The terms “power” and “energy” are often inter-changed in everyday speech but their meanings are very different. Before going any further, I think it is important to understand this difference.

When thinking about a journey, it is clear to everyone that “distance” and “speed” are not the same. After a few moments’ thought, most people will agree that:
Distance (in miles) = Average Speed (in miles/hour) × Time_of Journey (in hours).

In the world of electricity, “energy” is related to “power” in just the same way. So:
Energy (in kWh) = Power (in kW) × Time_forWhichTheApplianceWasOn (in hours).

If a 2 kW heater (its power rating) were to be switched on for a period of 1 Hour (time), then 2 kWh (of energy) would be consumed. Similarly, if a PV installation were to generate electricity at a constant rate of 1 kW (power), and this situation were to be maintained for a period of 17 Hours (time), then 17 kWh (of energy) would have have been generated.

In the same way that “Speed is the rate at which Distance is covered”, i.e. miles per hour, so “Power is the rate at which Energy is consumed or generated” i.e. kWh per hour, or just kW. In the material which follows about the measurement of mains electricity, it is often helpful to think of power as the rate at which energy flows. (from )

Looking today (Nov 2013) at my domestic bill I can see that I pay basic netto price of 17cents per unit of energy !*

*additionally standing charge 34 cents per day, PSO Levy and 13.5% of VAT

Those are simplified calculations based on assumption that you use all energy from your renewable energy system. If you understand that you can read Energy Balance chapter below.

1. If you are not at home when your PV system is making it’s maximum power, or if you sleeping at night when your wind turbine is spinning, energy will flow back to the grid.

2. If there is no wind or no sun and you have your washing machine, kettle and play station on then energy will flow from the grid.

3. If you have 3kW PV system on your roof giving 3kW of power (perfect sunny conditions ) and you have your 2kW kettle on that means 1kW of power is flowing to the grid.

Understanding those 3 bullet points is a key to understanding energy balance and designing system for maximum self consumption rate.

It is important as you pay 17 cents for each unit of energy imported to your house, and they pay you only 9 cents* for each unit exported to the grid.

*9 cents apply to domestic only from Electric Ireland until end of 2015, It is nothing! for export for others (shame as in UK and many other EU countries they pay you at least the same amount for import as for export).

If you monitor your energy usage (at least you note your meeter readings) your energy profile can be assumed and based on that more precise calculations can be made. Below you can see example of energy balance graphs for one of the systems we have design (health center with annual usage of 60MWh and air conditioning) . If you can use more than 50 % of energy your system is producing I would call it a good balance.

The answer is NO.
You can find more details here.


cost of 1kW of PV system is 1000 euro

cost of 5kW wind system is 10 000 euro

Installation costs from 100 euro per kW up to 500 e per kW (depends on location, type of roof, ground conditions, cable lengths etc )

Assuming that cost of energy is 17 cents* per unit

  • – 5kW PV system can make annually energy worth 850euro (5MWh),

– 5kW wind turbine in good location can make annually energy worth 1400euro (8MWh)
*Cost of electricity from the grid in Nov 2014: 17 cents per kWh (unit ) – excluding standing charge and other extras… all above exclude of VAT.

That simple calculations show that you have your PV system paid back in 8,8 years, and wind turbine back in 7 years. In reality energy rate of 17 cents per unit will get much higher in coming years. In reality calculations are not that simple…

P.S. Big banks are investing millions of euro in big wind farms have those payback periods longer ! (up to 15 years !!!)

P.P.S. Internal rate of return is describing investment better than payback period.



Assuming that:

your PV system will survive 25 years
you have paid 1500* euro per each kW
each kW in well design PV system should produce 1MWh of energy per year**
RESULT: in 25 years your system will make 25 MWh that means 1kWh (unit of energy) will cost you 1500e/25000 kWh = 6 cents !!!

*price 1500e per kW can be achieved but that depends on many factors like type of modules, cable length, type of mounting system etc.

**production yield of 1MWh per kW can be achieved in south parts or Ireland and coastal areas. Production yields will decrease with age of the modules (0.5-1% per year )

Solar irradiation in Ireland can be very different from place to place. Clouds play the major role in here. I live on the coast and can see that very often clouds are forming few miles in land when it’s still sunny on the coast. Same thing can happen with hills around. If you consider your area sunny you should definitely consider solar system.

Maps of solar radiation are not very precise but most of them show higher radiation farther south and in coastal areas.

Estimation of annual production can be very simple. For south faced, tilted at 30 degrees good quality system you can expect 1MWh of energy from 1kW. That could be more in more sunny areas and less.

I am using PV SOL Premium software to design system and estimate energy production for clients (takes shadow, inverter efficiency, loses, and many many other factors in to account ) but for your basic needs this online calculator should do.


If you want to read more check SEAI PV guide.

Any PV system needs sufficient sun access: a steady, unshaded six hours a day (generally, from 9 a.m. to 3 p.m.) is recommended. Several other factors come into play for roof-mounted systems: the roof’s pitch and azimuth, plus the roofing material itself. Standing seam, face-fastened, corrugated or shingled metal, asphalt (a.k.a. composition or “comp”) or wood shingles, slate, concrete, clay or spanish tile—whatever the surface, a manufactured mounting solution is available, although that doesn’t mean every installation will be straightforward or simple…

Check out sflex or schletter website for more details on mounting systems I have been using.


Assuming that:

  • your wind turbine will survive 25 years
  • you have paid 10 000 euro* for 4kW wind turbine
  • that wind turbine will produce 8 MWh of energy per year**

RESULT: in 25 years your wind turbine will make 200 MWh that means 1kWh (unit of energy) will cost you 10 000e /200 000 kWh   =  5 cents !!!

*that price can be achieved if you do installation yourself

** in good windy location with average wind speed of 6m/s and  low roughness that production can be achieved by good quality 4kW turbine (4kW means power at wind speed equals 11m/s)

Ireland has very good wind resource, on open areas, hills and coast we can find places with average wind speed around 6m/s at elevation 12 meeter above the ground. On SEAI wind map you can see average wind speed at 50m above ground and above.

Before installation of wind farm wind resource assessment is done based on one year precise measurement on up to 120meters high met mast. For small scale wind turbines that could be done too, but most often we do not want to wait a year for wind measurements. There is few important factors regarding placing a turbine (prevailing wind direction, shadow and turbulence are the most important).

To know how much energy your wind turbine will produce you need to know 2 things:

  • average wind speed of your site (at the hub height);
  • estimation of expected annual energy production (AEP), this can be done by independent test center or data sheet of wind turbine (manufacturers usually increase it).

Below you see AEP for windspot wind turbine (4m rotor, 6kW max power of generator) from test center and from manufacturer.

I hope that everyone understand that wind turbine is producing little power at small wind speed and higher power at higher wind speed. Power curve of the turbine describes that relation. Power curve also says about quality of the wind turbine (it’s starting wind speed and maximum wind speed). Again power curve can be measured by independent test center of you can find it on data sheet. Examples below.