• Graham Harris

One year later: results from my home solar PV system (Part 1)

It’s been a little over a year since I posted about having my home solar photovoltaic (PV) system installed and energized by Virtuoso Energy, and I’ve had a lot of questions about the performance of this system: has it performed as expected? What impact has it had on my electricity bills? What’s the payback? What is the impact on GHG emissions? So in this blog post, I’m going to dive into the data to start answering these questions.

Before I do, however, let’s briefly go over how the solar PV system interacts with the electricity grid and my home electricity system. This is shown in the illustration below.

The generated electricity from my solar PV system can go to one of two places – either it will be used by my home electrical system (self-used electricity) or, if the PV system is generating more than the demand, then it will send the excess to the electricity grid, where it will be used by someone else (exported electricity).

To track the performance of the system therefore requires two data sources and some simple math. The first data source is electricity bills, which show both the electricity drawn from the grid (imported electricity) and the exported electricity, based on meter readings from a bi-directional meter. The second data source is data downloaded directly from the solar PV system inverter. This shows the total generated electricity.

However, neither the meter nor the inverter tracks self-used electricity. However, this is easily calculated as generated electricity - exported electricity. For example, if the system generates 1,000 kWh in a month, but only exports 400 kWh, then that means 600 kWh must have been used internally.

Finally, the total electricity used by my household is also calculated as self-used electricity + imported electricity.

Now, with this understanding, let’s go through the system performance.

Question #1 - has the PV system performed as expected?

Pre-installation, Virtuoso provided me with a simulation showing that my 8.04 kW system should produce, in a typical year, around 7,735 kWh/year of clean electricity. By way of context, this was almost exactly 100% of my household’s predicted annual energy consumption (including the electricity needed for charging our fully-electric 2015 Chevrolet Spark [more on that here] – the vehicle we use to get around Calgary on a day-to-day basis).

Post-performance, the generated electricity between 23-Sep-2020 (the day the system was energized) and 15-Sep-2021 (not quite a full year, but this aligns with the utility’s billing cycle) was 7,785 kWh of electricity. By comparison, my total electricity use (between 22-Sep-2020 and 15-Sep-2021 - not quite the same time period, due to the utility company’s billing cycle) was 7,585 kWh.

So yes, the system has performed as expected! This is shown in the graph below.

Question #2 – what impact has the PV system had on my electricity bills?

The impact of exported electricity on my electricity bills is easily found as a microgeneration credit is shown directly on those bills. Over the course of the year, the system has generated 5,277 kWh in exported electricity, for which I have received $340 in credits.

A second impact on the bills come from the self-used electricity – every kWh of self-used electricity is a kWh of electricity that my household did not need to import, and therefore is an avoided cost.

And here’s where it gets interesting. A kWh of self-used electricity is worth more than a kWh of exported electricity.

How does that work? Well, it’s to do with the disparity between how you are charged for imported electricity versus how you are paid for exported electricity. You are charged a certain rate per kWh for imported electricity (say, $0.06/kWh). However, in addition, you are also charged variable transmission and distribution costs on every kWh you import (say, another $0.04/kWh). By contrast, when your system exports electricity you are only paid the same rate per kWh that you pay to import electricity – you don’t get any variable transmission and distribution costs. So, in our example, self-used electricity avoids $0.10/kWh of costs whereas a kWh of exported electricity only gains you a payment of $0.06/kWh.

Going through my own bills, self-used electricity is actually worth almost 3.5 times more than exported electricity!

Self-used electricity was 2,509 kWh, avoiding $581 in costs. In total, therefore, as shown in the graph below, the solar PV system saved me $921 in electricity costs over the year.

In my next blog post, I'll continue the exploration of the solar PV data to answer the remaining two questions: 1) what is the payback for the PV system? and 2) what is the PV system's impact on GHG emissions?

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