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Am I oversizing? Opinions sought!

  • 11-05-2022 11:34am
    Registered Users Posts: 22

    Hi, I’m looking for opinions on what size of a system I should have installed, considering the following:

    • Usage over the past 12 months was approx 11,000 units (3500 day, 7500 night). Avg daily DAY units in the summer months is 8 units, 12 Day units in the winter.
    • South facing roof at 35 degree pitch with enough space to max out with a 6.6KWp system if I wanted. However, I would like to leave a gap in the panels for a velux window in the event of an attic conversion in the medium/long term which would still allow up to 5KWp of panels.
    • A3 rated home, air to water heat pump, underfloor heating. No electric showers. Heat pump heats water so no need for an Eddie.
    • Someone at home most of the time to run appliances when there is excess being generated.
    • 64kwh EV sits in the driveway 5 days a week and can soak up excess as Zappi is already in place. I put on average 20kwh into the car each night due to a long commute. It would be nice to be doing some of this charging from solar during the day.
    • Definitely want a battery and the option of using this as back up power for the entire house in the event of a power cut of which we get 3 or 4 per year in our area. What suppliers install an automatic switchover as opposed to a manual switch?
    • Id like a fair bit of control over what happens with any excess solar. Ie what percentage it charges the battery to before switching to the Zappi or what percentage the storage battery gets charged to overnight, leaving a bit of capacity to be topped up by solar in the morning. What inverters have a good level of functionality like this?

    At the moment I planning on a 5KWp system with a 10kWh battery. The 10kWh battery would allow me to use night rate electricity in the winter along with the couple of units of solar generation and avoid using any day rate units at all most days.

    Am I oversizing or does that sound about right? There’s currently only two people in the house but we plan on adding a few kids so I want to factor in inevitable increases in usage over the years.


  • Registered Users Posts: 2,411 ✭✭✭bullit_dodger

    Might be oversizing the battery (maybe) and undersizing the panels.

    With 11,000 units per annum, I'd be trying to squeeze every available panel onto your roof. Yeah, I know you have a attic conversion planned, but I wouldn't factor that into the solar panels now. The reality is that you might never do the extension, or whatever "gaps" you leave now won't be quite right with the way the windows want to go in and you'll have to move the arrays anyway when you do the extension. It's easy enough to do this, an hour or two to take the panels up and move them.

    With the EV, you don't need a massive battery... 5Kwhr is usually "ok" as you have the EV as a bigger storage, at least at the weekend if your communting during the week with it. That said, 10kwhr does seem to be a sweet spot with 11,000 (I have that usage myself - albeit it's house usage which behaves a little different than 11,000 with EV in the mix - as the EV tends to be all night time)

    Do you have another roof that you can place a few panels on? even "semi-north" facing isn't the end of the world and will recoup their money over the lifetime of the panels. if you can get to 7-8Kwp in panels I'd try it.

  • Moderators, Sports Moderators Posts: 18,644 Mod ✭✭✭✭slave1

    Leave space for the future as outlined but as you have roofers on site and a sparky too I'd seriously consider panels on your N facing aspect, they really kick in from April-Sept and will give a good boost, you can go 8kWp on a 6kW inverter

  • Registered Users Posts: 10,801 ✭✭✭✭the_amazing_raisin

    Would it make sense to just put the skylights on the north side and max out the amount of panels on the south facing side?

    Also keep in mind you need at least 30cm between the panels and the roof edge AFAIK. That can cut down the number of panels a bit

    Otherwise I'd say you're in a very similar boat to me. I don't think you'll be able to fill the battery from solar every day buy you can also use it to store up some night rate electricity

    You could also look into getting an Eddi and maybe a bigger hot water tank to store up some heat? Plus it means you can have 30 mins showers guilt free

    "The internet never fails to misremember" - Sebastian Ruiz, aka Frost

  • Moderators, Home & Garden Moderators Posts: 5,752 Mod ✭✭✭✭graememk

    • 64kwh EV sits in the driveway 5 days a week and can soak up excess as Zappi is already in place. I put on average 20kwh into the car each night due to a long commute. It would be nice to be doing some of this charging from solar during the day.

    Yeah the car can soak up the power well. But if you have a long commute, and put 20kwh in a night (approx 100-120km?) Is the car there during the day? It needs to be there in the middle of the day to catch the high sun.

  • Registered Users Posts: 22 JimmyKane

    OP here, thanks for the replies.

    True, the attic conversion might never happen if finances don’t allow it. The location for the window is kind of set in stone as the house was built with the attic ready for conversion and lots of neighbours in the estate have done it already, so should be easy enough to nail down the exact location for the window. Regarding putting in panels anyway and moving them when the time comes say in 6 or 7 years, would the savings made from that extra kWp not be lost in the money I’d have to pay an installer to come out, set up scaffolding and move the panels? If it cost me an extra €1,200 for that additional 1kWp of panels which might generate at best 1,000kWh a year x 7 years = 7,000kWh. If I was to manage use every one of those units and the saving was at a unit price of say halfway between by current day (22c) and night rates (11c), that would be 16.5c x 7000 = €1,155. The extra panels have now finally just paid for themselves after 7 years when I want to remove them. And I still have to pay an installer a couple of hundred euro to come out and move them. So, although I’d like to max out the panels, I don’t think it’d make sense?

    The other side of the roof is due north so I don’t think that would be a good return on investment. What sort of annual generation can you expect per kWp of panels facing north?

    View from the south side is better and at the back of the house so I’d want one of those duplex cabrio window balcony’s. Would also make it easier for cleaning the panels every now and again! Would probably put in a couple of roof lights on the north facing side of the roof for extra light.

    Yeah the car is there most of the time during the day. On the couple of days I work often I don’t leave the house until around 3pm. I have it worked out that there’s 2 full days per week on average when the car isn’t there during the day.

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  • Moderators, Sports Moderators Posts: 18,644 Mod ✭✭✭✭slave1

    North will provide 50% of South generation but do not discount it, really kicks in late March through to September

  • Registered Users Posts: 10,801 ✭✭✭✭the_amazing_raisin

    I was about to say I checked the calculator for my location and got about half the generation on the North facing side than South for the same number of panels

    Given panels are relatively speaking one of the cheapest components in the system but difficult to install then there probably is some merit in putting them on the North side

    However, they're really no use in winter when you'd be scrambling for every kW. If you have a South facing garden then I'd almost say build a big shed or extension (or both) and put panels on those

    Or do all of the above 😁

    "The internet never fails to misremember" - Sebastian Ruiz, aka Frost