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Some basics questions about solar viability on my house

edited 8 June 2012 at 12:43AM in Green & Ethical MoneySaving
8 replies 1.2K views
TooledUPTooledUP Forumite
7 Posts
edited 8 June 2012 at 12:43AM in Green & Ethical MoneySaving
Hi,

I have a south facing house in the north west. I am interested in getting solar panels not really for the feed-in or anything...

I work from home mostly (ie telecomute), and because of the IT equipment I use in my job, ie mulptiple 24" monitors, desktop systems, test labs etc I could easily consume 1 - 1.5 KW hours per hour, 24 hours a day based on what my current servers consume.

At the moment I try to power down the servers and storage at night and up in the morning to try and minimise the cost but powering up 10 - 15 machines can be time consuming (clustered virtualised hosts but ISCSI storage systems etc etc)

Therefore, I'd like to know what sort of savings I could expect in the long run,

I think I would benefit more than most, but would like some general advice on it.

Thanks!

Replies

  • rogerblackrogerblack Forumite
    9.4K Posts
    TooledUP wrote: »
    I think I would benefit more than most, but would like some general advice on it.

    To clarify - your average load - measured - not estimated - is around a kilowatt?

    A couple of points.

    The simplest solution is to install a kilowatt (or whatever your base load is) of solar panels, and an inverter.
    A kilowatt of panels, and an inverter may set you back a little more than 1000 quid, if you're happy to install it yourself.
    It will produce around 900kWh/year.
    If your electricity is at 12p/unit - this is about 110 quid, for a payback time of the order of 9 years.

    Then it gets more complex.
    Feed in tarrifs - if you got the above panel 'properly' installed - it might cost you 1500 quid.
    But, you then would get 21.5p _extra_ per unit.
    So, 12+21 * 900 = 300/year.
    Payback in 5 years.

    If you install (say) a 4kW panel - you can't legally (as I understand it) backfeed the grid, and make your meter run backwards, even if it allows this.
    So, you would need to (if you do not go down the FIT route) arrange some means to ensure you never export, and either use all your power internally, or turn down the inverter automatically so it does not export.
  • edited 8 June 2012 at 9:50AM
    Martyn1981Martyn1981 Forumite
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    edited 8 June 2012 at 9:50AM
    TooledUp, I think Roger's numbers are spot on.

    I'd always suggest that for a medium+ sized system of 2kWp or more, most people would expect to save around £100pa (£70 to £150) off their electricity bill. Increased daytime consumption will push those numbers up, and with such a high daytime demand as yourself, I'd expect you to easily double that (with a 4kWp system).

    It will probably be possible to estimate more accurately if you can give a few details of your roof. Approx location and size. Also as it's south facing it's slope would also be interesting. A steep south roof generates a bit less in the summer and a bit more in the winter (total annual generation isn't affected much), so a flatter generation would suit your consumption and also lift savings a bit.

    For basic economics, the cost of an install falls proportionately with size, so the bigger you go, the lower the cost per kWp installed.

    Mart.
    Mart. Cardiff. 5.58 kWp PV systems (3.58 ESE & 2.0 WNW)

    For general PV advice please see the PV FAQ thread on the Green & Ethical Board.
  • EricMearsEricMears Forumite
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    rogerblack wrote: »

    The simplest solution is to install a kilowatt (or whatever your base load is) of solar panels, and an inverter.


    A kilowatt of panels will of course only supply a kilowatt of base load for a few days in June (but not this week ! :mad: ). You really need the full 4Kwp to stand a chance of getting a Kw for a reasonable proportion of the year. It might even be worth going up to 10Kwp and dropping a FIT band to get a Kw supply pretty much most of year.

    Then of course it maybe worth fitting a UPS unit - probably still not worthwhile on the raw electrical costs but factor in a couple of hours labour whilst you boot up all the computers and it might be worth considering.
    NE Derbyshire.
    4kWp S Facing 17.5deg slope (dormer roof).
    BEV : Nissan Leaf e+
  • rogerblackrogerblack Forumite
    9.4K Posts
    EricMears wrote: »
    A kilowatt of panels will of course only supply a kilowatt of base load for a few days in June (but not this week ! :mad: ). You really need the full 4Kwp to stand a chance of getting a Kw for a reasonable proportion of the year.

    Of course.
    But more than a kilowatt of output gets complicated.
    To output the same as the load would need around 10kW of panels over the year - but this implies infinite free storage - which even if your meter did go backwards, and the electricity company diddn't check when it was going backwards, is legally questionable. (if you use the grid as a large battery).

