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How many PV panels is enough?

  • 17-04-2019 7:00pm
    #1
    Registered Users Posts: 9,775 ✭✭✭ Effects


    What's the minimum number of panels to make it worthwhile?
    Is 3 300w panels sufficient?
    Coming from a green perspective, not about cost saving on electricity.
    Mostly self installed so no grant or high installation costs.


Comments

  • Registered Users Posts: 1,560 ✭✭✭ Prenderb


    If it's just a "green" perspective, and I don't mean to hijack this thread to open a big debate, but if it were me I would be considering whether I can actually generate enough electricity to counteract the emissions involved in manufacturing the PV panels. Admittedly this article is 5 years old https://spectrum.ieee.org/green-tech/solar/solar-energy-isnt-always-as-green-as-you-think but highlights some of the environmental side effects of the manufacture of photovoltaics. I look forward to a more positive view, if there is one!

    The renewable component of grid-generated electricity is increasing year on year anyway - was 30% in the last month.

    Other issues to consider are the amount of usable electricity you'll get - without storage (i.e. a battery system) you will only benefit from reduced grid usage when the sun is shining and when you're using electricity, i.e. during the day which will reduce the positive effect of your install. So if your house is not using electricity during the daytime, are you going to be able to reap the benefit? Of course if you're feeding into a heat pump which is on all day or something like that, that's usable.

    3x300 W panels would yield 0.9kW at maximum yield, which won't boil a kettle on it's own, but might run a PC and a television or something like that?

    I'm interested to see what other feedback you get in relation to the question. I still think solar PV has a role in new builds where there's a Building Regs requirement for renewables, but for me I'm still mixed as to it's own merits on an existing property.

    That could all change when feed-in tarriffs are introduced and you can sell unused electricity back to the grid as you generate it.


  • Registered Users Posts: 9,775 ✭✭✭ Effects


    It’s not my house, but a friends. My thinking was it would cover his base load during the day anyway.
    Possibly with view to a battery system when they come down in price.

    They aren’t going for a heat pump, just gas.
    Which is using more renewable gas each year.

    I wouldn’t hold out for feed in tariffs myself.
    I’m possibly getting an i3 in a couple of months, and with my own 1.5kw system, will probably add a couple more panels to keep output up.


  • Moderators, Category Moderators, Motoring & Transport Moderators, Regional East Moderators Posts: 58,818 CMod ✭✭✭✭ unkel


    Effects wrote: »
    What's the minimum number of panels to make it worthwhile?
    Is 3 300w panels sufficient?
    Coming from a green perspective, not about cost saving on electricity.
    Mostly self installed so no grant or high installation costs.

    Purely from a green perspective, the answer is the most panels you could physically fit.

    A typical semi detached house could easily fit 13 panels, producing 4000kWh per year (south facing). The average household in Ireland uses just 3500kWh per year. So you are producing more than you are using!

    Buy the parts well, install DIY and you can install this system for under 2 grand.

    "Wind is Ireland's oil" - An Taoiseach, 25/05/2022



  • Registered Users Posts: 9,775 ✭✭✭ Effects


    The house is on a west facing terrace. They are adding a two story extension that's south facing. This is the part that'll fit a few panels, but not a lot, as it'll have two skylights in it as well.


  • Moderators, Society & Culture Moderators Posts: 31,241 Mod ✭✭✭✭ Gumbo


    900w peak production won’t be enough to power base load and feed a battery in my opinion.

    I’ve a 3kw system. Would love to go bigger!


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  • Moderators, Category Moderators, Motoring & Transport Moderators, Regional East Moderators Posts: 58,818 CMod ✭✭✭✭ unkel


    Effects wrote: »
    The house is on a west facing terrace. They are adding a two story extension that's south facing. This is the part that'll fit a few panels, but not a lot, as it'll have two skylights in it as well.

    If they still have west facing roof left, they can install a second string on that. Many modern inverters are MPPT and can use 2 or more strings. And optimise both strings separately.

