Measuring my Solar Panels efficacy

[Jim Hare]

Okay, so never really knew how to figure out just how effective my solar panels are compared to our usage and bill.

Please tell me if my thinking is wrong here as I can be overly simplistic at times:

1. Our panels output an average of 30kWh per day
2. I'm assuming this is all during the peak period billing hours
3. During a billing quarter I estimate we generate 2700kWh
4. Last quarter we sold back 663kWh of unused power
5. I take this to mean we used the other 2000kWh to run the house during peak hours.
6. By NOT purchasing 2000kWh @ 40¢ I'm giving that a value of $800
7. Selling the other 663kWh @ 20¢/25¢ yielded a further $143

So does that mean our panels are saving us $943 per quarter? I appreciate this will vary season to season...

If that's the case our system would pay itself off in 6 years.

Now's the time to point out flaws in my logic... ;D

[Brian]

I'm no accountant, and don't know what your capital cost was. An accountant might cost-in the interest on capital.

But from those figures, you paid less for your bigger system than we did for ours.

12 months records show that our 5.5kW system generated an average of 24kWhrs per day, whilst we consumed an average of 18.1kWhrs per day, for a net export of 5.9kWhrs per day.

We buy at 25 cents per unit flat rate, and sell at 47 cents per unit.

I will be closely watching the monthly electricity ' Smart ' meter readings that I record to see the impact of the LEAF.

[Jim Hare]

Sounds like you got in earlier than us since your buy is the good old price. So guessing your system cost more but you are getting a much better buy rate.

We paid about $22k for a 6.1kWh system that has been in place about 15 months now. Our buy is now 20¢ but since we use peak power for our home businesses, I'm more interested in the amount we use and therefore don't have to buy at the peak price of 47¢.

I'm not concerned with calculating forgone interest on capital since it would be surpassed in the first year the system has paid itself off. In other words I consider it roundoff error.

Didn't think of looking at the smart metre, does it give lots of good info?

So you're actually in surplus with your system? That must be a great feeling! Our system covers about half our needs but we console ourselves with the fact that we aren't driving 2 cars to work and aren't consuming further energy in an office.

Would love for our system to cover 100% of our needs but there isn't enough roof space! Actually there is but the system would be cripplingly expensive.

[Brian]

We started with a 1.72kW system in August 2008. We upgraded to a total of 5.5kW in December 2010, as a result of the introduction of the 47cent feed-in-tariff. 5kW is the theoretical max allowed in the West.

Total cost after government subsidies was around 28k. Our household is just my wife and I - our power guzzling daughters have long since moved out. So yes, we are in surplus - part of the retirement plan. We have a solar HWS, and cook on gas, both of which reduce our electricity use.

I read the so-called smart meter on the last day of every month for my own records / comparison. It gives cumulative total imported and total exported, plus break-downs for ' Shoulder', 'Peak', and 'Off peak' and weekend. But I also have a portable remote indicator which shows what the inverter is producing at any time, a daily total, and a cumulative total. This, combined with a Cent-O-Meter that shows power being consumed in real-time, gives a really good picture of our production / use.

[Jim Hare]

Interesting. We have gas hot water on demand and gas stove as well. Our power guzzling kids are very much here, and with my wife and I both running businesses from home, we're lucky that half our bill is covered! For us it was more about carbon guilt than saving money, though the fact that the system will eventually pay for itself made the decision easier.

Thanks for the info. I've been watching the readouts on the two inverters but will start paying attention to the smart metre.

[Brian]

Carbon guilt was very much part of our equation, starting with the electric motor bike. I am aiming to reduce our carbon footprint to carbon tip-toes.

We have just one 5kW inverter.

[jeffjl]

Sticking with the title (and in an effort to get off the Tyre Kicker level).

At my inverter in previous years the voltage use to be 180V, It is now generally between 150 and 160V. Could this be a faulty panel and if so how do I check the panels to confirm this? (21*60W panels, 1700W inverter, Solar Shop)

Brian. I have just changed to SmartPower*. Checked with my 'Shoulder', 'Peak' 'Off Peak' and 'Weekend' rates since I got the panels installed and worked out that I had given Synergy some $500 that I did not need to. The Feed In Tariff is not affected. Now I am going to be charging "Coal Burner" (name for my Leaf) at the Off Peak rate of 14.6c. It might improve your retirement plan.

[riddick]

I started with a 1.5kW system in 2010. Upgraded to 4.6kW in Sept 2011 when the Premium Feed in was about to be phased out. Initial system cost about $300 with government subsidies + RECs. Upgrade cost about $7k with RECs. System fully paid for itself in May 2013. Since then it earned nearly $3k as a result of:
1. Not having to pay as much for power
2. getting some cold hard cash back

Now with the LEAF things are different, I am yet to establish a pattern.

These comments may be a bit late, but here we go:

Jim, do not assume that daylight hours are your peak!! Peak is generally late afternoon, often running till 10-11pm! Your solar will not help here. Also, be careful when calculating returns, as a solar plan will give you different consumption rates from a non-solar plan. What I did was, I rang my retailed and explained that I want to calculate the returns on my solar system and would they be kind enough to let me know what rates I would be paying on my property without solar. Believe it or not, every property is charged differently, so you really need to ask this question. Even the service charge may be different (mine was lower initially for solar, so there was a bit more saving there.). But the rest of your logic is ok.

Brian, generating 24kWhrs a day and consuming 18.1kWhrs a day does not mean that your net export is 5.9kWhrs per day. It greatly depends on when you consume!! For example if you consume nothing during day light hours and then you buy it back during the dark hours, you would be net feeding in 24kWhs per day. I think your figures are much better then you think!

jeffjl, your panels are either aging or you may have an issue (I am assuming that you checked it under the same conditions, ie full sun, similar season). The only thing you can do is to turn off all circuit breakers and disconnect the panels and measure them one by one. Be careful if you don't know what you are doing, get someone else to check it.

[Brian]

Thanks, riddick, that is encouraging, and could explain some anomalies between my calculations and my power ' bills ' ( I do like to see those 'CR ' ( for in credit ) notifications.