Electronics > Power/Renewable Energy/EV's

UPS run from wall socket? (like a powerwall for a single appliance)

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This question could fit in a bunch of categories, so I apologise if this is not relevant enough to solar. It has some solar-related questions though, so I decided to put it here. I have a random idea and a bunch of questions about whether it is feasible.

So I am getting solar panels installed tomorrow. The sales person said that during the day the inverter would inject the power straight into the house. If there is an excess, this would go back onto the grid and earn 8c / kwh. If there is a deficit from the panels, the inverter would supplement the supply from the grid (I pay 26.4c / kwh). So during the day I should mostly use generated power, which is free. The excess would go towards paying for night-time usage.

Question 1: Is this correct? That during the day I effectively have "free power" (not including the cost of the system).

I have a server that runs continuously and draws a fairly constant 60w. Would it be feasibly to have a UPS that charges up during the day and powers the server overnight. Effectively this would be

mains socket -> timer ~(8am - 4 pm) -> charger -> lead acid battery -> inverter -> server.

So here are the questions -

Question 2: Am I going to burn my house down? The wife would probably be annoyed by this.
Question 3: Given I am going from DC solar -> 240 AC -> DC battery -> 240V AC, is this efficient enough to be worth it? I am effectively looking at if I put power onto the grid and then access it at night, that power is ~30% efficient, because I only get ~30% of the price to sell the power vs buying it. To put this another way I gain 18.4c/kwh by storing the power and using it versus sending it to the grid.
Question 4: Is this a terrible idea? I can't find anyone else asking about it, which makes me weary.

The Solar system is 3.5kw, if that helps.

Thanks for any replies.

A server that draws 60W? That isn't a server ;)

Personally if I was looking to do what you are doing, I'd eliminate the inverters etc, and make or modify a PC power supply to run from the battery DC.

I suspect you won't make your money back however, battery charging etc. is not massively efficient, a suitable battery pack would be expensive and need replacing every few years with that kind of cyclic use.

That is a decent size system.  Batteries will run you about 13 cents a KWH to use over their life. Batteries are not free storage. I have avoided them as much as possible.  I live totally off grid at my summer house. The system is all my design and not conventional.   There is only one  bad thing about your system now. If the grid goes down,  you likely won't have any power.  If you are having to ask these questions now, you likely don't have the technical experience to create a better system.  The best place to store a little extra energy is in making hot water, running a water heater extra hot during the day.  A controller that sends excess energy to the heater, using a blend valve to make usable water for use in the home, and only using grid power when desperate.  This is not available commercially.

1: yes. Watch the eevblog episodes on the solar power installation.
2. Not if you use certified tested equipment. Maybe if you bodge your own kit.
3/4. Bad idea. You'll need about 1 kWh of batteries for just your server to run the night.
You could store some energy to use in the evening to lower grid use during peak hours. But I don't think this is an economically feasable solution yet.

Another thing to consider is that your solar inverter does nothing when there is no grid.

For a 60W load, the power used by the electronics in the UPS is going to be noticeable.
10-15W would be my guess, but it could be higher. Also consider the round trip efficiency, eg how much you lose in the battery during charge/discharge.

A better idea is to have an extra solar panel, then a conventional offgrid 12V DC system, which powers the server either via an inverter, or using a 12V DC input ATX PSU (as used for people's custom car PCs).

Another option if your server is actually a laptop, or uses an external PSU like one, is a LiIon powerbank, assuming on exists with enough capacity.


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