    "Wind is Ireland's oil" - An Taoiseach, 25/05/2022



  • Registered Users Posts: 9,775 ✭✭✭ Effects


    It might not charge enough to power the entire house but it’ll keep the base load covered through the night, wouldn’t it?
    Battery is only a future thought for now anyway.


  • Registered Users Posts: 9,775 ✭✭✭ Effects


    unkel wrote: »
    If they still have west facing roof left, they can install a second string on that. Many modern inverters are MPPT and can use 2 or more strings. And optimise both strings separately.

    West would be front of house. I can’t remember what rules apply for that?


  • Moderators, Category Moderators, Motoring & Transport Moderators, Regional East Moderators Posts: 58,818 CMod ✭✭✭✭ unkel


    Effects wrote: »
    West would be front of house. I can’t remember what rules apply for that?


    No different rules for front of house / road facing. Still the same old rules apply of max 12m2 of panels and max 50% of total roof space. Use more and you will need planning permission. These rules are antique and ridiculous at this stage, will probably change soon I would expect, but they are still here...

    "Wind is Ireland's oil" - An Taoiseach, 25/05/2022



  • Registered Users Posts: 9,775 ✭✭✭ Effects


    unkel wrote: »
    max 50% of total roof space. These rules are antique and ridiculous at this stage, will probably change soon I would expect, but they are still here...

    Is that 50% of the front roof space, or total roof space? i.e. All the front could be covered and none of the back.


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  • Moderators, Society & Culture Moderators Posts: 31,241 Mod ✭✭✭✭ Gumbo


    Effects wrote: »
    Is that 50% of the front roof space, or total roof space? i.e. All the front could be covered and none of the back.

    Subject to a max of 12 square meters.


  • Registered Users Posts: 8,287 ✭✭✭ n97 mini


    kceire wrote: »
    900w peak production won’t be enough to power base load and feed a battery in my opinion.

    I’ve a 3kw system. Would love to go bigger!

    Here's my base load for a day. I have 2 x 300w panels which are currently peaking at about 350w combined as they're not optimally positioned.

    As you can see we're mostly between 250w and 500w in daylight hours. That's the low hanging fruit and can be cheaply replaced with solar.

    The spikes are mainly kettle, toaster and microwave during daylight, and it's pointless chasing that as their demand is very large and very brief. So the grid handles that.

    The really big loads are charging the car, the dish washer and washer/dryer, which are all done on night rate and would be prohibitively expensive to try to supply with solar and batteries.

    Estimated ROI for current setup is about 2 years and 9 months. Until a universal FIT is introduced, my advice is go after the low hanging fruit and stay small.

    478279.JPG


  • Closed Accounts Posts: 3,364 ✭✭✭ rolion


    n97 mini wrote: »
    Here's my base load for a day. I have 2 x 300w panels which are currently peaking at about 350w combined as they're not optimally positioned.

    As you can see we're mostly between 250w and 500w in daylight hours. That's the low hanging fruit and can be cheaply replaced with solar.

    The spikes are mainly kettle, toaster and microwave during daylight, and it's pointless chasing that as their demand is very large and very brief. So the grid handles that.

    The really big loads are charging the car, the dish washer and washer/dryer, which are all done on night rate and would be prohibitively expensive to try to supply with solar and batteries.

    Estimated ROI for current setup is about 2 years and 9 months. Until a universal FIT is introduced, my advice is go after the low hanging fruit and stay small.

    478279.JPG


    Is this reply or graph ... a joke !??
    As I really dontvwanna take it as non knowledge or fcuking worse ... an offence ! ;)


  • Registered Users Posts: 8,287 ✭✭✭ n97 mini


    rolion wrote: »
    Is this reply or graph ... a joke !??
    As I really dontvwanna take it as non knowledge or fcuking worse ... an offence ! ;)

    :confused:

    Instead of being offended, why don't you refute my points, which are actually based on real data?


  • Moderators, Category Moderators, Motoring & Transport Moderators, Regional East Moderators Posts: 58,818 CMod ✭✭✭✭ unkel


    n97 mini wrote: »
    Estimated ROI for current setup is about 2 years and 9 months.

