A long time ago I wrote a post on how much money my solar panel was saving me. In the post I promised to write another post in November 2022 with updates. Well, lots of things happened and I completely forgot about it until one reader reminded me requesting for an update. So here it goes. It has been more than 2 years since I installed a 320W solar panel and I have been diligently collecting data from my panel, our consumption and how much we are pulling from the grid. Very recently I finally finished my code to fetch the data, massage it and used some nice libraries to draw the graphs so I can analyze the data.

The history

Let me go over the history of why I installed only a 320W panel while we consume an average of 250 units per month. The idea was to chop off the highest slab of units because that costs the highest per unit. For example, any consumption above 200 units are charged at Rs. 8.20/unit where I live. So if I can save the top 50 units, I would cut my bill by Rs. 8.20 x 50 = Rs. 410. As a result I did not go for a solar panel farm to offset all my units.


The general guideline is that a 1KW panel will generate about 4-5 units of electricity a day under good conditions. Based on that, my 320W panel should generate about 1.28 to 1.6 units per day. Which is about 38 to 48 units a month if there are no clouds. In a year we should have 467 to 584 units generated from the panel under ideal conditions. So how did we do then?

About the data

Before we do the reality check, I want to let you know that the data is only as good as the sensors I used and my code. The sensors have some issues like temperature drift, error due to unstable power supply, finicky bluetooth, etc which I mentioned in a few of my posts earlier which you can read by following the links below:

That was the reason I wrote about all the issues with my setup so you know the data is inaccurate. I had to write a bunch of code to clean it up. As a result there are some gaps in the data. For example if my bluetooth connection failed from 6 am to 10 am, then there is no solar data for that much time. So what I did was indicate all such errors with a lighter bar color as shown below.


Using the past 1 year worth of data, the highest I have seen my panel generate is 1.7 units in a day on Feb 24, 2023. Remember that all the numbers are only as good as my sensors. So the reality may be slightly different but it shouldn’t be too far. This matches our expectation.

The best month for the past 1 year has to be March 2023 with a record 43 units generated by the solar panel. Again within our expected range for a month. Finally, the solar panel generated 393 units for all of 2022. A bit lower than expectation but it is understandable because there will be some cloudy days and hence the solar panel cannot generate full power.

You can make some interesting observations from this chart. First, the months with lowest solar power generation are from July to September. Next, the best months seems to be from January to April, both in 2022 and 2023 (so far). Which corresponds with the cloud cover.

Energy usage

In 2022, we consumed a total of 1375 units (only 5A load is measured) and we pulled 1027 units from the grid (5A data again). That means, we saved 348 units of electricity thanks to the solar panel. Notice how the panel generated more than 348 units. That is how much we are losing in various losses due to UPS efficiency, solar charge controller efficiency, some measurement errors, etc.


Assuming that the 338 units saved mostly fall in the top slab of Rs. 8.20 (in 2022), we can assume a saving of Rs. 8.20 x 348 = Rs. 2853. That number is a bit lower than what I expected we could be saving when I wrote my earlier post. At that time, I expected to save Rs. 3424 and solar panel investment recovery to happen in 4.4 years. With the new information, it seems like it will take about 5.25 years. Of course, now that I have data, I don’t have to guess. I will just wait a few more years and see how many units were saved and thus the energy bill savings.

I will try to keep you updated once a year about the savings due to solar panel. If I forget, please remind me via comments :).