Solar PV Questions and Answers

We get asked a lot of questions about solar PV in Ireland. So we’ve decided to share some of the most interesting questions and answers below…

NB: The questions here are mostly niche/advanced. Consider reading our Overview of Solar Panels in Ireland first.

Question

I’m considering installing 18 PV solar panels on my SE roof. Should I move a portion of them to NW side? Oversized system with 5 kW inverter and 5 kWh battery.  SE roof 167 Deg and NW roof 339 degree. The idea was to get most of the system with afternoons sun.

Answer

Very good question! Your idea to get more generation in the afternoon/evening is a good one.  And I am a fan of East-West PV arrays. However, 339 degrees is more NNW than NW and 167 degrees is more SSE than SE, so the difference in energy output will be significant. You can play around with PVWatts to see the difference in energy generation for yourself… My quick check assuming 30 degree roof slope shows 50% more energy generation with SSE rather than NNW in Ireland.

So overall, my advice is go all panels on the SSE side of the roof.  Especially as all households will be eligible to sell surplus solar electricity to the grid from the end of July.  Plus you’ve got a battery to store energy for use later in the day.

Tl;Dr:  Put all your panels on the SE roof.

Question

Hi Dominic, I’m looking to either retro-fit a power diverter or battery to our solar panel units to optimise them. Hoping you can provide me with some advice on the best option.

Answer

Thanks for the question. Here’s what I would advise:

• Get a power diverter, especially if you use lots of hot water for mixer/pumped showers.  ROI on power diverters is generally good. Expect to pay around €500 + installation for a good one (I can recommend the MyEnergi Eddi).
• Don’t bother with a battery.  ROI on a battery would not be worthwhile in your case.
• Call ESB networks ASAP so and ask to get a smart meter (it’s free).  This way you’ll be ready to sell your surplus electricity as soon as the smart export guarantee scheme goes live (July 2021).
• If you want to invest more in solar then some extra panels will probably give you a greater benefit than adding a battery, especially once the smart export guarantee starts.
• Also, if you don’t already have an online monitoring app where you can see both your solar generation and your electricity consumption then get a monitoring device like the Emporia Vue installed.  They only cost about €150 + installation.

Question

Hi Dominic,
I’d appreciate finding out what you would recommend for my situation.
I’m in Cork and my house has a south facing roof with plenty of space for panels.
My current electricity bill is around 130/month, but I’m on a 40% discounted rate from Energia for 12 months, so that will increase.
There are 4 adults in the house.
Heating is currently by oil fired Stanley range which also heats the water.
I also have an immersion.
I have an electric car with just a granny charger at present.
I suppose what I am wondering is what size of system would you recommend:

1. Output of panels?
2. Battery size if required?
3. Unit to power immersion?
4. Car charger?

Answer

Good question! I’ll answer it step by step:

1. That’s a lot of electricity considering you’re using oil too. No surprise with 4 adults though. I’d advise getting as many panels as your budget allows, up to the limit of around 8/9kW which is about the most that it’s feasible to install given ESB regulations.
2. If you’re eligible for the grant then no harm to get a small battery (~2/3 kWh) but it’s not too important. You can sell your surplus electricity beginning in July anyway. In my opinion it’s not worth a battery if you’re not eligible for the grant.
3. & 4: I’d recommend the MyEnergi Eddi power diverter and MyEnergi Zappi car charger along with the MyEnergi hub to integrate them.

Extra advice: Make sure to call ESB networks and ask for a smart meter, so you’ll be ready to go with selling your surplus electricity beginning in July. ESB are prioritising customers with solar PV. Also, make sure to get day/night metering when your electricity contract is up for renewal (if you haven’t already). Day/night metering is a great match with solar PV.

Question

I’m building a house and want to install solar panels on it at a later stage. How can I make sure the design of the house will be suitable for solar?

Answer

For laying out solar panels on a roof, the main considerations are:
Size of the panels: Nearly all solar panels are either approx 1m X 2m or approx 1m X 1.65m
Distance from edge:  Building regulations state that the panels must be at least 500 mm from the edges of the roof.
Orientation:  Anywhere from E/SE/S/SW/W is fine.  Of course, due South gives you a few percent more energy output. Half east and half west is also a good option.
Portrait:  It’s generally more practical to install the panels in portrait if possible.  However, landscape orientation can work if need be.
Area: Try to provide an area for panels of at least 40 m2 + a border around the edge with no obstruction (e.g. roof windows, chimneys, etc.).

PS. If you might be installing solar panels on a detached garage, make sure the electric cable from the house to the garage is at least a 16-square.

It’s also good to get a large, well-insultaed hot water cylinder with a slot for an immersion heater. This allows you to store surplus solar electricity in the form of hot water for a long period of time. It doesn’t need to be a “solar cylinder” though (those are only required for solar thermal).

