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

[kimbecause]

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.

[SteveyG]

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.

[Seekonk]

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.

[Jeroen3]

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.

[station240]

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.

[kimbecause]

Thanks for the replies everyone. I had a feeling it wouldn't be viable, but it is good to hear it from people in the know.

I am a pretty avid watcher of eevblog on youtube, but mainly the mailbag, teardowns and repairs. I had forgotten about the solar system episodes. I'll definitely go back and watch them now.

In case you are interested, the "server" is this all-in wonder -http://www.asrock.com/mb/AMD/QC5000-ITXWiFi/. It has 4 mechanical drives and 3 SSDs. It also has a fairly high-end raid card, but doesn't need a graphics card. It is pretty much just a file server, but does run 3 virtual machines. It has been a few years since I have bothered to plug a keyboard, mouse or monitor into it, not that a keyboard and mouse would waste much power.

I have investigated Pico-PSUs in the past because they are supposed to be more efficient, but couldn't find any that had enough sata plugs. It is also like you say - the small amount you gain from saving power can be insurmountable compared to the initial cost.

Last time I checked this computer was using 60 - 70 watts, but that is with a cheep watt meter so I'm not sure about the accuracy of that. If this UPS-ish seemed useful I was going to build the system for 150 watts to cover my bases. I guess UPS would be the wrong acronym here though because I was only interested in making more efficient use of the power, not for uptime. I am in the 'burbs so we only get the occasional outages.

Thanks again for all your replies. They were very helpful.

[David Hess]

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.

What kind of utility metering is the power company using?

At least where I am, the power company only pays a lower rate (actually zero unless special arrangements are made) for power produced in excess during the billing cycle.  So unless the utility meter runs backwards, the solar power displaces watt-hours used at the normal billing rate.

If that is the case, then there is zero advantage to using a battery system to operate the server versus using utility power except for backup reasons if your utility power regularly fails.  There is no reason not to treat the utility grid as an infinite AC battery.

[Red Squirrel]

You could look into an inverter-charger, it is basically the heart of a UPS but designed for longer run times and you can connect as many batteries as you want.  I have one for my server setup and with 4 deep cycle marine batteries it gives me about 4 hours.  (batteries are a bit older so probably not 100% capacity anymore).




What you could do is setup a MCU with a contactor that cuts power to the inverter-charger to force it to switch to battery.   The MCU would also monitor voltage so if it gets too low it would go back to AC power.  Have it on a timer so that it goes on battery around peak time.  Or maybe near the end of peak time, so that you don't end up having to charge the battery on peak time if it does not run long enough.  Keep in mind efficiency losses when charging a battery, running an inverter etc... 

A dual conversion setup would be even better, and with your small load more viable.  So instead of inverter charger you'd have a charger + inverter.  the inverter is always running, but the charger is toggled on/off as required (ex: off during peak hours)

I've actually been pondering this with my own server room, but also pondering just doing it with the whole house, and adding solar to the mix. 

[NiHaoMike]

Maybe a hybrid car battery pack or two would be a good start? The voltage (200-300V or so) is right in the operating range of a modern switching PSU. To charge it, use a boost converter such as a hacked PFC stage. Or for something simpler, a voltage doubler with the capacitors sized to provide a current limit and a contactor to switch it on/off.

[Circlotron]

60 watts @ 26.4C / kWhr = $138.75 per year. If you have 10 hours of sunlight that could get as low as ~$81 per year. Probably easier to find other places to save, as others have mentioned.

[Red Squirrel]

If his rates are like ours here and keep going up several times a year, then it ends up being a bit more per year.

That said, I would not put such a small load on solar just to save money as the difference is going to be too small.  But if you want to do it for fun or with plan to expand then it does make a neat project.

[Circlotron]

But if you want to do it for fun or with plan to expand then it does make a neat project.

100%. No problem with spending money to have fun. People do it all the time. And if you end up with something useful and also learn stuff along the way then great!  Things don't always have to have a practical justification.

[LukeW]

"The sales person said"

There's the first problem. Be careful, be skeptical. They're not interested in making the world a better place, mitigating climate change, or giving you cheap energy. (Well maybe some of them genuinely are). They're interested in getting your money.

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

Well kind of yes, but "free excluding the system cost" is a big caveat, and you only have so much power up to a point, and it's not completely reliable and predictable when you want it.

"Would it be feasibly to have a UPS that charges up during the day and powers the server overnight."

In principle yes, but what's the watt-hour capacity of your UPS? Mostly they are only designed for a short run, enough to safely shut down and power down the downstream systems.

"Question 2: Am I going to burn my house down? The wife would probably be annoyed by this."

Not if you have reasonably good quality gear, quality wiring etc.

"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."

Direct DC use at the appliances is more efficient. Many big datacentre systems and companies like Google are seriously getting into this concept with 48V distribution to all the servers.
But is the time, labor, engineering that is going to be needed worthwhile for a small-scale system?

"Question 4: Is this a terrible idea? I can't find anyone else asking about it, which makes me weary."

It seems like it's not bad as an experiment in small-scale, low-cost, "try before you buy" home battery storage.