Choosing an inverter

[mikepepler]

We've had our off-grid solar working for over a month now, and I'm getting a feel for how much power we can use from it while keeping the batteries mostly full (ready for an emergency...). We've currently got a 180W Victron inverter, which has been running the solar water heating controller/pump, the router, up to 2 laptops, and occasionally charging an electric bike battery. It's a good quality inverter, but it can't actually consume all the power the PV generates. The main reason is that it can only take a surge to 360W for a few seconds. Our fridge and freezer, while each drawing less than 100W in operation, draw about 500W for a few seconds on start-up, so I can't plug them in (I've tried) - this is a shame, because on sunny days when we go out, I'd like to leave at least one of them running off the PV.

So, I've been looking at getting a second inverter. I plan to keep the 180W one in operation, as it's standby power is pretty low, so it can be left on 24hrs for the solar water controller, but I want a bigger one that can be used in addition to it. Here's the factors I'm considering, and would appreciate anyone's thoughts on:

1. While the grid is working 100%, the job of the new inverter is just to make use of the excess power from the PV, to save some money. As such, it will probably be used mostly during daylight hours on sunny days, as it's more efficient to use the power directly, rather than it going through the battery.

2. If the grid was off for up to a few days, it would be good to be able to run the fridge, freezer, or both. Of course, if it was winter, the fridge food could just be moved to a secure outdoor area, saving some power.

3. If the grid was off for a prolonged period, I'm not sure what I'd use it for on a daily basis - is there much need for a working fridge AND freezer if the grid’s gone and our infrastructure is crumbling? I'm thinking the PV and inverter would then be best used for power tools, computer (looking up info I've saved), lighting, and charging batteries for friends and neighbours (perhaps in return for something?)

4. Availability of power. My current estimates indicate that I could run the fridge off the system all year, though in the winter it might be better to just put food in a cold place. The freezer uses a bit more, so I could only run it indefinitely from Feb-Oct. I could only run both fridge and freezer from Mar-Sep.

5. Is it advisable/practical to use timers to alternate the fridge and freezer, so they don't both start up at once? Both running at once is about 160W, but both starting at once is over 1kW for a few seconds!

6. What brand to get? The Victron (Dutch made) is a good brand, but a bigger version costs a lot. There are cheap ones made in China, but I'm wary of quality, considering this could be something that I'm depending on one day.

Given the above factors, I basically need to decide:

- which appliances should I be trying to run from it in the case of grid outage
- do I get a 750W inverter, or something much bigger, say 1.5kW
- how much do I spend on it (i.e. what brand)

A few other points:
- Fridge is 117kWh/year, no ice box.
- Freezer is a small chest version, 150kWh/year
- I’m 99% certain I don’t want a modified sine wave inverter.

Any thoughts?

[adam2]

I would avoid modified sine wave, especialy for motors.
I would avoid cheap chinese inverters.

I used a studer inverter for years, purchased from wind and sun, this eventually failed but only after many years heavy use.
I am now useing an old Trace Engineering (now Xantrax) inverter model 1255 I think. This works fine and copes with a large portable A/C unit.

I would get an inverter big enough to start the fridge and the freezer together, timers and changover relays add cost, complexity and another point of failure.

[PowerswitchClive]

Victron make great quality inverters, but sine wave inverters don't come cheap and also you lose energy in the conversion.
Since you will need to spend around £500-700 for an 800 watt pure sine wave inverter, perhaps it would be worth thinking about replacing your fridge/freezer for energy efficient 12volt versions

http://www.boatfridge.com/index.asp?Col ... oductID=28

Shoreline also allows you to add their power convertor for £80... which allows automated switching between 240v and 12volts... Run the unit on mains power and in a power cut, it would automatically switch to your 12volt supply.

