Solar UPS from existing inverter

[karlb]

Hi,
I have been looking at some UPS circuits online but most of them seem to be based around building the inverter circuitry as well as the control circuitry.
I have some solar panels, a 300w inverter, some relays, and other bits, so I should be able to put these together to make this.

The most basic version I can think of is a relay which, once the voltage from the panels goes above a certain level (adjusted by a pot) switches the inverter on and switches the 240v power to the devices from the mains to the inverter output.
But how do I prevent excessive toggling, eg. when the sun is not quite up enough to power the devices fully, and the voltage keeps going over the threshold, but then below again once the inverter turns on?
For the AC, can I get away with using a SPST relay on only the live line or do I need to use a DTDT relay to switch both the live and neutral?

[Bruce S]

From reading your post and your questions, and that you happen to be in the U.K.; I'm inclined to say this may not be an undertaking you should attempt right off.
UPS's are flow through devices. They are charged by the mains power and only switch to battery power when the mains is no longer available.
Let's start with the basics ; what do you intend to have attached to this 300 watt inverter? Knowing that a 300 watt inverter will not even power a mains powered kettel.
How proficient are you with electronics? Including mains powered devices?

[karlb]

I intend to power the fridge and maybe the computer, these are my main base loads and as I'm just starting to invest in some panels and have a turbine in the process of being built, these would be the loads that would make sense to target first.
I'm not worried about kettles and such for now, they are a big load but are not switched on for long throughout the day. I'm focusing more on targeting the loads which are constant but still draw medium amounts of power.
Why does living in the UK make this more difficult? Regulations?
I'd say I'm quite proficient with electronics, I often make small DC circuits that will serve a purpose around the house. I go into AC mains less often, but I still know what's what and how to be safe.

[Bruce S]

Regulations is the main issue.
The kettle though a steady and easy load is 1/10th of what you'll run into.
A fridge will take a very heavy initial current draw each and every time the compressor starts ( typically 10x the listed rating).
The computer can be a very different story. I have a fairly small setup compared to people like DamonHD (UK based) yet I do run my DSL and laptop from this bank. The items I replaced so I would have a lower overall draw is the mains to 12Vdc wall-wart. Mine is strait from a 12Vdc plug into router.
Laptop is charged via wall-wart and set with power management to max battery saving.
Hope this helps
 

[dnix71]

Grid tie should not be 'too easy' no matter where you live. You are connecting to someone else's equipment.

Actually the UK makes it easier to connect than some places, ask Damon. You just have to have the money and do it their way. An adhoc system with uncertified components and homemade electronics isn't going to pass inspection anywhere.

I'm off the grid in my apartment, so it doesn't matter. My computer is connected to a small sine inverter and the lights, fan and Engel 40 fridge I have are all 12v direct. 5 deep cycle 12v marine batteries store power and keep things running when the sun doesn't shine. I have a small modified AirX in the back yard that is making a few watts now in the summer breeze.

You could easily build an adhoc system if you included batteries, but running a full sized mains power fridge from solar isn't a good idea in that case. It takes over a kwh a day to run a modern fridge. That's a lot of power to store, considering how a few days overcast weather could wipe out your power storage and ruin your food.

[karlb]

Regulations is the main issue.
The kettle though a steady and easy load is 1/10th of what you'll run into.
A fridge will take a very heavy initial current draw each and every time the compressor starts ( typically 10x the listed rating).
The computer can be a very different story. I have a fairly small setup compared to people like DamonHD (UK based) yet I do run my DSL and laptop from this bank. The items I replaced so I would have a lower overall draw is the mains to 12Vdc wall-wart. Mine is strait from a 12Vdc plug into router.
Laptop is charged via wall-wart and set with power management to max battery saving.
Hope this helps

Thanks.
The inverter is rated 300w but can give a 1000w surge for 30 secs so should be OK for the fridge.
Kettle - seems more of a challenge so I will tackle that at a later date.
What are the specific regulations that this breaches? I'm not trying to modify my mains wiring (the wires in the walls), I'm not trying to grid tie, I'm just trying to use a relay to switch a mains appliance. It's basically the same as using a relay to switch on mains lighting from a circuit eg. arduino or similar. I'm sure a lot of people make timed lighting / clap switch circuits? Is that against regulations?
Sorry about the rant, it's not directed at you, but at the strict regulations.

Grid tie should not be 'too easy' no matter where you live. You are connecting to someone else's equipment.

running a full sized mains power fridge from solar isn't a good idea in that case. It takes over a kwh a day to run a modern fridge. That's a lot of power to store, considering how a few days overcast weather could wipe out your power storage and ruin your food.

I want the fridge to be powered from the solar panels and inverter during the day, then at night the relay switches over to power the fridge from the grid. So no storage needed and no grid tie.

[DamonHD]

Hi,

1) I think that you probably have to allow at least 10x surge over your fridge rating for the inverter not to trip.  And that's thick battery wiring too.

2) If you have something that is interconnected with the mains, and worse, has the possibility of back-feeding it if you make an error, then you will likely need at least a part-P-certified electrician on the case, to be legal and to avoid (say) invalidating you house insurance if it happens to burn down once you've set your stuff up.

3) I have 5kWp+ on my roof, fully grid-tied.  I have about 200Wp off-grid which mainly manages to power my super-efficient *4W* SheevaPlug Internet server.  Daily insolation in the UK averages about 2.5Wh per Wp of panel, down to 1 in winter, up to 5 in summer; less if not pointing due south and with optimal tilt.  Work out the actual consumption in Wh/day to work out what fraction of time they would be solar powered.  Appliances can be astonishingly energy-hungry unless designed otherwise.  A minimal cup of tea takes about 30Wh to boil, as a data point for you.

4) Voltage without a load is, I think, an unreliable measure of available power from solar panels.

5) I have from time to time used an off-the-shelf (CE-marked) power adaptor that will accept power either from mains or 12V (eg designed to work in car), and switched the 12V or 230V side (eg with SSR) when the battery voltage has been high enough, eg see here: http://www.earth.org.uk/low-power-laptop.html

Rgds

Damon

[OperaHouse]

The inverter is rated 300w but can give a 1000w surge for 30 secs so should be OK for the fridge"

My fridge draws 123A off a 12V battery to start.  That inverter just won't work.   On knowing when to switch to solar, the best thing is to buy a 5W solar panel, place it where your main solar panels are, and put it into a low resistive load.  That will track very well what the main panels are doing and allow you to decide when to put loads on the inverter..