The world of solar panels can get pretty technical. There’s lots of jargon to confuse the poor homeowner: Watts, Watt-hours, efficiency, temperature coefficients, tilt angle, orientation etc. etc. Amongst all this noise, the bottom line is, how much money can you save? The answer to this is critically dependent on how much energy you get from your panels.
Let’s start with some basics. Probably the first number you’ll see for your panels will be the power rating, given in watts. The panels might be 300 W, for example. Watts is a unit of power. The power rating of your panel is the amount of power that the panel will generate under standard test conditions (STCs). STCs are a level of sun intensity of 1000W per m2, a cell temperature of 25oC, and no wind. The power rating is used to compare different panels: the actual power you get from your panel depends on a whole host of factors including actual sun intensity, orientation and tilt of your panels, and the real temperature of the panels (in the sun panels can get a lot hotter than 25oC!). But more about this later.
So is a 350W panel more efficient or better than a 300W panel? Not necessarily. A low efficiency 300W panel will give you exactly same amount of power (300W) as a high efficiency 300W panel under STCs. The lower efficiency panel would be larger to deliver the same power. Panel manufacturers typically make panels with a range of powers, e.g. 250W, 300W, 350W etc. While there may be some difference in efficiencies, the main difference is often that panels with higher power are simply larger than the panels with lower power. Installers might like to use larger, higher wattage panels since they need to carry and install less panels for the same system power. Alternatively, your roof may have space that works better for smaller size, lower watt panels.
One outcome of having panels with different powers is that you can’t compare different solar systems just based on the number of solar panels. For example, one installer might give you a quote for 20 x 300W panels. It is the total system power that is important. In this case, the total system power is 20 x 300 = 6000W. Another installer might quote for only 18 panels, but these are 370W panels. The total system power is 18 x 370 = 6,660W. Therefore the second system has more power, even though it has less panels.
So total system power is a good first-pass comparison of different system quotes.
Power vs Energy
Getting back to the basic question, i.e. how much money will you save? When you pay for electricity, you pay for the amount of energy you use, not power. Energy equals power x time. The units of energy are watt-hours (Wh) or kilowatt-hours (kWh). This is what you will see on your electricity bill. A watt-hour is the amount of energy gained by producing 1 W of power for 1 hour.
The amount of energy you will get from your system, and therefore how much money you will save, is dependent on quite a lot of factors.
The most obvious factor is how big your system is. As we’ve discussed, the total system power is a good indication of this. More power = more energy produced in a given time. Simple. The other factors are a bit more complex.
Where you Live is Important
Sunlight is more intense the closer you are to the equator. But it’s not that easy. Obviously weather plays a key role as well, i.e. how many cloudy, rainy days will decrease your solar output? Good solar companies will use sophisticated software to give you an estimate of your solar energy. This software uses a year of real weather data to calculate your solar energy. This means rainy days are factored into your energy estimate.
To give an idea of how much the amount of sunshine can vary with location, the table below gives some total solar radiance numbers for different cities in the US. There certainly is a large variation. People living in Phoenix will pay their solar systems off much faster than people in Boston, and if you’re way up in Anchorage, expect to take nearly 2.5 times longer to pay off your system than your friends in Phoenix.
|Location||Incident Sunlight (kWh/m2/year)|
The second major factor is orientation of the roof. In the northern hemisphere, it is best to mount solar panels on a south-facing roof (towards the equator). In the southern hemisphere north-facing roofs are better. But how big a difference is there for different roof orientations? And if you live in the northern hemisphere, is it worth it to put panels on a north-facing roof?
The diagram below shows a simple, model roof on a house located in Florida. It has a roof with areas that face north, south, east and west. The different colors show the different amounts of sun that hit each face. Brighter colors indicate more sun, darker colors less sun.
The southern face is brightest, as expected. It receives 1830 kWh of sunshine per m2 per year. The east and west faces get 1640 kWh/m2/year while the north face 1440 kWh/m2/year. So compared to a north face, east and west faces will produce about 10% less energy, and the south face about 21% less energy.
Many people seem to think that putting panels on a north face (or a south face in the southern hemisphere) isn’t worth it, however that is often far from the case. In good areas, a system on a south face will pay for itself in 3-4 years, depending on upfront cost. The warranties on panels are generally 25 years, so that means 21 to 22 years of making money. If the panels are on the north face, the payback will simply be about 20% longer, 4-5 years instead of 3-4. Still an outrageously good investment with 20-21 years of making money.
The next factor that is very important for energy output is the angle of tilt of the panels. There is a common ‘rule’ that the best tilt angle is the latitude of the location, however this is only approximate. For example, the latitude of Phoenix is 33.4 degrees north, so about 33.4 degrees is the optimum tilt for panels. But how big an effect is this?