    To do this with actual batteries for most of the year would need perhaps 25kW of panels, and 100kWh of batteries.
    This would cost perhaps 35K, and will not pay back under most estimates, especially once you take into account maintainance.

    One option that I omitted would be an inverter selected (or configured) for a baseload of (say) 1kW, and 2kW of panels.

    This (at least in my location), especially if the panels were somewhat split in facing - say ssw and sse - pays back almost as fast as 1kW of panels, but produces more total electricity.
    As the panels both produce for longer each day (being better able to catch morning and evening sun), and catch more sun due simply to their area.
    Here, the loss in doing this is surprisingly low - around 20%.
    Leading to payback in perhaps 12 years.

    In short - FIT payoffs are _significant_, and unless you have some really compelling reason for avoiding them, they drastically reduce payback time.

    One such reason is that you're willing to DIY, and don't have a lot of money to invest.
    A couple of 220W or so panels, and an inverter setup in some manner so it doesn't backfeed the grid can be done with care for under about 500 quid.

    It would be nice to know the legal position WRT backfeeding.
    (assuming you have an inverter that will isolate on grid failure).

    As I understand it - in principle - if you have a meter that goes backwards, and pump electricity in - in principle you could be done for theft of the difference between your retail price for electricity and the export tariff (say 8p/kWh).
    Previous threads have reached the conclusion that there isn't a power company that will accept small generators not on the FIT scheme, and supply them export payments.
  • edited 8 June 2012 at 12:34PM
    grahamc2003grahamc2003 Forumite
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    edited 8 June 2012 at 12:34PM
    Much of the time, even with a 4kw system, you'll be using most/all of the panel output, assuming a 1.8kW baseload in your situation. This should make the electricity bill saving quite decent (relative to the rest of us), say £200/£250 as an informed guess (estimated by taking the panel output on a 12hr daylight day of power vs time as a gaussian distribution, and estimating the total curve integral (which is the total energy generated), minus the integral of the curve above 1.8kW (which is the exported energy). You only save this relatively large amount because you spend so much anyway (about £1600 just for your computers).

    I think your only realistic legal choice is via the fit system whether you want to or not - and that will cost you about £6k (anyone got current prices for 4kW?). I expect the fit plus export would come to around an additional rough £600/700 (if you're quickish), so, as a first cut approximation, a return of £900ish for you for a £6k investment (making lots of obvious assumptions). Knock off £100pa towards a new inverter after 10 years - making a cleanish £800ishpa.

    To get a better estimate of the fit - feed in your details into one of the sites designed for solar predictions.
  • TooledUPTooledUP Forumite
    7 Posts
    Hi,

    Thanks for all the info. I have already read it twice, but need to do it a few more times to understand it fully.

    There is an element of feeling not very green as well, which does concern me as I like to be environmentally friendly where I can.

    I don't mind feeding back into the grid, but I suspect during the daytime I would actually consume more than I would use.

    Regards elevation, that is something I will need to find out.

    In an ideal world, if I could get this load offset on a permanent basis, that would be a fantastic achievement. I realise that would require batteries, and cost a lot (although I didn't realise around 35K!)

    As to why I am getting to 1 KW, consider the fact that I have more IT stuff than the average office powered on. These servers consume 200W and I have 3 of them! That doesn't include the storage systems and other desktops, routers and switches.

    I guess I need to go away and get some solid data first on site and exact usage :)
  • EricMearsEricMears Forumite
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    TooledUP wrote: »

    In an ideal world, if I could get this load offset on a permanent basis, that would be a fantastic achievement. I realise that would require batteries, and cost a lot (although I didn't realise around 35K!)


    I think the figure of £35k included the solar installation and a rather generous 100Kwh of batteries.

    If your base load (just the computers) 1Kwh then your worst case of battery requirement would be for 20 hrs standby running or (very roughly - there are other things affecting the calculation) 20Kvah. At this time of year there would only be about ten hours of non-generation time
    NE Derbyshire.
    4kWp S Facing 17.5deg slope (dormer roof).
    BEV : Nissan Leaf e+
  • larkimlarkim Forumite
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    What's so complicated about all of this?

    Surely if a FIT installation is considered to be economically viable (which many still believe is the case), then a FIT installation with a double-whammy that you can manage to use most of the generated solar electricity is a complete no-brainer?

    The whole point of he FIT system is to subsidise a generation approach that otherwise doesn't make economic sense; so why would anyone consider a non-FIT system (if they can afford an installation in the first place).

    I must be missing something!

    (If all you want to do is be green and save on your current consumption, some might say that a RaR freebie scheme is worth considering. Just saying...)

    Matt
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