to use for several cloudy months).

Is there any good website to show how many months on average we would get enough daylight to basically reduce our grid electricity usage to zero? Even if we could do it for 80% of May to September, that would be enough to take about a third off our electricity bill - though I guess that's not that much more than the calculators are suggesting...  

That's a shame, as for us it limits the attractiveness immensely - all the calculators we use seem to suggest a bill saving of about £450py, sell back to grid about £150py, which given the costs gives a lifetime benefit after 25 years of only about £5k (so about £200py) - with such tiny benefits, I'm not sure why anyone bothers for financial reasons. (Perhaps calculations are very different for others?)

QrizB

scarletjim said:

Is there any good website to show how many months on average we would get enough daylight to basically reduce our grid electricity usage to zero?

Yes, but you need to do a bit of work.

There's an EU-run website called PVGIS that has solar data for (almost) the whole world:

https://re.jrc.ec.europa.eu/pvg_tools/en/

Find where you live on the map and click on it. Then complete the table with details of the solar array you want to model - the total rated power of the panels in kWp, the tilt angle and the direction they face (azimuth). Then click "visualise results".

You'll get a bar chart showing a pretty good estimate of how many kWh you'll generate per month, in an average year. Without a battery you'll be able to use perhaps a third of what's shown (the rest will be generated at times you don't want it), with a battery maybe two-thirds.

You were talking of a solar PV system that would generate 6000kWh/yr. In the UK that's likely to need a 6-7kWp array which will cost you £8-10k at present prices. Adding an optimally-sized battery will roughly double that price.

If you need any help, just ask.

Evan3020

""Why is it cold in winter?"

Well its nothing to do with cloud, its the axial tilt of the earth tilting the northern hemisphere away from the sun so it strikes us at an oblique angle.

Alnat1

@scarletjim The simplest and cheapest way to reduce your import from the grid would be to make efficiencies in the amount you already use, which is double that of the average household.

QrizB

Alnat1 said:

scarletjim The simplest and cheapest way to reduce your import from the grid would be to make efficiencies in the amount you already use, which is double that of the average household.

scarletjim says:

scarletjim said:

we use slightly less electricity in the winter because we don't have the air-con on

... I expect the air-con will account for a good part of the excess.

Alnat1

I guess my method of putting a few damp tea towels in the fridge on a hot day, so I can grab one to put around my neck when I'm starting to overheat wouldn't cut it with everyone. 

scarletjim

QrizB said:

scarletjim said:

Is there any good website to show how many months on average we would get enough daylight to basically reduce our grid electricity usage to zero?

Yes, but you need to do a bit of work.

There's an EU-run website called PVGIS that has solar data for (almost) the whole world:

https://re.jrc.ec.europa.eu/pvg_tools/en/

Find where you live on the map and click on it. Then complete the table with details of the solar array you want to model - the total rated power of the panels in kWp, the tilt angle and the direction they face (azimuth). Then click "visualise results".

You'll get a bar chart showing a pretty good estimate of how many kWh you'll generate per month, in an average year. Without a battery you'll be able to use perhaps a third of what's shown (the rest will be generated at times you don't want it), with a battery maybe two-thirds.

You were talking of a solar PV system that would generate 6000kWh/yr. In the UK that's likely to need a 6-7kWp array which will cost you £8-10k at present prices. Adding an optimally-sized battery will roughly double that price.

If you need any help, just ask.

Wow, that is incredibly interesting / useful, I've just spent the last hour playing with it and constructing a spreadsheet to try to understand my surplus / deficit each month. Because we have quite a high electricity usage in summer, and slightly lower in winter, then if we had a battery that enabled us to use the generated electricity at night, we should be able to achieve zero electricity usage cost March to September (very small surplus in March, August and September, much great in April, May, June, July). We would still have usage to pay for in the period October to February, but it would still be a huge saving - seems to me to be about £1,200 per year. I'm assuming the online calculators don't generally have options for cost benefit analysis for systems with batteries (does anyone know of any that do?), so I guess that's why I can't confirm my findings using them - it's all very well me saying I'd save £1,200 a year, but as you say, the additional cost of the battery would pretty much eat up the additional savings value.

So online calcs say I'd benefit by about £600pa (£450 bills, £150 sell back) without a battery at a cost of (say) £10k, meanwhile my with-battery calc says I'd benefit by about £1200pa at a cost of (say) £20k - so I guess it makes sense.

Thanks for the little tutorial and link - I've certainly learned a lot more than I normally would on a Friday afternoon, that's for certain!

Magnitio

scarletjim said:

That's a shame, as for us it limits the attractiveness immensely - all the calculators we use seem to suggest a bill saving of about £450py, sell back to grid about £150py, which given the costs gives a lifetime benefit after 25 years of only about £5k (so about £200py) - with such tiny benefits, I'm not sure why anyone bothers for financial reasons. (Perhaps calculations are very different for others?)

I don't know what your electricity rates are, but if we assume 35p/kWh and you export at 7.5p/kWh, then if you use 30% of a solar installation that generates 6000kWh per annum, then your annual saving would be (6,000 * .30 * .35) + (6,000 * .70 * .075) = £945. If you are using air conditioning, then I presume that is in use on the hotter (i.e. sunnier) days of the year, so you are likely to use more than 30% of the solar panel output. Many people who get solar installed without a battery also adjust the use of major consumers (e.g. washing machines / tumble driers / cookers) to coincide with decent solar output. For a lot of households, based on current/projected energy prices, payback for a solar installation should be less than 10 years, certainly not 25 years.

scarletjim

Alnat1 said:

@scarletjim The simplest and cheapest way to reduce your import from the grid would be to make efficiencies in the amount you already use, which is double that of the average household.

Well yes of course that is true to an extent, but 'average household' is surely a pretty meaningless measure as it ignores the size of your house compared to the average. If you have a big house with lots of rooms and lots of people living in it, of course it's going to be more expensive to run. We do have the luxury of the air-con which we use for the hottest 6-8 weeks of the year, but otherwise we're very careful - lights off when not in rooms, LED bulbs throughout, devices off when not needed to be on etc. We do our best with most elements - it's just a big house!

Dogbyte009

scarletjim said:

Is there any good website to show how many months on average we would get enough daylight to basically reduce our grid electricity usage to zero? Even if we could do it for 80% of May to September, that would be enough to take about a third off our electricity bill - though I guess that's not that much more than the calculators are suggesting...  

These are the generation stats for my system (3Kw panels plus a 2.5Kwh battery) for last year. Between November and February you can't generate enough to fully charge the battery. May to August you stand a good chance of zero grid usage, June and July I usually generate more power than I buy. You do have to learn to build your life around it bit, using high consumption appliances like the washing machine when the sun is out for example.