[PowerswitchClive]

Just a thought also.... if you are running your system as a 12volt battery.. your charge controller is not beefy enough as it is only rated for a 200watts solar on a 12volt system

[emordnilap]

Since you will need to spend around £500-700 for an 800 watt pure sine wave inverter, perhaps it would be worth thinking about replacing your fridge/freezer for energy efficient 12volt versions

http://www.boatfridge.com/index.asp?Col ... oductID=28

Shoreline also allows you to add their power convertor for £80... which allows automated switching between 240v and 12volts... Run the unit on mains power and in a power cut, it would automatically switch to your 12volt supply.

Would it make sense to replace fridges and freezers with the above even in houses with ordinary grid power supplies? Run through appropriate adaptors?

[adam2]

Just a thought also.... if you are running your system as a 12volt battery.. your charge controller is not beefy enough as it is only rated for a 200watts solar on a 12volt system

Should not be a problem, the limit on the charge controller is the size of PV array that can be used, not the load.
Most charge controllers have a protected output for lighting etc, this is of limited capacity, sometimes a bit more than the solar input.
An inverter however is normally connected direct to the battery, not via the charge controller.
The only limit on inverter size is how much current the battery can supply.

[PowerswitchClive]

quote adam2 Should not be a problem, the limit on the charge controller is the size of PV array that can be used, not the load.

Yes that's my point... it's a 15Amp charge controller, plus what to me looks like a very impressive array of seven 40watt panals - this equals a peak out put of 285watts - The controller is rated for 200watts of solar

Emordnilap - If a new sine wave inverter is going to cost £600ish... why not spend that money on a new 12volt fridge/freezer - add in the £80 power pack and the fridge will run on either 12v DC or 240 AC. Keep the 180 sine wave inverter for laptops etc.
Stepping up the voltage is not very efficient... and i bet you the 12volt fridge/freezer unit lasts longer than an inverter.

[emordnilap]

Emordnilap - If a new sine wave inverter is going to cost £600ish... why not spend that money on a new 12volt fridge/freezer - add in the £80 power pack and the fridge will run on either 12v DC or 240 AC. Keep the 180 sine wave inverter for laptops etc.
Stepping up the voltage is not very efficient... and i bet you the 12volt fridge/freezer unit lasts longer than an inverter.

Thanks (though I think your answer might be for a different question )

To repeat in another way: if a household only has grid (240 AC) supply, would it still make sense to buy one of these fridges or freezers? This is a very valuable thread; apologies for sqivelling off topic.

[PowerswitchClive]

If you are not planning to run the fridge / freezer on 12 or 24 volts, then no it would make no sense at all. Shoreline fridges are hand assembled in the south of England using quality parts, but they are at least twice the price of a regular fridge freezer.

[adam2]

quote adam2 Should not be a problem, the limit on the charge controller is the size of PV array that can be used, not the load.

Yes that's my point... it's a 15Amp charge controller, plus what to me looks like a very impressive array of seven 40watt panals - this equals a peak out put of 285watts - The controller is rated for 200watts of solar

Emordnilap - If a new sine wave inverter is going to cost £600ish... why not spend that money on a new 12volt fridge/freezer - add in the £80 power pack and the fridge will run on either 12v DC or 240 AC. Keep the 180 sine wave inverter for laptops etc.
Stepping up the voltage is not very efficient... and i bet you the 12volt fridge/freezer unit lasts longer than an inverter.

I misunderstood the post, I thought that it was being suggested that the charge controller was too small for the proposed load.
In fact it was being suggested that the charge controller was too small for the available PV array.
This would appear to be the case.
If the controller is designed for 200 watts of solar, as stated, then I advise against use with a 280 watt array.
If however the controller can handle 15 amps, as also stated then it should be OK, just. A 280 watt array is unlikely to produce more than about 14 amps in practice. A good "rule of thumb" is one amp per 20 watts of rated output.
It might be worth measuring the current in bright sun.

If the charge controller is overloaded, the obvious answer is to buy a larger one, but there is a cheaper approach.
Connect about a third of the modules directly to the battery, via a fuse, blocking diode and switch.
When the battery is fairly well charged, turn the switch off, the battery will still reach full charge via the other modules and the charge controller.
When the battery is low, turn the switch on, overcharching is unlikely for a day or two, especialy if current is being used.

