How Solar panels really perform

Solar panels are pretty much the norm and for most of you it is a choice between the cost of power and the cost of solar installations. Solar is more a longer term investment, but not always. our micro systems usually pay back within the year to 18 months, meaning that from that time forward its all money saving.

Solar panels are sold by businesses who may not know fully how solar works, its a cash wagon with over charging and a lot of frauds as we reported the news on our facebook page. but solar panels themselves can be a lot misleading.

Solar panels are tested using STC – Standard Test Conditions, this means in a lab, with a temperature of 25oC with 1000Kwm2, However in the real world these can be very different results.
NOCT – Normal Operating Condition Tests show a different picture of how solar panels really perform.

To find out this important data, we need to look at the data sheets of the solar panels.

bifacial Solar panel 440w — RenewSolar

As you can see we have STC and NMOT, As you can see there is more than 100W difference in the panels and we are using 800Wm2, the temperature is lower at 20oC, but often the solar panels have a much higher temperature.
This is where the Temperature Coefficients come into play. We lose Voltage .25% per oC and this is around 0.28W per oC. so from 20 – 50oC is 30 degrees difference – thats 30 x 0.28 = 8.4% which is around 40Watts loss.

Now if we look closely we can see the 440W on the power curves (right lower line graph) we can see the top line, which is NOT equal to 1000Wm2 gives us around 440w and the 1000Wm2 gives us about 375w.
we also take into account that this is at a temperature of 25oC Therefore the 30 degrees loses us 40W from the 375w of the panels. giving us around 335watts, but we also need to consider weather. so we use the 800Wms which is around 11 amps together with the voltage drop would give us around 350w panels.

Typical installations on the roof are subject to around 35% power loss, so you then drop to around 202W per panel. or about half the power as advertised. but these are bifacial, so you do claim back a lot of the 35% losses.

while this sounds bad, the panels performance varies over the year as the sun angle differs and the temperature changes, as we get indirect sun in winter the temperature drops meaning you have more power and less losses. but you may have other factors such as the tilt and angle play a larger part in how much solar and for how long you can yield power.

To give some real terms of power our 870w array in spring to early summer will put out up to 900W normally around 675W as a average, in the summer this drops to around 625w and normally around 375W. Typically in summer we use less power in the home and also the sun is out for longer. so in effect this cancels itself out.

In summer we get sun from 4:25 until 21:35 (17 hours) however this will vary depending on your install and location. it is far more like 12 hours for most installations roof top south facing ( this amounts to 180 degrees)

Check out this power on how the position of your solar panels work. Sun Hours and Seasons: How to Angle the Panels — RenewSolar

MONEY MAKER

Solar installations take many forms, from the simple grid tied solar, which is a matter of “you only save what you use”. There is the Hybrid which has batteries and stores the power for later use, and there are the ATS or off grid, kinda hybrids.

grid tied, means that it works in parallel with the grid a costly endeavor due to certificates and a lot more work. but hybrids just have a battery and can be DC or AC coupled and combined.
with grid tied you just produce power, anything you use you save and anything more gets exported. We tend to think of these as off-sets.
The hybrid takes away the export and stores the power you make. it also buffers clouds to give you a smoother power output and covers the drops in power. which when you look at live data shows a different picture.

As you can see here, this shows around 30 minutes of solar power, if we have a load of 400wh when we are over 400W we are exporting, when we are below we are importing. Most apps will show you the peaks line giving you the false impression of being above for most of the time which is FALSE.
We can flip the power above to fill the gaps of the power below our load, this is what a hybrid system will do and take away the charges for drawing from the grid.
with the grid tied, every time the solar drops below the load I will be charged.

The cost of the install, the off set or off grid will depend on what system you have and what you really desire.

lets say we have the 800W grid tied micro inverter system, thats £488.
The current price of power is 0.21pkWh. 488/0.21 is 2323kWh
We can work this out over 18 months gives us a power requirement of 4.5kWh per day.
And just to prove the point – here it is making that power on a average day.

While this shows you the pay back is in 18 months it is grid tied so you only save what you use. but for the purpose of this article we will pretend you do. There is also season difference which will change the amount of power you get over the day. and therefore its best to buy and install by late February so that the return is much shorter as it will get much longer over winter adding just under 3 months to the pay back time.

While many will say you Save with Solar, you have “free energy” of course that is ONLY after you have paid back the cost of the hardware. but on this system you would be “making” £1.00 per day or just under based on the current cost of power. That was £1.71 per day not long ago as a comparison, that then changed to a pay back time of just 285 days!!

The big cost of solar batteries

Solar energy storage is good if you make enough power to charge the battery and cover the power that you use. but the cost of the batteries can be massive, we were the first in the UK to sell the 15Kwh battery for £2200, even Fogstar cannot keep up with us. but they are technically on par.
So lets look at this in numbers 2200/0.21 =10,476kWh (699 days)

You do need to make sure that any solar array can charge the battery and supply the load, therefore you need to take care in the sizing of both the array and the battery. You wont discharge ( or you should not) therefore the charge kWh would need to be divided by the time of year yield per hour and the daily load.
for example a 6 hour @600w would yield 3600Wh (3.6kWh), 2000w over the same period would yield 12,000wh it is always – load ( minus load) if you have 300w running then this goes to the home and not the battery.

Please see our consultation services to talk about how to install and set up solar. you may even want to take up our project management services and you can take benefit of hardware savings too.