    Can you elaborate on that calculation? How much did you pay for the panels each and for your inverter and how many kWh do you expect to produce per year?

    "Wind is Ireland's oil" - An Taoiseach, 25/05/2022



  • Registered Users Posts: 8,287 ✭✭✭ n97 mini


    unkel wrote: »
    Can you elaborate on that calculation? How much did you pay for the panels each and for your inverter and how many kWh do you expect to produce per year?

    2x Eco Delta 300w panels, Solis inverter, cables, plugs came to €450.

    Currently producing 2.5kw/h per day. Electricity costs €0.21 per kw/h, so €0.52 coming from solar per day.

    450/0.52 = 865 days = 2 years 4.5 months.

    Weather is particularly good at the moment, so I'm allowing a bit for "normal" weather.

    We're also exporting some amounts as the meter has detected it, but I can't tell how much or when as having an inverter plays havoc with the CT clamp on the energy meter. Battery powered (e.g. AA) CT clamps don't work at all well with inverters, they must be AC powered due to incorrect assumption of power factor.

    BTW, thanks for your initial advice, I'm compiling more question for you ;)


  • Registered Users Posts: 8,287 ✭✭✭ n97 mini


    n97 mini wrote: »
    We're also exporting some amounts as the meter has detected it

    Just to elaborate, the display on the meter cycles between "rEd" and the number of units.

    rEd = reverse energy detected

    Apparently the meter also records export power, but doesn't display it. Need the manufacturer's handheld scanner thing that they only supply to ESB etc.


  • Moderators, Category Moderators, Motoring & Transport Moderators, Regional East Moderators Posts: 58,818 CMod ✭✭✭✭ unkel


    n97 mini wrote: »
    2x Eco Delta 300w panels, Solis inverter, cables, plugs came to €450.

    Currently producing 2.5kw/h per day. Electricity costs €0.21 per kw/h, so €0.52 coming from solar per day.

    450/0.52 = 865 days = 2 years 4.5 months.

    Solis mini inverter? You did well buying. I presume your costs include all mounting bits and pieces (which can be expensive enough :()

    From your other posts I reckon your install is pretty recent? You said you have peaked at 350W. Given the time of year, that means you will peak at an optimal day at about 380W. This means your system will generate roughly about 360kWh per year. All at peak times of course. You said you switched to EI, so have I and afaik their peak rate is about 18c/kWh

    So your revenue is 360 * .18 = €65 per year (presuming you use 100% of your production and you never feed the grid - this is a bit optimistic even with a very small system - you state yourself that it looks like you already exported)

    With zero interest and zero opportunity cost of money, this represents a payback period of 7 years (flattered). Pretty good, but nowhere near as good as you thought.

    Happy PVing anyway :cool:

    "Wind is Ireland's oil" - An Taoiseach, 25/05/2022



  • Moderators, Category Moderators, Motoring & Transport Moderators, Regional East Moderators Posts: 58,818 CMod ✭✭✭✭ unkel


    n97 mini wrote: »

    BTW, thanks for your initial advice, I'm compiling more question for you ;)

    You're very welcome :)

    I'm only a novice myself in this whole area. Started with my own small DIY setup on my shed at the start of last year. But it has grown quite a bit since then. Turned into a bit of a hobby even :D

    "Wind is Ireland's oil" - An Taoiseach, 25/05/2022



  • Registered Users Posts: 8,287 ✭✭✭ n97 mini


    unkel wrote: »
    Solis mini inverter? You did well buying. I presume your costs include all mounting bits and pieces (which can be expensive enough :()

    Yeah, I took a last minute notion to go for a certified inverter and got a brand new one at a good price. The ancillary stuff was a bit more costly than I expected, but my install was very simple.
    unkel wrote: »
    From your other posts I reckon your install is pretty recent? You said you have peaked at 350W. Given the time of year, that means you will peak at an optimal day at about 380W. This means your system will generate roughly about 360kWh per year. All at peak times of course. You said you switched to EI, so have I and afaik their peak rate is about 18c/kWh
    All came together in the last week or 10 days.