Question

Hi,
If it would not be trouble I would like to ask about the Solar panels:
SOLAR THERMAL How long it will take to heat 300 litres cylinder (Joule Cyclone with dedicated solar coil) for domestic hot water use (4 people) from 15° to 50° : – in a summer sunny day? – in a summer cloudy day? – in a winter sunny day? – in a winter cloudy day in Ireland?
How many panels we might need ? How long is the lifespan? What would be the costs of the complete system including installation?
What would be the difference in costs  between the Thermal panels vs PV panels (average 4-person household)?
What would be the required space in the hotpress room for plumbing stuff?

Answer

I’m not really an expert on solar thermal panels, but I can give you some pointers:

• PV panels generally cover a larger area of roof space than thermal. Perhaps 25 m2 for PV vs 4 m2 for thermal.
• PV panels can be used in combination with a power diverter to give hot water as well as electricity.  This setup typically supplies all the hot water a family needs from around March-October.
• November-Feb the PV panels provide enough energy to get the water lukewarm, so the immersion has only half the work to do. The same goes for solar thermal.
• With PV panels, no change to plumbing is required because the solar energy goes into your hot water via your already-existing immersion heater.
• You will need a special hot water tank if you go for solar thermal. It’s generally big (~1.5 m tall and ~ 0.7 m wide). Solar thermal also requires extra pipework, a pump, etc.
• Solar thermal warranties are generally 1-5 years. Warranty on PV solar panels tends to be much longer at 10-25 years. Other PV system components (battery, inverter) generally have medium-length warranties (5-10 years).
• Generally, you would be looking at ~€10,000 for 4kW of solar PV plus battery storage.  This would be a good amount for a family of 4.  You may be able to get a grant of €3,000 which brings the total cost down to ~€7,000.  Annual savings on energy bills would be in the region of ~€800.
• I don’t have cost estimates for solar thermal panels. They’re usually slightly lower-cost than PV initially though.
• PV panels require much less maintenance than thermal because they have no moving parts. So longer-terms costs will be lower for PV.
• PV panels give greater savings on energy bills because they provide electricity for general use as well as hot water

You might also find our article on solar PV versus solar thermal panels useful.

Question

How much do solar panels weigh? Will my roof be able to support the weight of the panels?

Answer

Solar panels are typically ~11 kg per square meter, or ~15 kg per square meter in the case of glass-glass panels. Exact numbers are available from the solar panel data sheets or your solar installer.

This is a modest load for a roof to support, and domestic trussed roofs are almost always structurally suitable for installing solar. If you have a large commercial flat roof you may need to get a survey done by an engineer to make sure the roof is suitable to support the extra weight of the panels.

Question

Hi Dominic,
I have a South facing rear roof, and it looks like a maximum of 12 panels can be installed.
Most installers have been quoting with a battery but one guy yesterday told me that I would not need a battery with 12 panels and that the battery would charge itself off the grid if there was no excess electricity generated to charge it.
I found this alarming as this would be costing 18 cent to charge per unit. Is this the case ?. My bills are €320 per 2-month period.

Answer

Thanks for the interesting question!

The battery definitely shouldn’t be charging itself from the grid with expensive day-rate electricity.  Either the rep was wrong or it’s a very silly design of battery!  The only exception would be if your system came with blackout protection, and you wanted to charge your battery in expectation of a power outage. On the other hand, some batteries can be set to charge from cheaper night-rate electricity.

Overall, batteries are sometimes worthwhile if you’re eligible for the grant.  Otherwise, just get yourself a smart meter and start selling your surplus electricity to the grid from July.  And make sure to get a power diverter whether you’re getting a battery or not.  The only reasons not to get a power diverter would be if you don’t have an immersion heater or if you have a heat pump.

€320 is a high bill, so it would be nice to find space for a few more panels somewhere as you seem to be suggesting.  6 kW would be nice 9 kW would be even better.  Solar carports, solar canopies, ground mounting, east-west mounting, and even putting a few panels on the north side are all worth considering.

Question

I plan to install a string inverter + battery solution. Should I consider microinverters instead? I have heard it is 4 times less risk of fire but 10% loss on energy conversion to the battery.

Answer

Micro-inverters vs a string inverter is a tough choice.

The 10% loss isn’t such a big deal because that only applies to electricity going to the battery, whereas a lot of your electricity can be used directly.  Plus there’s a 5% loss with a standard inverter+battery anyway, so you’re only losing an extra 5% on battery electricity. This probably translates to about 2% of your overall electricity generation, i.e. very little.  Furthermore, these losses will be offset by the slight performance gain of having a dedicated micro-inverter for each panel.

On the other hand, if you do get microinverters, you’d better be sure they’re from a top-quality brand (Enphase or SMA) because they’ll be very difficult to replace if they ever fail.  The upside is that Enphase microinverters have a great reputation for long and trouble-free lives, even more-so than good string inverters.  The cost of microinverters is also generally higher than an equivalent string inverter.

Correctly installed solar PV systems from reputable installers have very low fire risk regardless.  But you are right that microinverters provide a very strong extra layer of safety.  It really boils down to a personal decision rather than a technical question:  How much you value the extra reassurance that microinverters provide?