Grid tie inverter voltages and batteries

[Simon]

I'm thinking of getting a grid tie inverter as the standalone inverter really is a waste of time unless I'm really in the sticks. I'm not sure if I have this wrong but to run such an inverter from a battery it has to be rated for twice the battery voltage ?

Basically I have a pair of panels so I can have 24 or 12 V output and can easily put them in series to run the inverter directly or in prallel to charge the battery which can be turned onto the inverter later (so I could store the power for when I get home and make use of it).

Is my reasoning flawed somehow ? though I'd best make sure before i press the spend button.

[NiHaoMike]

No, match voltages. Make sure the cut-in point can be adjusted so it can be used as a "regenerative" shunt regulator.

[Simon]

well the 14-28V models say not to be connected to a 12V battery. In the past when I've ask sellers the question I've been told to use a 28-55V inverter on a 12V battery, something to do with the built in MPPT

[Zero999]

well the 14-28V models say not to be connected to a 12V battery.

That's because it's most likely designed for a 24V battery so connecting to 12V would cause it to undervoltage cut out.

In the past when I've ask sellers the question I've been told to use a 28-55V inverter on a 12V battery, something to do with the built in MPPT

What does MPPT mean?

I hate clueless sellers. I would suspect a 28V to 55V inverter is designed for 48V operation not 12V. For 12V you need an inverter rated to 10V to 15V.

[Simon]

MPPT = mean power point tracking, it was widely discussed on electrotec, in the end I've bought a 200W GTI from hong kong that say's it can be connected to a battery and is rated down to 10.5 V, I guess it will not take more than 200+over head from the battery, I've got a 74Ah battery so should cope nicely and as I've only got 40W of panels so far it is just the right size as I'll get a few more panels later, electric is going up and to be honest I see power becoming VERY exspensive in the future. Solar panel prices are falling, last year I got panels for £3/W now they are at £2.5/W, of course as soon as fuel goes up heavily they may also go up due to transport

[scrat]

MPPT = mean power point tracking, it was widely discussed on electrotec, in the end I've bought a 200W GTI from hong kong that say's it can be connected to a battery and is rated down to 10.5 V, I guess it will not take more than 200+over head from the battery, I've got a 74Ah battery so should cope nicely and as I've only got 40W of panels so far it is just the right size as I'll get a few more panels later, electric is going up and to be honest I see power becoming VERY exspensive in the future. Solar panel prices are falling, last year I got panels for £3/W now they are at £2.5/W, of course as soon as fuel goes up heavily they may also go up due to transport

MPPT = MAXIMUM power point tracking. Since the output V-I characteristics of a solar panel change depending on temperature and illumination (and between one panel and another), input stage is controlled to dynamically find the V-I point of maximum power, basically an equivalent "DC" impedance matching.

What do you mean with grid tie inverter run from a battery? Grid-connected photovoltaic inverters are quite complex objects that usually have no battery and must be certified following severe rules.
A friend of mine, which is an eletrician, uses his wind turbine and solar panel power to feed almost "constant power" electrical appliances like electric water-heater. The idea is to use the batteries the less possible.

[Simon]

well my thought was that if I'm not home i let it charge the battery and use the power when I do get home

[bill.rowland]

i was told by a friend who insalls grid tie inverter generators you have to have a new meter fitted as the power you do not use will go in to the grid and this will be deducted from your power bill he also recomends 1.2v storage batterys he uses the type used on forklift trucks if i can get a photo he has a 6 kw lorry mounted inverter generator which uses 48v panels to charge the batterys he uses this for eco gigs



bill

[Simon]

yes in theory you need a new meter, in actual fact if you are matching your own needs there's not much point. That's why i wanted to put it on a battery, I'm only getting a 200W inverter so I only need to fire up my computer to amply use that in my home instead of taking power from the grid. Yes if i leave it connected during the day I'm just giving power away

[scrat]

You could switch on the battery usage only when you know that you will use power.
Another option is to use a battery/grid two-way automatic switch. When the battery is low, you use the grid, and viceversa. Something similar is done on boats (battery/engine generator switch).

