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  • ariarnia
    ariarnia Posts: 4,225 Forumite
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    i'd be more concerned about the fridge than the freezer. i read somewhere that you should get rid of high risk foods (rice. meat. etc) that have been in an unpowered fridge for more than 4 hours (i think with freezers its a couple of days) because they dont maintain temps as well as freezers do. 

    but i think even freezers. when they come back on the motor will start up again at full pelt and it could stress it. there not designed to gain and lose power so i can see it could reduce the life. but how much it would reduce it (if its supposed to last 10 years maybe it would last 8?) and how that would work for your inverter cost vs the savings. 
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  • 12 kWh losses in the inverter each day sounds unlikely, you'd be able to toast muffins from that much heat.
  • Miser1964
    Miser1964 Posts: 283 Forumite
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    edited 12 November 2022 at 7:25PM
    >Where did you get 12 kwh from?<

    OK, I can see you meant the invert losses would be 20W
  • Alnat1
    Alnat1 Posts: 3,269 Forumite
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    Most people will have only one inverter and there will be numerous household appliances that will have a fairly constant but small power draw besides the fridge and freezer.

    Thinking of wired in smoke alarms, burglar alarms, CCTV systems, internet router, bedside clock etc. so if it's 30w to keep all that going, it's not so bad.
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  • mmmmikey
    mmmmikey Posts: 1,618 Forumite
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    Alnat1 said:
    Most people will have only one inverter and there will be numerous household appliances that will have a fairly constant but small power draw besides the fridge and freezer.

    Thinking of wired in smoke alarms, burglar alarms, CCTV systems, internet router, bedside clock etc. so if it's 30w to keep all that going, it's not so bad.
    That's fair comment and if you NEED to keep it on then your need to. But in my case it's just the fridge and freezer so it's hugely inefficient. If I turned my inverter off for just 8 hours a night that would save 8 x 30 = 240 watts. I might even get away with 12 hours on, 12 hours off - which would be 360 watts a day.

    Which isn't huge but in Autumn / Winter when solar is scarce every little bit helps. 

    Also - to be clear - it's not 30w to keep that all going. It's 30w per hour. So if you're out or sleeping for 20 hours in a day those little bits and bobs have an overhead of 600w. Which means you're wasting a huge chunk of the solar that you collect. 

    EDIT: You can keep things running on DC 24/7 without overheads.
    Agree with what @Deleted_User is saying and that reflects my real-world experience, measuring this with Tapo plugs on the charger and output of the inverter charger that drives my fridge and freezer. If I run just the fridge and freezer I get something like 63% efficiency (fridge and freezer use 300Wh/day and 600Wh/day respectively.

    So what I do, as @Alnat1 suggests is connect various other small load bits and bobs to the inverter (e.g. garage door opener on standby, philips Hue equipment, reclining bed on standby, water softener on standby), and also use it for small but fairly consistent load devices like the TV, computer and lights. In normal use I average ca. 1.6kWh output for 2.2kWh input, i.e. about 73% efficiency. This is very consistent from day to day as this particularly inverter/charger runs known, consistent loads.

    Still not good but perfectly adequate for me because I am charging the battery with a consistent source of cheap electricity. I pay 6.9p/kWh for the input (E7 night rate) and save 47.31p/kWh for the output (E7 day rate). So input costs 2.2*6.9=15.18p and output saves me 1.6*47.31=75.70p. Daily saving = 60.52p, annual saving = £221 for a setup that cost £950 and has a life expectancy of about 12 to 15 years.This includes a 2.5kWh battery and 1kW inverter/charger. All fully automated - everything just works as normal, saving me money without any intervention or management.

    Note that in the summer, I can use solar power to charge the battery instead if I choose, but this makes little difference financially (at the moment with EPG rates) because using 1kWh of solar power reduces my export income by 5.99p - i.e. only a 2p(ish)/day saving in total on the input side to be gained by changing from E7 to solar charging.


  • mmmmikey
    mmmmikey Posts: 1,618 Forumite
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    Flipping back to original question, I did experiment with switching the freezer off for short periods at one stage when I was running it off a smaller battery. Only comment is that in order to make the food keep freash for as long as possible, it's recommended to set the thermostat to -18C. If you turn it off, in my case the temp rose to -16C over a 4 hour period, so I reset the thermostat to -20C and allowed it to rise to -18C. This added about 50Wh to the daily energy use, which is something you need to factor in somewhere. I don't bother doing it now because as soon as you've got 1 device that needs 24h power you've got to take the hit to the standby load anyway. So now it's just dead simple and the freezer runs as normal.
  • BikingBud
    BikingBud Posts: 1,730 Forumite
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    If you are really concerned, as food safety relies upon refrigeration and sustaining the cold chain, the only way to be certain is to record temperatures and see what happens.

