Pros and cons of flat to the roof vs. tilted

The PV installer is constrained by many factors when trying to design the most appropriate system for a customer’s needs. These constraints include: the physical nature of the roof; its orientation; existing structures that take up valuable room; the consumption profile of the site; and the aim to minimise excess solar production being exported to the grid.

This blog looks at:

• Flat to the roof versus tilt framing
• Cost comparison of materials
• Labour comparisons
• Output comparisons
• Other factors

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Orientation Consumption Profile

Commercial solar systems are designed in such a way that the site loads are addressed as efficiently as possible. In the southern hemisphere, panels are oriented towards true north (or towards the equator) and the right panel angle will maximize output per kW installed. However, because of the discrepancy between the price of electricity that is negated by the PV array, say $0.25/kWh and the credit received for sending excess production back to the grid at, say, $0.09/kWh this may not be the best direction to go. Sometimes an East-West configuration will be OK as it may, more efficiently address the site loads.

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System Panel Design

For example, we have a large flat roof - it provides lots of options! We could go East-West as the ridge is North-South or go North facing but this would necessitate the use of a tilt frame.

Important Factors

So from initial analysis the system size is determined to be 99 kW and ideally we would like to maximize output in regards to roof area. We also need to remember that the number of panels relates to point of sale discount, i.e. overall system cost, STC’s etc when dealing with sub 100 kW systems.

So, looking at the following:

• Installation: flat to roof versus tilt, associated materials and installation costs?
• Tilt: higher output - but less overall space available due to row spacing considerations?
• Matching consumption profile to system output
• Material costs versus installation costs for both methods

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Modelling

Modelling is required on many levels to ascertain the best possible system.

Determine your installation cost per watt for flat to the roof (if an option) and your installation cost per watt for tilt frame (if also an option) and in this case it is. Then compare overall outputs of both systems. Be sure to factor in the point of sale discount scenario for the STCs and the differential between flat and tilt installs.

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Let’s have a look at outputs North, 10.5 degree tilt:

All North facing, 10.5 degree tilt, so will need a tilt frame. Output per watt/year installed is 1.345 kWh.

Total for a 100 kW system per year is 134,463.50 kWh/year

( Near Maps conservative output Melbourne, Australia)

Total for a 100 kW system per year is 134,463.50 kWh/year and this is 368 kWh/day on average for a 100 kW system. We will assume  $0.22/kWh cost of electricity so total saving is $29,581.97/year assuming all is consumed! 

The reality is that not all solar production will be consumed on site.

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Let’s have a look at outputs North, 15.5 tilt

All North facing at 15.5 degree tilt but now have an increase in production to a total for a 100 kW system per year of 136,647.62 kWh. Again assume $0.22/kWh so total saving is $30,062.48/year.

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Let’s have a look at outputs North, 17.5 tilt

Now if we increase panel tilt to 17.5 degrees output increases and this results in savings of $30,205.65/year assuming all is consumed.

Comparisons

So we can see in the above examples an increase in panel angle with a North facing aspect increases production and, of course, savings.

North facing comparisons:

• Difference in saving between 10.5 and 17.5 degrees is approximately $615/year  
• (Given x amount of roof area less panels installed @ 17.5 degrees than 10.5 degrees
• This is due to row spacing restrictions
• Difference between 10.5 and 15.5 degrees is around $480/year

Let’s assume the cost of tilt framing is the same regardless of degree of tilt (which is fairly realistic), so it’s more bang @ 17.5 than 10.5 degrees but more space for the same sized array because there’s a bigger distance between rows and potentially more cable tray, more cable and more time?!

All things to consider.

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Let’s have a look at costs, North facing tilt

We have 350 watt panels x 285; a total 600 metres of rail; and feet  every 1000 mm (so the total number of feet is say 600).

Will make some assumptions about the cost of these components and we come to a total of around $10,825. So our cost for tilt framing is around $0.11/watt.

Let’s have a look at outputs: EW FTR

One alternative is to forget about a tilt frame and go flat to the roof ( FTR). Output per watt/year  installed is 1.248 kWh.

Total for a 100 kW system per year is 124,780 kWh/year and, again, assume $0.22/kWh cost of electricity.

Total saving is $27,451.80/year assuming all is consumed.

Can see difference in savings compared to 10.5 degree North facing is *$29,581.97 - $27451 = $2,130.97

So if we design all East-West facing we get:

• Less than maximum output per watt installed,
• Peak output in morning or EW, morning and afternoon
• Could more closely match the consumption profile of business?

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Let’s have a look at costs: EW

Costs will be similar but there will be no utilisation of a tilt framing system and this drives down the cost of a FTR configuration to approximately $0.07/watt

N versus EW: Output versus Materials and Labour

Install Rail and Feet Cost

Now material costs are one thing but what about labour? We will assume a labour rate of $0.35/watt installed for the job and with our  99 kW system, this is nearly $35,000 and time spent laying rail and feet is 20% of labour cost for the whole job.

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Install rail and feet cost  FTR vs. Tilt

So here comes some more maths! 20% of $35,000 is $7000 but with tilt this adds an extra 25% in time, minimum, compared to FTR (a lot more time consuming installing tilt than FTR; yes lots of head nodding!)

This means cost to install tilt is $8,400

An extra $1,400!

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Install rail and feet FTR vs. Tilt

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Total difference is in  favour of FTR EW configuration is $3470 but the cost savings on material and labour are quickly negated by the reduced output of the EW configuration compared to a North facing array. Also we have assumed all solar has been consumed on site.

Conclusion

The are many factors to consider when designing these system and these include:

• The physical nature of the roof will determine system design
• A commercial solar system is offered as a financial proposition
• Installing systems FTR is cheaper, quicker but are these savings negated by less production

The savings long-term based on current import export tariffs seem to favour north facing arrays but each site is different and needs to be individually assessed from all perspectives.

Good luck on your next project!