    I don't understand how you arrived at 360kw/h annually, which is 1kw/h per day? The inverter says today's output was 2.7kw/h, but considering the good weather, it's still spring, my panels in their temporary position are slightly west facing (at a 15 degree angle) and are overshadowed from about 6pm (and partially for about an hour in the morning around 11am, by a fecking tree!), I'm expecting their output to peak in summer at over 3.0kw/h and to trough in winter at 1.0kw/h or less, especially when they eventually get mounted in a south facing aspect.
    unkel wrote: »
    So your revenue is 360 * .18 = €65 per year (presuming you use 100% of your production and you never feed the grid - this is a bit optimistic even with a very small system - you state yourself that it looks like you already exported)
    EI is currently €0.21 (price just went up). With an annual daily average of 2.0kw/h (which I think is slightly pessimistic), that's €153 a year
    unkel wrote: »
    With zero interest and zero opportunity cost of money, this represents a payback period of 7 years (flattered). Pretty good, but nowhere near as good as you thought.

    Happy PVing anyway :cool:

    We'll chat again in a year :)


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  • Moderators, Category Moderators, Motoring & Transport Moderators, Regional East Moderators Posts: 58,818 CMod ✭✭✭✭ unkel


    n97 mini wrote: »
    I don't understand how you arrived at 360kw/h annually, which is 1kw/h per day? The inverter says today's output was 2.7kw/h

    Experience and generally available statistics.

    A south facing 600w system in the best parts of Ireland generates about 580kWh per year. But your system is far from ideal, the best you have seen is 350W. I saw about 90% of my max production in March (3.6kW from a 3.8kW system), so your max production is about 380w. So your annual production is 360kWh or so per year at best

    I made about 25kWh today myself. But today was far from a typical / average day, probably one of the best days of the year :)

    Some days you make almost zero

    Still your pay back period of realistically about 10 years is pretty good. My own first system, with very cheap end of line panels I sourced and a GBP70 eBay inverter and 100% self use (cryptomining made sure of that) had a calculated pay back time of about 5 years. I sold all the parts for more than I paid for them within a year, so my actual realised pay back time was 0. In other words, I had all the benefits, but less than zero costs :cool:

    "Wind is Ireland's oil" - An Taoiseach, 25/05/2022



  • Moderators, Home & Garden Moderators Posts: 10,101 Mod ✭✭✭✭ BryanF


    rolion wrote: »
    Is this reply or graph ... a joke !??
    As I really dontvwanna take it as non knowledge or fcuking worse ... an offence ! ;)
    Rolion, cut it out. Just explain the graph lacks info, without the abuse. Thanks
    n97 mini wrote: »
    :confused:

    Instead of being offended, why don't you refute my points, which are actually based on real data?
    Attack the post not the poster. Please clearly label the data.

    Let’s leave it there. Thanks


  • Registered Users Posts: 66 ✭✭✭ Bohser


    n97 mini wrote: »

    We'll chat again in a year :)

    How did you get on?

    I looked at this today and for 3rd party install of LiFePO US2000 storage with 10 Longi 350kW panels I reckon we're looking at a little less than 16 yrs payback factoring a feedback tariff to be applied.

    Our usage is low for a 5 person house with people wfh (3,200 kW per annum).
    10 panels at 5.33kW per day during winter = 160kW p/m when we're using 350kW p/m for last year's January (70 day) bill would have been €135 instead of €207. So saving possibly €70 in winter months and up to €90 (high usage) in summer months.

    So averaging €80 per two months (probably on the optimistic side) I'm estimating savings of around 480 p/a before a feed tariff. I'd actually go for it if we thought we were staying in this house but we're not sure & will need the cash - €8K.

    Wondering if a 3rd party 6 panel battery-less install for €3K may be OK and good for the environment? (Going 3rd party as not sure if I've the time or tools install myself).


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