[adam2]

To repeat in another way: if a household only has grid (240 AC) supply, would it still make sense to buy one of these fridges or freezers? This is a very valuable thread; apologies for sqivelling off topic.

Possibly, it depends on how doomerish you are !
If there is no likeleyhood of ever running the fridge on 12 volts, then I would advise against it.
It would be best to spend the money on an extreme efficiency mains voltage fridge. No 240 volt to 12 volt converter to buy, no risk of this item failing, and no paying forever for the (small) losses in the converter.

If however you have only a mains voltage supply AT PRESENT, but might use 12 volts in some future emergency, then yes I would buy the 12 volt appliance. This would facilitate future use in long term power cuts.
A properly designed PV and battery system would be best, but in the short term a vehicle battery would serve.
If running a 12 volt fridge on the mains via a converter, it would be well to have a spare converter against failure.
If useing a 240 volt AC to 12 volt DC converter not supplied by the fridge manufacturer, ensure that it can supply enough current. Some 12 volt fridges use as much as 30 amps, very briefly when starting.

[mikepepler]

Sorry for my absence, been busy! Some replies:

- our fridge and freezer are only 6 months old, small, and efficient. For the price of a shoreline fridge we could by an inverter. Nice appliances though!

- although there are 7 panels on the roof, only 6 are connected to the main system. One is connected to a secondary system which runs small DC loads (night light for our pet birds, chargers for phone, ipod, camera, etc.) and also keeps a smaller deep cycle battery on float charge as a backup.

- the 15A controller I have is an MPPT, and the manual says that if you give it more power than it can deliver to the batteries, it will just shift the panel voltage away from the MPP to reduce the power. In case anyone's not familiar with this terms, basically the problem is that a PV panel is most efficient at a set voltage, say 17V, but a standard controller connected to a 12C battery will operate it at 12.5-14.7V. An MPPT uses a DC-DC converter to step the voltage down, and the gain in panel efficiency more that outweighs any loss in the converter.

- I was originally going to run the system at 24V (the controller can auto-switch), in which case the power delivered would have been fine, but I changed my mind and went for 12V. The bulk of the day is spent charging at 14.3V, so 15A gives about 215W. After a time it drops back to 13.7V, so that's 206W. The peak panel output is 240W, but that's only going to be the case for a short time on the sunniest days, so I don't think I'm missing much power.

- The controller I have on the backup system can handle 30A at 12 or 24V, but it does not have an MPPT. So here's an intersting question... Which is best: the 15A MPPT (which can't take all the peak output, but will run panels at optimum voltage the rest of the time), or the 30A standard controller (which can take the full power, but will not be at optimum voltage).

Going back to the inverter question:
- I've not bought one yet - I've been measuring the consumption of key appliances around the house, averged over a day or two, to see what I could really run in practice, to inform my purchasing decision.

- I could still rewire my pair of batteries for 24V operation, and get a 24V inverter. This would let my MPPT controller make use of the full output of the PV panels, and I could still use 12V appliances using a 3-wire system as described elsewhere by Adam. However, were one battery to fail in a situation where it could not be replaced, I'd be stuck with a useless inverter, while at present if one battery failed I'd carry on with the other one but at redduced storage capacity.

I'm actually waiting for a "free" fix of a water leak in our garden before spending money on things like inverters, just in case the work crew have to move our shed (which would then break) or something stupid, so it doesn't end up being zero cost to us. Once that's out the way I'll be buying the inverter...

[adam2]

Question one, I would suggest that the 15 amp MPPT controller is better than the larger non MPPT unit.
As you point out, the MPPT will waste some of the energy, buy not very often, and then only at times of plenty when it probably does not matter.
On cold winter days, the MPPT controller should give significantly more energy into the batteries, just when it is most likely to be needed.

Question two, a 3 wire DC system operating at 24 volts might be best, as described elswhere.
The drawback of one battery failing is probably more theoretical then actual. Batteries dont often fail suddenly and complety, gradual deteriotion is more likely.
It is likely that both batteries would reach end of life at about the same time.

