4 ms·

Most other commentators here have hit the nail on the head, in summary: - Backfeeding into the grid is unsafe - Backup generators will throttle to not produce

Most other commentators here have hit the nail on the head, in summary: - Backfeeding into the grid is unsafe - Backup generators will throttle to not produce too much electricity for your home, so you can safely use a transfer switch - Solar can't be throttled, excess power needs to be used, stored somewhere, or sent to the grid

toomuchtodo 8y ago
Newer inverters can throttle solar, as there are some markets where no export is permitted to the grid (the inverter optimizes the MPPT to effectively shut certain solar modules down). https://www.solaredge.com/us/solutions/feed-in-limitation-and-metering-solution#/ https://www.solaredge.com/us/solutions/feed-in-limitation-an...
- sathackr 8y ago
  I have such an inverter(SolarEdge HD Wave)[1], but you have to specify a specific power level to throttle to -- and it requires some additional supporting hardware to measure what it is feeding the grid with, and still requires at least a sync signal from the grid. I have not found an off-grid mode in its settings yet. [1] https://www.solaredge.com/us/products/pv-inverter/single-phase#/ https://www.solaredge.com/us/products/pv-inverter/single-pha...
  - toomuchtodo 8y ago
    It’s not user friendly (export limit setup), as it should be set by your solar installer at system provisioning. There’s only a few markets in the world it applies to (Arizona, Hawaii, parts of Australia). In short, newer inverters can’t be overloaded if there isn’t enough load and too much sun.
    - sathackr 8y ago
      I kicked the solar installer out after he did the physical work, I did the rest of the provisioning and configuration after the inspections cleared and the meter was swapped. He was insisting on installing a cellular modem on my inverter and maintaining remote access and control over it. I did not want this, so I did not allow them to.
      - bonzini 8y ago
        Here they required installing a data logger which is really a Raspberry Pi running a web server + a USB to RS-485 adaptor. It was required for warranty beyond the legal minimum because they want to monitor the environment temperature in the room where the inverter is installed. However it is not on the public network and does not call home.
stephen_g 8y ago
Backfeeding isn't really a problem practically unless you're (dangerously) hacking a system together yourself without knowing what you're doing. Any inverter/charger designed to work in this kind of system either has a transfer switch built in, or interfaces to an external one that stops backfeeding when the grid is down.
namibj 8y ago
Actually, solar cells are perfectly fine if you don't connect the outputs. Your inverter has to handle the float/no-load voltage (20~50% higher than at peak power), but that's it. Now, the inverter in this case has to be throttled by the secondary, instead of just blasting as much amps as it can with the solar power. You might want to have a nice-ish clock reference for the phase of your micro-grid, if you care about mains-phase-locked clocks keeping their accuracy. You'd want to count cycles and compare against those in the main grid, as well as adjust the frequency (much stronger than the large grid, as there won't be anything that initiates an emergency shutdown at the odd frequencies you might need to catch up/slow down to the main grid phase) so that the cycle count matches up when you sync back to the grid. If your inverter is sufficiently fast, and your transfer switch can at least disconnect external power from the internal system within less than half a period, you might be able to keep the internal power stable without any hickup at all. And it's not like the required control logic for the inverter is too hard to integrate nowadays with microcontrollers having WiFi etc.

toomuchtodo 8y ago

Newer inverters can throttle solar, as there are some markets where no export is permitted to the grid (the inverter optimizes the MPPT to effectively shut certain solar modules down). https://www.solaredge.com/us/solutions/feed-in-limitation-and-metering-solution#/ https://www.solaredge.com/us/solutions/feed-in-limitation-an...

sathackr 8y ago
I have such an inverter(SolarEdge HD Wave)[1], but you have to specify a specific power level to throttle to -- and it requires some additional supporting hardware to measure what it is feeding the grid with, and still requires at least a sync signal from the grid. I have not found an off-grid mode in its settings yet. [1] https://www.solaredge.com/us/products/pv-inverter/single-phase#/ https://www.solaredge.com/us/products/pv-inverter/single-pha...
- toomuchtodo 8y ago
  It’s not user friendly (export limit setup), as it should be set by your solar installer at system provisioning. There’s only a few markets in the world it applies to (Arizona, Hawaii, parts of Australia). In short, newer inverters can’t be overloaded if there isn’t enough load and too much sun.
  - sathackr 8y ago
    I kicked the solar installer out after he did the physical work, I did the rest of the provisioning and configuration after the inspections cleared and the meter was swapped. He was insisting on installing a cellular modem on my inverter and maintaining remote access and control over it. I did not want this, so I did not allow them to.
    - bonzini 8y ago
      Here they required installing a data logger which is really a Raspberry Pi running a web server + a USB to RS-485 adaptor. It was required for warranty beyond the legal minimum because they want to monitor the environment temperature in the room where the inverter is installed. However it is not on the public network and does not call home.

stephen_g 8y ago

Backfeeding isn't really a problem practically unless you're (dangerously) hacking a system together yourself without knowing what you're doing. Any inverter/charger designed to work in this kind of system either has a transfer switch built in, or interfaces to an external one that stops backfeeding when the grid is down.

namibj 8y ago

Actually, solar cells are perfectly fine if you don't connect the outputs. Your inverter has to handle the float/no-load voltage (20~50% higher than at peak power), but that's it. Now, the inverter in this case has to be throttled by the secondary, instead of just blasting as much amps as it can with the solar power. You might want to have a nice-ish clock reference for the phase of your micro-grid, if you care about mains-phase-locked clocks keeping their accuracy. You'd want to count cycles and compare against those in the main grid, as well as adjust the frequency (much stronger than the large grid, as there won't be anything that initiates an emergency shutdown at the odd frequencies you might need to catch up/slow down to the main grid phase) so that the cycle count matches up when you sync back to the grid. If your inverter is sufficiently fast, and your transfer switch can at least disconnect external power from the internal system within less than half a period, you might be able to keep the internal power stable without any hickup at all. And it's not like the required control logic for the inverter is too hard to integrate nowadays with microcontrollers having WiFi etc.