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Comment by bushbaba | Hacker News Reader

I paid ~15k to install solar in 2019. After tax rebates it was ~11k. I’m generating ~12MWh per year. My break even period is less than 5 years.

I’m based out of California, and generate excess energy. My electric bill is now just ~$100/year.

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Have you experienced any grid power outages where your solar panels saved your day?

Solar won't keep you online through a power outage. The inverter is required by law to cut power from solar panels when there is not power from the grid, so as to avoid feeding electricity back into the grid lines (which technicians may be working on!) - this is known as anti-islanding protection. If you want to be online through a grid outage you need a battery array.

From a circuit perspective, it seems trivial to open a switch back to the grid to prevent grid-feed, while still feeding the internal circuit that is the house. Can you help me understand why this is hard?

The problem is that its not a DC connection to the grid.

Not only do you have to match voltage on re-connection, but you have to match frequency AND phase.

Its "doable" but to do so reliably without a stable reference can be challenging. That's why the DC requirement is there. They want some form of reliable power to well tested electronics to match phase and frequency when the grid comes back up.

A heavily loaded AC line will drift in voltage and frequency stability (say when your AC kicks on and browns out your solar panels).

I've read that before, and I have a further question if you could indulge me

What if, when the grid goes out the inverter keeps powering the house (and disconnects from the grid).

Then when the grid comes back on, kill the output of inverter (which results in outage to the house), reconnect to grid, then power up the inverter again, which skirts all the sync issues.

The house will see a few second outage... but it will have been "on" for the entire grid outage, which I think is much preferable.

The overall problem is more or less solved, it just adds equipment to the installation, both to make the power usable and also to ensure that it is isolated from the grid.

It's doable, but it requires a bunch of extra expensive equipment. Most people that want to accomplish that goal just install a battery array instead, since you're spending the money and it also gets you power coverage when the sun isn't shining. But the point stands that the typical grid-tied solar system just turns off when the grid goes down.

It's not hard. There is type of inverters that called "Hybrid inverter", I have one that can keep power off-grid or on-grid or even on-grid in import mode only (so only import any needed power without exporting...), and it can work off-grid even without battery (from solar) but that is not recommended unless you know that your load is much smaller than the solar output, even small lead-acid battery can mitigate this issue. The way the inverter work is that when the grid goes off it will automatically switch the power export off and keep the load running from solar and/or battery in isolated mode.

It is possible to do this without battery, but you need some special inverters, like the Enphase IQ8.

Not just that, but an ATS needs to be installed like the Enphase Enpower. That's usually at least a few thousand to have an electrician install.

No as I haven’t lost power since installation. However during a power outage I can configure the SMA inverter to detach from my breaker box and operate a single 15Amp outlet

Do you have a battery or are the excess directly dumped to grid?

Excess sent to the grid using the NEM 2.0 TOU-C rate plan.

if you generate excess energy, can you store it for a later time when you need more energy (presumably summer time)?

You would need batteries which are quite expensive at this time.

There are no sensible storage methods that would allow you to store enough energy from summer to make any reasonable contribution towards winter.

Batteries a great for powering your house for a few days. Not really beyond that. Unless you buy a ridiculous amount of batteries.

Batteries are also great for using at night!

While you wait decades for home battery storage technology to improve, you can make a water battery in the meantime with a few things from the hardware store.

https://hackaday.com/2021/10/08/power-your-home-with-a-water...

This system only had enough power to run a small light, IIRC.

You can buy a residential battery today that provides real value. The limitation on batteries is for grid scale supply.

You have to be joking, right? That's a 55 gallons of water as a battery that require 5.5 hours to charge and store 3.9 Wh of energy. Not enough to even power a 10W LED lamp for half an hour. A 10,000 mAh power bank is 37Wh.

It's a lot more efficient to sell back to the power company. I'd be more comfortable with having a dual fuel generator for apocalyptic situations vs a bunch of batteries.

Some (all?) electric companies in CA do net metering, where you can sell them your excess electricity at any time and storage essentially becomes their problem.

> presumably summer time

Assuming you have batteries, you discharge and charge them on a daily cycle, not a yearly cycle, to make the most money/have the most benefit. There are predictable time of day based troughs and peaks in the price that you can take advantage of.

Why would I? I’m on NEM 2.0. The grid is my “battery”. Every KWh I generate that isn’t used is sent back to the grid. In exchange I get an IOU for the equivalent $/KWh price during that time. This is the retail $/KWh which includes transmission costs.

~12MWh or ~12KWh ?

12kWh is what a typical rooftop solar system can produce in a day

so OP's number of 12MWh checks out for a whole year

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