    It is good practice to have a thermometer in your domestic devices, a legal requirement for some foods in commercial premises, but this might cost you as much as the electric you are hoping to save, 
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  • mmmmikey
    mmmmikey Posts: 1,618 Forumite
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    Thanks for this.

    Can any of you see any potential for it being bad for the appliance? Not sure how - I mean they're designed to go on and off frequently so presumably unplugging it won't be any different?

    If someone does a lot of batch cooking and is constantly opening and restocking the freezer that will increase temperature more than switching off for 12 hours or so in each 24.

    The load on the compressor is much more doing an hours stint every four hours to raise the temp 2 degrees than it is if it runs in lots of short burts to raise it 0.1 degree - or so the argument goes. Maybe something to do with the compressor getting hotter running for longer periods and therefore wearing out sooner? A bit like saying much easier to slowly walk up a hill at a gentle pace for an hour, than leave it for 55 minutes and then run like hell to get to the top in 5 minutes - again, or so the argument goes. Sounds reasonable but haven't seen any evidence to back it up - e.g. online reports of folks trying it and killing their freezer in a few years.

    Very subjective of course and the only way to know for certainty is to run it like this and check back in 15 years. But (being a bit paranoid about wrecking it) I looked at the on-off cycle to see if it was runing for hours on end after coming back on, and it didn't seem to be. When we enjoyed (endured?) the 40+C temps in the summer and very hot days for a prolonged period, and the freezer seemed much more stressed then than it did in normal conditions doing its turning off for a few hours cycle. So personally, I'd take a bet on it being OK to run it, say, 4 hours on and 4 hours off over a day. But the words I'm using a deliberately loose - it's a gamble.

    Maybe you could only do this during the winter months when the freezer needs to do less work and there's less sunshine, and in the summer when you've got spare power from the solar panels run the freezer as it's designed?

  • mmmmikey
    mmmmikey Posts: 1,618 Forumite
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    edited 13 November 2022 at 1:34AM

    All I'd add to that is to say that when you're not able to get enough solar in winter it makes sense (if your system allows it) to put all your low power, long duration stuff onto the mains and keep your solar for high draw, short duration - even if it means only using your generators for half an hour every other day. 

    For example if you only get 500 wats of solar on a winters day consider these two options:

    1. Run a 5w lightbulb all day (you might assume that it would run for 100 hours) but if your inverter uses 30 watts per hour your battery dies after about 15 hours. You then need to power your lightbulb for 9 hours off the mains (45 watts). 

    2. Run a high power appliance for 15 minutes (inverter cost = 7 watts)
    Now imagine you want to run both - you can't run both off the solar as you don't have enough. So consider the cost of mains electricity according to the two possible scenarios:

    1. If you run your lightbulb from solar and your high power appliance from mains you will have to purchase 545 watts from the electricity company. (500 to run the high draw appliance and 45 to continue running the bulb after the battery goes flat)

    2. If you run your high power appliance from solar and your lightbulb from mains you will have to purchase 60 watts from the mains to power your lightbulb.

      The difference in mains electricity cost per day is 485 watts. 




    Makes sense but my scenario is different...

    (a) Cheap rate overnight mains electricty for charging the batteries costs 6.9p/kWh

    (b) Any unused solar is exported (and metered) and sold for 5.99p/kWh, so for every kWh I use myself reduces my income by 5.99p/kWh and in this sense solar is not free - it costs me 5.99p/kWH

    So there's very little difference cost wise between charging the batteries from the mains or through the panels, so my batteries always start the day fully charged. It doesn't matter how much the sun shines as the batteries are big enough to last through to the next cheap rate electricity period.

    Your point is still perfectly valid though, but not having enough cheap energy to charge the batteries is not a scenario I face.

    It does all come back to your central point about wasting as little as possible in inverter standby. I've tackled that by concentrating all the "always on" stuff onto one battery system. I have a completely different battery system (a bit like one of your Bluetti generators) that I use for high load short duration things like the toaster, air fryer, etc. and that isn't left on standby, I just turn it on when I need it, so that's fairly efficient - typically about 87%.

    And as you also point out, most efficient by far is running DC appliances directly which is how my router works, and I get 95% efficiency there because there's only charger and battery losses (plus a minimal loss from the voltage regulator which pales into insignificance compared to using an inverter)

  • Ectophile
    Ectophile Posts: 7,314 Forumite
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    A lot of the confustion in this thread is people mangling their electrical units.  What does "you can save 250 watts a day" even mean?  Is it that you can save 250 watts for a day, or 250 watt-hours per day, or 250 watt-days per day?  Who knows?
    If it sticks, force it.
    If it breaks, well it wasn't working right anyway.
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