What happens under variable loads with excess solar when grid is down?

[Billme]

I believe I have the right sized PV system for normal daily use of power. I’m able to fully charge a Powerwall and Model 3 during the morning, and in the afternoon I turn on a Spa pump which primarily uses the solar to run until dusk, and which point the Spa turns off and the loads are serviced by the Powerwall.

The problem is my load tends to vary at times during the day, when for example the water heater turns on, or a cloud passes over. In the early morning, I’m only drawing 400 watts. By 9am that goes up to around 1KW, but it will spike from time to time up to 6KWs. In the afternoon, my Spa draws 6KWs just on its own, and the loads can spike as high as 11KW for a few minutes. The grid, acting as a buffer, handles all of this nicely.

What happens when the grid is not available? So, if I have a power failure before the battery is fully charged, but the PV is generating 8KW, what will my single Powerwall do? Once it’s fully charged, I know it can turn up the frequency forcing the PV offline, but when the PV is putting out more than the battery can accept, will it do the same? I’ve heard the Powerwall can actually accept more than 5KW for short periods of time; how short, and how much?

 

[jjrandorin]

It depends on how your system is installed (whats connected to the powerwall and what isnt). With one powerwall, I doubt your spa and everything is backed up.

what is actually backed up by your powerwall? Its entirely possible for a partial backup configuration to still supplement loads that are not actually "backed up" when there is no grid.

If your PV is putting out too much energy for your powerwall to take, then it will shut the entire "system" down, with the system being both the PV and the powerwall. This can be avoided depending on whats backed up, how the solar is configured, etc.

The problem / issue with this stuff is, each install is different, because every homes electrical composition is different. This is something you will need to test (and testing it now may not give you the same result as testing it in april, either).

In general terms,. there cant be any "excess solar" when there is no grid, because there is no place for it to go. Your solar would be shut down until such time as the powerwall (or powerwall + home load) could absorb it.

 

[slcasner]

It also depends what PV inverter you have. I have the Tesla inverter that is part of the Powerwall+ so there is a direct control communication path among the Gateway, Powerwalls and PV inverter. Because I don't have PTO yet the system is configured as non-export which is kind of like being half off-grid: I can draw energy from the grid but not send excess energy to the grid. When the two Powerwalls get full then the PV inverter modulates its output to match the home load so there is no excess. I have not operated in a truly off-grid situation to observe if the behavior differs at all.

 

[RKCRLR]

This is how my system works:

When the grid goes down the Powerwalls will take over and provide power to the backup circuit. The speed of the switchover seems dependent on the load. Light load the switchover is quick, heavy load takes a little longer. If the load is greater than what the Powerwalls can provide then they will shut off for about 5 seconds and try again. This cycle will repeat. At some point it may be longer before the Powerwalls attempt to restart but I've never let it go that long before remove the excessive load when I'm home.

Solar production will turn off for about 5 minutes whenever the grid goes down or the Powerwalls go offline when the grid is down. If the Powerwalls are fully charged when the grid goes down then the solar won't be commanded on again until the Powerwalls get down to around 90%-95%. The Powerwalls will then recharge commanding the solar to reduce output as 100% is approached if production is greater than demand and the system supports curtailment. A string system will just shut down before 100% charge is achieved.

If the batteries aren't charged when the grid goes down solar production will resume in about 5 minutes unless the Powerwalls are tripped.

 

[arnolddeleon]

When my sister's system didn't have PTO yet they basically ran it "off-grid" most of the time. They just turned the main breaker off. You can't have excess solar. The system will modulate the solar (if it has the capability) to match the load even if the battery is full. If the solar doesn't have that feature then the solar will be shut down and Powerwall will take the load. Obviously the maximum load you can have is total of what Powerwall can put out plus what your solar is putting out.

My sister has a whole house backup configuration with one Powerwall. There are main panel and load center configurations where this is the most straight forward (and cheapest) configuration. They have an electric dryer and electric oven/cooktop. During the pre PTO run they accidentally tested the limit of the system will put out. An electric dryer and electric oven will pull more than what one Powerwall can handle.