However, your electric company probably doesn't allow you to do some of these things.

[Simon]

Well the grid tie inverters sold currently plug straight into the wall. I did put in an interest for my supplier to quote me on a full blown solar setup and of course it was too much but i did ask the guy about using my own GTI, he said that providing I was not installing it that was fine, I explained that the unit would just plug into the wall and he said that was fine. My control over it would be the socket switch, the GTI will only supply power to the grid when connected so if I operate the switch that is on the mains socket i can turn it on and off at will, the only other addition I may make is to disconnect ther battery when I am home so that it stops being charged and i use the panels direct, when the sun goes down I bring in the battery that was charged up while I was at work.

By the way, solar cells have dropped in price over the last 18 months from £3/W to £2/W - we are winning, I'm seriously considering of getting another 50W to add to my current 40W

[Uncle Vernon]

Well the grid tie inverters sold currently plug straight into the wall.

How does that work? Is the power lead (and more importantly the power plug) supplying back into the mains? Sounds incredibly dangerous? Shutting off the main switch would not be any guarantee of isolation.

Grid-tie inverters sold here must be hard-wired, with the switchboards labelled to identify any additional source/s of supply.

How exactly are you connecting (plugging in) these inverters? 

[Simon]

the inverter is plgugged into the wall like any other appliance, it will only output when it senses the grid connected for safety and because it has to mimic the grid signal anyhow. I think the pug in idea is to avoind problems with regs. in the UK you can do your own hotspurs but not your own wiring, but then in theory if you hostspure the whole house you can wire the whole lot yourself. Like i said I asked my electric board and they said it was ok providing the equipment met the current standards and I was not wiring it in myself.

[Uncle Vernon]

the inverter is plgugged into the wall like any other appliance, it will only output when it senses the grid connected for safety and because it has to mimic the grid signal anyhow.

That idea still terrifies me. It's not the first time electronics has false sensed a live mains plug would be result.

More important though is isolation and identification. A main switch is supposed to be just that.

I think the pug in idea is to avoind problems with regs. in the UK you can do your own hotspurs but not your own wiring, but then in theory if you hostspure the whole house you can wire the whole lot yourself. Like i said I asked my electric board and they said it was ok providing the equipment met the current standards and I was not wiring it in myself.

We have more than enough nanny state regulations here in AU to. I'm sure such an inverter could not receive C-tick certification for sale here.

We have had piggy back plugs outlawed because some moron lit himself up using one as a cord socket. But on this occasion I'd have to agree the idea of any plug being a power source is an electrocution waiting to happen.

[Simon]

Well in the name of safety I'll be playing skeptical like you and always disconnect the source before the mains plug. once I have it up and running I may well "wire it in", I could put an RCD in line with it, at any rate a double pole switch will be installed at some point.

As sockets in the UK have a swirch on them I won't have to unplug it to turn it off but just operate the socket switch, but ultimately a double pole switch is going in there

[Uncle Vernon]

Well in the name of safety I'll be playing skeptical like you and always disconnect the source before the mains plug. once I have it up and running I may well "wire it in", I could put an RCD in line with it, at any rate a double pole switch will be installed at some point.

As sockets in the UK have a swirch on them I won't have to unplug it to turn it off but just operate the socket switch, but ultimately a double pole switch is going in there

That's fair enough, I cannot see you having problems, I'm just amazed at the concept. Most of our electrical rules are BS ones with the BS rubbed out and AS written in in it's place. And the mandatory C-Tick to give the government another gouge. Mind you all the free trade concessions now have any old rubbish being accepted. We can no longer be trusted to wire and fit piggyback plugs correctly but we must accept Chinese product with green active wiring. Go figure.

We've got a couple of 2.5KW grid tie systems installed under contract at 60c/KW-hr they are on course for a four year payback. Now the tariff only offers 20c/KW-hr so the sums don't work for new systems. Government incompetence at is usual heights.

[Simon]

To be honest I was amazed that the uk grid supplieris happy for me to just plug my own equipment in providing it's certified (yea I'm sure we all know what certified from china means), but apparantly it is fine