Automatic switching from Battery to Grid on a small system

[Bratan]

First of all I apologize if this topic was already covered, I'm not sure exactly sure what this thing is called so I couldn't search for it
Basically I'm looking for a UPS like device that will switch power from batteries that's low on charge to regular AC outlet power. It's a very small solar system (100W or so) and a single AGM battery. I want to be able to power a computer off Solar/Battery until it runs low and then switch back AC power, until battery recharged. What would such device be called, are there any sold, or perhaps there are DIY plans how to convert existing UPS to this?

[Sunking]

It can be done but why would you want to do this? UPS system work exactly opposite. They use inexpensive commercial power and only switch to battery power in the event of a power failure at which time a generator starts and takes over.

A 100 watt off-grid solar can only power a regular desktop PC for only a few short hours in a day. So if this computer was running say 24 x7 or even say 10 hours per day a 100 watt system is not going to keep it powered up and will be running on AC most of the time.

Much less expensive and more reliable to use a conventional UPS using AC power to charge the batteries, and only use the battery when power fails. No solar required.

[billvon]

Basically I'm looking for a UPS like device that will switch power from batteries that's low on charge to regular AC outlet power. It's a very small solar system (100W or so) and a single AGM battery. I want to be able to power a computer off Solar/Battery until it runs low and then switch back AC power, until battery recharged. What would such device be called, are there any sold, or perhaps there are DIY plans how to convert existing UPS to this?

The old Trace SW series of inverter/chargers will do this, either in energy management or LBX (low battery transfer) mode. You can get them for around $1000 used on Ebay. You probably want the SW2512 version (12 volt.)

However you may be unhappy with the result. Cycling the batteries like that will result in a lifetime of less than two years.

[Bratan]

A 100 watt off-grid solar can only power a regular desktop PC for only a few short hours in a day. So if this computer was running say 24 x7 or even say 10 hours per day a 100 watt system is not going to keep it powered up and will be running on AC most of the time.

Oh I see, thanks for the info. I wanted to save on electric bill by running my server PC off battery charged by solar during the day and off AC during night, but I guess 100 Watt is not nearly enough for this...

billvon, that's way too expensive I was hoping to find something for around $50.

Thanks guys for replying!

[Mike90250]

To save on "store-bought" electricity (from the Grid) you either conserve, or install a Grid-Tie system. No batteries needed (batteries introduce more losses and expense)
Grid -tie systems start about 500W for very small ones, and the prices go up. If you have good sun, and a willing energy provider (meaning that they allow it), payback periods can be 10 years or so.

[Sunking]

Oh I see, thanks for the info. I wanted to save on electric bill by running my server PC off battery charged by solar during the day and off AC during night, but I guess 100 Watt is not nearly enough for this...

Sorry but that is not even remotely possible. Going off-grid will cost you at least 10 times more for electricity over buying it from the POCO.

[billvon]

Oh I see, thanks for the info. I wanted to save on electric bill by running my server PC off battery charged by solar during the day and off AC during night, but I guess 100 Watt is not nearly enough for this...

Nope. A typical PC may take 100-200 watts average but peaks at much higher than that. You'd need around 400-500 watts. To run all day you'd need around 300 amp-hours of batteries at 12 volts. You will end up paying far more for solar to do this.

billvon, that's way too expensive I was hoping to find something for around $50.

Given that the batteries alone will cost around $300 and will have to be replaced about once every two years - you're probably better off just plugging into the wall.

If you want to play around with solar and partially offset your energy usage here's a bargain basement option:

Enphase M215 inverter $200
Kyocera KD235GX-LFB 235 watt solar panel $400

Wire up a 240V outlet and plug it in. You'll generate a few hundred watt hours a day and learn about solar. It will NOT be approvable or rebatable, you won't be able to get money back for power, the utility will be most annoyed if they find out you're doing it - and you shouldn't try it unless you are 100% familiar with basic AC and DC wiring. But it would get you started.

[Mike90250]

....
If you want to play around with solar and partially offset your energy usage here's a bargain basement option:

Enphase M215 inverter $200
Kyocera KD235GX-LFB 235 watt solar panel $400

Wire up a 240V outlet and plug it in. You'll generate a few hundred watt hours a day and learn about solar. It will NOT be approvable or rebatable, you won't be able to get money back for power, the utility will be most annoyed if they find out you're doing it - and you shouldn't try it unless you are 100% familiar with basic AC and DC wiring. But it would get you started.

Not legal to do in North America, and will void your building's insurance coverage, if there is any claim. They keep your preimum payments anyway....

And if you don't have a bi-directional billing meter, the power you generate and feed back to the grid, will show on your bill as power consumed ! Wheee...

[billvon]

And if you don't have a bi-directional billing meter, the power you generate and feed back to the grid, will show on your bill as power consumed ! Wheee...

