Sizing the system will depend on your current usage, the amount of roof space you have, your budget, and how much time you want to spend optimizing the system.
Using the basic numbers from the PVWatts site and from our bill it looked like we would be good asking DTE to approve a 7,000 Watt DC rated installation. A couple of installers confirmed that range should be approved so I knew my baseline at that point.
However, I know the marginal cost of installing a system goes down as the system gets larger. This is a critical point to understand. A number of the quotes I received were derived and justified based on a certain cost per watt. But the actual cost per watt gets lower as the size of the system gets larger. (Think cheaper by the dozen.)
My desire was to utilize the maximum amount of the DTE renewable energy credit which paid $2.40 per DC watt, and also size my system so I wouldn’t need to come back later to add additional capacity. (Which would probably cost a lot more money per watt.)
I also knew that we currently use wood as our primary heat source and that as we get older we might want to use it less. And, we are planning on installing some super efficient mini-split heat pump systems to supply heat and air conditioning in the future.
I did some basic heat energy calculations and determined that our electric energy use could easily increase by 25 to 30% over the coming years as we used less wood and more electricity for heating.
So, I targeted a larger system for my proposal to DTE. I spoke to a couple installers who agreed it made sense to plan this way. The target number I ended up with was a 10,000 watt DC system.
I also have a 3,000 square foot roof so I know that about 1,500 square feet is facing South West. That gave me 1,500 square feet to play with which would turn out to be plenty to install our system.
Note that there are at least two different ways to describe the power capability of a solar system:
1. The maximum possible output of the solar panels themselves. (This is referred to as the DC Watts output or the “Nameplate Watts.”)
2. The maximum possible output of the system. (This is referred to as the AC Watts output.)
So I targeted a 10,000 DC Watt system, knowing that it would rarely if ever be possible to actually get a full 10,000 watts of output from that set of panels because it would take perfect conditions as far as the sun’s location in the sky, the temperature on the roof, shading, and the weather.
I ended up with what Detroit Edison called a “9.66 DC/ 8 AC kW Solar System” on my roof. And, today as I write this there have been a couple minutes where conditions have been good enough to hit the 8 kW level of system output, so I can confirm it is actually working as designed.
Since DTE is currently paying $2.40 per DC watt after installation, this means that when the installation was complete and had met all the approvals, DTE would write me a check for $23,184 to help pay for the system. (And after a few nervous weeks I actually did receive it!)
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