[emordnilap]

To repeat in another way: if a household only has grid (240 AC) supply, would it still make sense to buy one of these fridges or freezers? This is a very valuable thread; apologies for sqivelling off topic.

Possibly, it depends on how doomerish you are !
If there is no likeleyhood of ever running the fridge on 12 volts, then I would advise against it.
It would be best to spend the money on an extreme efficiency mains voltage fridge. No 240 volt to 12 volt converter to buy, no risk of this item failing, and no paying forever for the (small) losses in the converter.

If however you have only a mains voltage supply AT PRESENT, but might use 12 volts in some future emergency, then yes I would buy the 12 volt appliance. This would facilitate future use in long term power cuts.
A properly designed PV and battery system would be best, but in the short term a vehicle battery would serve.
If running a 12 volt fridge on the mains via a converter, it would be well to have a spare converter against failure.
If useing a 240 volt AC to 12 volt DC converter not supplied by the fridge manufacturer, ensure that it can supply enough current. Some 12 volt fridges use as much as 30 amps, very briefly when starting.

Thanks adam2. A very comprehensive reply and plenty to think about.

[mikepepler]

I've been doing some number-crunching. I'll spare you the details, but what I found was this:

- The efficiency of my MPPT charge controller is proportional to the current flowing through it, NOT the voltage it's working at. So because switching to a 24V system would halve the current, the efficiency would actually drop. Also, I've currently got 24V input and 12V output, and from looking at the efficiency graphs in the manuals it looks like if I require the panels so its 12V in / 12 out I'll be about 1% better off overall, allowing for increased losses in the cable from the higher current between the panels and the controller.

- 24V input inverters are 1-2% more efficient than 12V input. However, their no-load current draw is usually higher as well, more than offsetting this gain if they're left on 24 hours a day. Some have power-saving modes, where they turn on briefly every few seconds to see if there's any demand, but that won't work for our solar controller, as at power-on it runs the pump, so it would be doing it all night...

- One of the Victron inverters I've been looking at draws 14W no-load. Alarmingly, this adds up to about 10kWh a month, which could use all the output of my PV in Dec and Jan!

Having monitored various appliances round the house, it now seems that my current system and 180W inverter (which only draws 2.6W no-load) is fine for leaving on 24hrs and still having surplus power to run both the solar thermal pump and circulation pump for the woodburner (which only comes on intermittently). This is true even in Dec.

From looking at the fridge and freezer consumption I concluded that on top of the solar thermal and woodburner (when applicable) pumps:
- I could run the fridge full-time Mar-Sep
- I could run the fridge and freezer full-time May-Aug
- in the event of a short power outage, I could run fridge and freezer for 2-3 days even with no sunlight (allowing 50% battery discharge)

So, given that I don't really want to spend any more money on PV or batteries at present, the conclusion I think I've come to is this:
- I'll keep the system at 12V
- I will probably rewire the panels for 12V to gain a bit more efficiency out of the MPPT
- I'll buy an inverter that is able to run the fridge OR freezer, but not both.

The plan will then be:
- under BAU conditions, I'll use the new inverter to run the fridge or freezer on sunny summer days, maybe just during the daytime. This is just to save a bit of money on elec usage.
- in the event of a short power cut, I'll run the fridge, and eat up what's in the freezer as it defrosts. I could also swap the fridge and freezer over for a few hours, to keep both running, as this is only a short-term situation.
- in the event of a prolonged power outage, I would run just the fridge (as long as there's food to go in it). In the months where there's not even enough power for that, it's cold enough not to need the fridge anyway, as we have a cold room we can leave food in.

Any thoughts?

The inverter I'm considering is this one (they seem to be about £50 cheaper here than elsewhere):
http://www.kuranda.co.uk/chandlery/vict ... erter.html
There's a datasheet here:
http://www.victronenergy.com/upload/doc ... -%20EN.pdf

Any comments on better alternatives at a similar price?