eepete
Platinum Member
Dan's post got me thinking about the phrase "total system efficiency".
Like any energy system, you have to "follow the juice". On a tractor, you start with the maximum theoretical energy in a gallon of Diesel. Then you have the efficiency of the engine, then the transmission, then tractor, and finally you get energy "on the ground". All such measurements need units, such as HP or Watts.
For solar, you start with the energy of the sun on some amount of area. The panels then convert that to electricity, and that's already a pretty bad number along the lines of 10 to 15 percent. You now have DC. Then there is a box (single inverter) or boxes (micro inverters) that converts the panel's DC output to either AC that can go on the grid or DC that can charge batteries.
If you have batteries, there is the efficiency of charging, the efficiency of getting current out of the batteries, and then the DC to AC efficiency on the "back end" inverter.
Like any good technical thingie, you can go nuts with the details. I like to start out with the DC power output of the panels. Mine are rated 7.7KW, but when they are new you get a bit more (I've seen 8.5KW of _output_ from the panels around this time of year when the sun is 90 degrees on the panels and it's a low humidity day). The published DC power rating for a panel is set up for an "after 10 years of use" number driven by the 10 year warranty on the panels. The panels get a little less efficient each year, but they will have 25+ years of life. Between OC detail Geeks and paranoid Lawyers, is it any wonder this is too complex :laughing:. At any rate, the DC output of the panels is my starting point.
My inverter has a maximum efficiency of 96%, and I've seen it very close to this. Typical is around 94%. The output power is then off to the grid. You get better efficiencies at higher power outputs. Not unlike a tractor which is often most efficient when used to it's maximum ability.
So at a max output of 8 KW at 96% efficient, I loose 320 watts. But at a lower output of 2KW, my efficiency is worse at 93% but I only loose 140 watts. Note that efficiency very much changes with different manufactures and inverter architectures. My Fronius 10KW inverter has 3 parallel stages, so at low power only one is running, which helps with the efficiency. Efficiency also changes with temperature. In the winter time, I see the near 96% efficiency. At the peak of summer, that will go down a few percent because it's hot. Nothing is simple....
So for a battery system, I'd want to see the DC power out of the panels, and then add up the efficiency of the "DC to Battery charger" (easy to measure/know), the efficiency of "energy charged and stored in the batteries" (this seems like a hard one to figure out) and then the efficiency of the DC to AC inverter (this is also easy to measure).
The grid tie system will be cheaper and more efficient, but is worthless when it's dark. The battery "off grid" system will be more expensive and less efficient, but you have power whenever you want it (as long as the batteries have a charge in them).
Clear as mud?
Pete
Like any energy system, you have to "follow the juice". On a tractor, you start with the maximum theoretical energy in a gallon of Diesel. Then you have the efficiency of the engine, then the transmission, then tractor, and finally you get energy "on the ground". All such measurements need units, such as HP or Watts.
For solar, you start with the energy of the sun on some amount of area. The panels then convert that to electricity, and that's already a pretty bad number along the lines of 10 to 15 percent. You now have DC. Then there is a box (single inverter) or boxes (micro inverters) that converts the panel's DC output to either AC that can go on the grid or DC that can charge batteries.
If you have batteries, there is the efficiency of charging, the efficiency of getting current out of the batteries, and then the DC to AC efficiency on the "back end" inverter.
Like any good technical thingie, you can go nuts with the details. I like to start out with the DC power output of the panels. Mine are rated 7.7KW, but when they are new you get a bit more (I've seen 8.5KW of _output_ from the panels around this time of year when the sun is 90 degrees on the panels and it's a low humidity day). The published DC power rating for a panel is set up for an "after 10 years of use" number driven by the 10 year warranty on the panels. The panels get a little less efficient each year, but they will have 25+ years of life. Between OC detail Geeks and paranoid Lawyers, is it any wonder this is too complex :laughing:. At any rate, the DC output of the panels is my starting point.
My inverter has a maximum efficiency of 96%, and I've seen it very close to this. Typical is around 94%. The output power is then off to the grid. You get better efficiencies at higher power outputs. Not unlike a tractor which is often most efficient when used to it's maximum ability.
So at a max output of 8 KW at 96% efficient, I loose 320 watts. But at a lower output of 2KW, my efficiency is worse at 93% but I only loose 140 watts. Note that efficiency very much changes with different manufactures and inverter architectures. My Fronius 10KW inverter has 3 parallel stages, so at low power only one is running, which helps with the efficiency. Efficiency also changes with temperature. In the winter time, I see the near 96% efficiency. At the peak of summer, that will go down a few percent because it's hot. Nothing is simple....
So for a battery system, I'd want to see the DC power out of the panels, and then add up the efficiency of the "DC to Battery charger" (easy to measure/know), the efficiency of "energy charged and stored in the batteries" (this seems like a hard one to figure out) and then the efficiency of the DC to AC inverter (this is also easy to measure).
The grid tie system will be cheaper and more efficient, but is worthless when it's dark. The battery "off grid" system will be more expensive and less efficient, but you have power whenever you want it (as long as the batteries have a charge in them).
Clear as mud?
Pete
