3 Reasons Your PV System Load Profile is Overinflated and How to Fix It

The article below is a discussion from our course, Batteries in Solar PV Systems, led by Christopher LaForge. Christopher LaForge is the CEO of Great Northern Solar and a NABCEP certified photovoltaic installer. He has been designing, specifying, and installing systems since 1988.

Takeaways:

• The Energy Master Program is a great tool to work out a design for a low resource PV system load profile. It uses various Kw of PV to find out home much run time a generator will require to meet demand during low resource times
• It is cost prohibitive to get all of your power from your PV system during low resource times
• The actual loads of new variable speed fan motors and high efficiency appliances are difficult to determine. Use a clamp meter to see what amps hardwired appliances are actually pulling.
• 3 reasons a PV system load analysis may be overinflated
• Experience makes PV system load analysis easier over time and you really have to put time in on each profile
• The amount of time it takes to track measurements of a PV system load profile

DISCUSSION BETWEEN STUDENTS AND CHRISTOPHER LAFORGE 

STUDENT 1:
I’m totally on board with load profiling. It is tedious as Chris (Laforge) explains, but 100% necessary. My question relates to load profiling an off-grid PV system. It would seem that in a state like Wisconsin, where solar resource is relatively low in November and December, and in general the electric load is higher. Shouldn’t we be giving special attention to the load profile during these ‘critical design’ months? And how would we effectively go about that?

CHRISTOPHER LAFORGE:
Very good question and yes – I work out the worst case load profile in this case (in my area near Lake Superior it is actually November) and I build the design from it.

However, to meet the demand in these low resource times it is cost prohibitive to get all the power from the PV. This is where the design in the Energy Master Program uses various Kw of PV to find how much run time the generator will require to meet the demand.

Load shedding is a great option but one not often thought of in the USA. Avoiding too much PV but keeping the generator run time down becomes the dance of design. Off grid is challenging. If you choose an off grid design for your capstone project I will be able to help you work through a few options to see how I go at this problem.

STUDENT 1:
Thank you Chris. I have been working a little with the Energy Master Program and I can see how that balance works. I’m working on my off-grid load profile right now – in the dead of a central-WI winter. Thanks for the offer – I would appreciate your eyes on my load profile and your insight into the PV/Generator balance. Thanks for the call last night – maybe we can converse another time.

STUDENT 2:
I find that when I do a load analysis that it tends to be overinflated instead of undersized as Chris warns about in the early lessons. For example, I know that my existing house uses about 7 kWh a day average. For the exercise I did a load analysis just on the first floor of it and I came up with 9.8 kWh average per day. I think part of this is using the name plate rating on many appliances and they are showing a worst case and, in my experience, are rarely accurate. Another part may be overestimating the daily use or guessing wrong on some unknown loads. Anyway my point is that it must take a lot of experience and/or really putting in the time, maybe with a Kill-a-Watt meter, to get these things accurate.

STUDENT 3:
A clamp meter might be handy to see what amps some hardwired appliances are really pulling.

CHRISTOPHER LAFORGE:
(The clamp meter) suggestion is good and can be accomplished in the Main load panel with care (live measurements are very dangerous). Using a good amp clamp with recording capability can help as well. Very good units can be used in record to get usage over time – but these are very pricey.

In addition to the Kill-a-Watt, I use the Brand watt meters. It’s a very good product, but these are all load specific continuing the tedium I’ve mentioned load profiling really is!

The over-estimation is most probably a combination of the reasons you list:

1. Name plate rating on many appliances and they are showing a worst case
2. Overestimating the daily use
3. Guessing wrong on some unknown loads…

Figuring out the duty-cycle of refer and HVAC equipment takes a good deal of observation and then it (of course) changes seasonally! Your final thought is accurate – experience makes it easier over time and you really have to put time in on each profile (this is no joke – our design and success is dependent on it).

STUDENT 4:
I agree that load profiling is very difficult. The actual loads of new variable speed fan motors and high efficiency appliances are difficult to determine. I look forward to getting as much battery knowledge from someone with the experience that Chris has.

CHRISTOPHER LAFORGE :
Be sure to look at Brand’s various options. These can save time – although it all takes time.
Specific questions will help a great deal as well! For monitoring circuits – CT’s make it simple.

STUDENT 5:
Does anyone have a recommendation for types of data logging equipment that are typically used for load profiling? How long would the measurement period be? Days, weeks or months? Looking for something that would survive outdoors. I’m looking myself and I can share what I find here. Thanks!

STUDENT 6:
There are meters (and revenue grade which may be required) with software that logs this data for a period of time. I think the amount of time is up to the designer and customer.

CHRISTOPHER LAFORGE:
The length of time depends on how accurate you want to be and remember that many loads are affected by weather – ambient temp. wind etc…

When looking at loads with variable duty cycles (compressors for refers etc…) I like to get several days (5+) data with the appropriate Brand meter or equivalent – if the variability is great then the longer the better (2 weeks+).

This may sound excessive but I want to nail the design and not get caught by inaccurate data.

STUDENT 5:
I’ve decided to go all in on some CT’s and a multi-channel remote logger. Also will get a pyranometer and some voltage divider inputs. Still looking for one that also has GPS and a Google Earth feed. Data will dump to a web portal via a modem. Not cheap, but with a moving target, probably the safest option. I’ll have to do some estimates for my class project but plan to verify operation long term with this gear on the prototype. Thanks!

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ADDITIONAL RESOURCES

• Free Lecture: Battery Capacity – The Basis of Storage
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