We used our modelling software to look at the effect of tilt angle on the amount of incident sun, for a flat roof in Phoenix. The panels are tilted towards the south. Results are shown in the table.
|Tilt angle (degrees)||Incident sunlight (kWh/m2/year)|
Similar to orientation, the differences are significant but nowhere near enough to wreck the economics of solar if you don’t have the optimum tilt. If your roof has any sort of angle, installers will normally mount the panels flush as putting tilt on the panels isn’t worth the extra cost. If the roof is flat, however, installers may recommend at least a 10 degree tilt to help with self-cleaning (rain runs off a tilted panel but pools on flat panels) as well as get some extra energy.
What about further away from the equator? Does that make a bigger difference? The table below shows results for Anchorage, Alaska, which is at 61.2 degrees north.
|Tilt angle (degrees)||Incident sunlight (kWh/m2/year)|
In this case the model is showing an optimal angle a bit less than the latitude. Again, the difference with different tilts is significant but not huge. However in areas such as this where the incident sunlight is low, customers might need a lot more panels to look after their power needs. The roof might not have enough area for all these extra panels. In that case, putting panels on a tilt to get more energy may be worth the extra money.
Shading is a double-whammy for solar output. You get less energy because there is obviously less sun hitting the panels. But it’s worse than that. Because of the way individual panels are electrically connected in an array, shade doesn’t just affect the cells that are shaded, it can bring down the performance of the whole sections of the array. There are ways to ensure that the second part of this double-whammy doesn’t occur, or at least is greatly reduced, by putting in either microinverters or DC optimizers on the panels. DC optimizers are becoming popular since they can be fitted to only those panels that are affected by shade, whereas microinverters need to be on all panels. DC optimizers and microinverters enable each panel to operate at optimal voltage and current. Therefore the shaded panels can operate with different conditions to the unshaded panels and do not drag down the performance of the unshaded panels.
The image below shows output from a sophisticated model (Aurora Solar) that has excellent shade analysis. The large tree is clearly reducing the amount of sun hitting panels on the roof that are closer to the tree. In this instance, DC optimizers might need to be fitted to at least six panels to help decrease the effects of shade.
If there are objects near the solar panels that create shade it is important to perform a good shade analysis to make sure DC optimizers are fitted where needed, and also get an accurate forecast of energy output to give an accurate estimate of savings from your investment.
Panels can get pretty hot sitting out in the sun all day, up to 80oC in some areas. The performance of solar panels decreases as they get hotter. This is shown by a drop in voltage. The amount the voltage drops is calculated using a ‘temperature coefficient’ that is given on the datasheets for solar panels. For example, a panel reaching 60oC might have a voltage drop of about 10%, reducing the energy from the panels by about the same 10%. Different panels can have different temperature coefficients. However nowadays these differences are normally pretty small and output might vary by only a percent or two among different panels. This whole issue is obviously more important in hotter areas.
We hear a lot about efficiency, and this can vary a lot among the different types of panels. The top (and most expensive) panels are up around 22% efficient, whereas most panels range between 16 to 18% efficient. Won’t we get more energy from more efficient panels? Well yes, per unit area of panels. You can get the same energy from lower efficiency panels as high efficiency panels, you just need to put up more of the lower efficiency panels. If you have plenty of space, it probably costs less to put up an extra, low efficiency panel than to use high efficiency panels. If space is at a premium, however, high efficiency panels might begin to look more attractive.
So there it is. The amount of energy you get from your panels, and therefore the amount of money you will save on your electricity bills, is a complex question. Luckily there is excellent modelling software available now that can account for the important variables and can give an estimate of energy production that is within 10% accuracy. However, it’s important that good software is used. Cheaper software may not take into account all factors and may give an erroneously high estimate. This looks great on a project proposal, but only leads to disappointment when the actual energy output is less than promised.
Proper estimates should be provided by installers so the customer shouldn’t need to worry about chasing down all these issues. But the customer should make sure that the estimates are being done properly. If multiple quotes are sought, and some give much higher energy output than others for the same size system, then something’s not right!
No! While it’s true that you will get more power from a south-facing roof, you’ll still get plenty of power even from a north-facing roof. It will just take a bit longer (about a year) to pay the system off, which isn’t a long time given the 25 year life of the system.
Not necessarily. You can get the same amount of energy from lower efficiency panels, just by putting up an extra panel or two. In many cases this can be more cost-effective. In situations where space is limited, high efficiency panels may be a better choice.
Yes! Shade can be handled using DC optimizers or microinverters. It’s important your installer does a proper shade analysis to make sure you get the most out of your system, and that you get an accurate estimate of energy output.
Not necessarily. If you have a decent angle on your roof, it may not be worth the extra cost of putting up tilted panels. If you are in a location with low sun, and have limited roof space, tilted panels may be worth the extra cost.