My first home-based system (in 1995) was guerilla, using an array of surplus panels and Microsine grid tie inverters. I had a regular old mechanical meter. Spun forwards when I used power, backwards when I generated power.

My first permitted system used the same kind of meter with no mods.

[Sunking]

Nope. A typical PC may take 100-200 watts average but peaks

He is not talking about a home PC. He i stalking about a SERVER 24 x 7. which means a lot of power and goodies to make it work. It is not an application for solar.

[MadnessGrid]

My question was basically the same as this. I saw that some controllers have AC input and assumed there must be a model that could be configured like this.

For my small aquaculture garden I was considering adding my new 12v DC pump on a timer that only turned on the "load" during the usable sun hours, then the timer for the AC pump would kick in with some delay to prevent flooding. Would this cycle my battery in a negative way?

Isn't any energy harvested from the sun money saved and better for the environment?

Can't I add more panels, batteries, and gradually switch my entire garden to 12v devices?

Both things are hobbies of mine driven by the notion I can somehow save money on food and electricity, with less waste, and to eat healthier fish and greens.

[Mike90250]

When you add batteries, the cost of the batteries and their replacements, mean you will never save $. To save $, you use a plain simple PV inverter and n agreement with your power company.

[davecook89t]

When you add batteries, the cost of the batteries and their replacements, mean you will never save $. To save $, you use a plain simple PV inverter and n agreement with your power company.

Alright, let me start by saying that I know at this time investing in solar will be uneconomic, except in a few very limited circumstances.

We have a house in SW Florida with good sun exposure that we only occupy during the winter months. When we are not here, I like to keep an eye on things using a set of security cameras and DVR. We have service from Florida Power and Light which is very reliable and inexpensive, but we do get the occasional power outage, particularly during summer months when there are storms in the area. Because I do not ever want to lose access to my security cameras, I was using an inexpensive battery powered UPS to hopefully keep the system active when the power goes out. Sometime last summer the UPS died and I lost access to the security system for several months. Apparently, when the power went out, my modem/router/DVR needed to be rebooted. Rather than replace the UPS when I came back to Florida, I decided to take the plunge and buy a 300 watt solar kit from Renogy, with a 40 amp MPPT Controller and a 50 Ah LiFePO4 battery. Well, of course the battery turns out to be of insufficient capacity to keep the system going 24/7. During the day, the components are drawing 20 to 25 watts, during the night time when the cameras switch over to Infra Red mode, the energy usage is 30 to 35 watts. If we have even partial cloud cover during the day, the battery will not keep the system up and running all night. Rather than junk the battery I have and buy a new one, I decided to try to make use of it in whatever way I can. After doing a bit of reading, I decided to try to utilize a 5 prong relay from an auto supply store that would hopefully switch the security system from battery power over to my AC powered DC adapter when the battery voltage falls below 12V. All of the components in the security system require a 12V power source. To be on the safe side for battery longevity, I have now changed the Low Voltage Disconnect level on the controller to 12.5V. I thought that if I were able to cut off power completely on the solar powered circuit, the relay should switch over to the utility powered circuit, but what I'm finding is that when the controller cuts off the power, the relay does not irrevocably make the switch. Instead, I can hear the mechanical switch clicking rapidly as it goes from one circuit to the other. It will do this for hours, and I have not determined if it will ever stop. I am guessing there is a residual charge in the solar circuit that is still supplying enough power to keep a magnetic field in the relay's solenoid. I have a manual switch wired into solar powered circuit, and when I flip that open, the relay does connect utility power to the security system. During the time the relay cannot make up its mind which circuit to use, there is not enough power flowing continuously to keep the security system operational.

Possibly that there are electronic geniuses in this forum that will know how to solve this problem. Is there a type of relay or other electronic switch that will be absolutely activated at a certain user-specified voltage? I am hoping not to have to spend too much more money to fix this, but as I stated earlier I did not go into solar expecting to reap financial benefits. I value the learning opportunity, particularly since I know renewable power is the wave of the future. In the meantime, I would like to have something that does not need to be micro-managed, since we are away from the house for months at a time.

Thanks for any help you can provide.

[J.P.M.]

Alright, let me start by saying that I know at this time investing in solar will be uneconomic, except in a few very limited circumstances.

Actually, at this time, and in many areas, if you can get NEM from a POCO with a grid tie system, your chances of a reasonable economic ROI are reasonably good.

Just stay away from off grid and batteries if you're looking for a sane economic solution to providing power for a residential application if grid power is available.

[Mike90250]

How many days do you want the security system to stay up, in case of blackout or storm damage ? 3 days? 8 days ?

Just build a big super sized UPS with enough battery. Forget the solar, if you are not there to keep the leaves/debris/palm fronds/lawn chairs off of it