Solar power generator as emergency backup power solution?

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boland

547 posts

Ultimate Geek

#250830 27-May-2019 14:14

We've got solar panels without batteries, so no power during an outage which I want to fix. However a hybrid battery solution is too expensive for us. So I've been looking at alternatives and just found this one. A Thunder Battery Box, 105Ah battery, and 100w panels.

I only need to run the fridge and charge our phones with it, so seems to be enough. The panels may be a bit on the low side, if I'm not mistaken 105Ah is about 1.3kWh, so 100w panels would take 13 hours to charge the battery?
Should be able to run the fridge during the day and hopefully most part of the night with it.

Can also use it during camping which is very nice.

Can't find a lot of reviews on this brand, but I see it being mentioned in some other forums.

Anyone got experience with it?

/Edit: The Thunder one is only 300w. I found this Chinese one, which is advertised as 1000w: https://www.trademe.co.nz/Browse/Listing.aspx?id=2161279066

Seems like a better choice as a fridge is probably more than 300w. However, can't find any reviews about it...

tripper1000

1618 posts

Uber Geek

#2246566 27-May-2019 15:32

That most likely would not run a house hold fridge at all. Reason being the the inverter is too small at only 300 watts (and 900 watts surge) and a fridge needs approx 4x its running current to start. You'd need an inverter closer to 800 - 1000 watts for reliable operation. Relying on the surge capacity of the inverter (900 watts) is a risky strategy because it may not be able to surge as long as it takes the fridge to start. Not long enough I'd wager as the surge power is coming entirely from capacitors as shown by the 12 volt fusing: The inverter 12v side is fused at 30 amps which is only 360 watts less it's 90% efficiency means the highest possible load supply-able from the battery is 324 watts.

Also modified sine inverters causes the fridge motor to run hotter, wasting battery power. Pure Sine is preferable for fridges.

I find my fridge/freezer (without opening the door) plus my ONT & Router uses 1kw/h per day (on pure sine), so this battery size would keep you going for around 24 hours at best. If you open the fridge door a lot, power consumption will rise and hours to run will diminish.

You are right, even in the most favourable sunshine conditions (let alone real world NZ conditions) the solar panels would be too small to keep up with the demand of a household fridge.

boland

547 posts

Ultimate Geek

#2246586 27-May-2019 16:00

tripper1000:

That most likely would not run a house hold fridge at all. Reason being the the inverter is too small at only 300 watts (and 900 watts surge) and a fridge needs approx 4x its running current to start. You'd need an inverter closer to 800 - 1000 watts for reliable operation. Relying on the surge capacity of the inverter (900 watts) is a risky strategy because it may not be able to surge as long as it takes the fridge to start. Not long enough I'd wager as the surge power is coming entirely from capacitors as shown by the 12 volt fusing: The inverter 12v side is fused at 30 amps which is only 360 watts less it's 90% efficiency means the highest possible load supply-able from the battery is 324 watts.

Also modified sine inverters causes the fridge motor to run hotter, wasting battery power. Pure Sine is preferable for fridges.

I find my fridge/freezer (without opening the door) plus my ONT & Router uses 1kw/h per day (on pure sine), so this battery size would keep you going for around 24 hours at best. If you open the fridge door a lot, power consumption will rise and hours to run will diminish.

You are right, even in the most favourable sunshine conditions (let alone real world NZ conditions) the solar panels would be too small to keep up with the demand of a household fridge.

Yeah I need a bigger solution. What do you think of the other one I linked? https://www.trademe.co.nz/Browse/Listing.aspx?id=2161279066

There's also a 1500w version.

Aredwood

3885 posts

Uber Geek

#2246606 27-May-2019 17:08

Just buy a generator to run the fridge. And get a 12V UPS to keep your Internet working, charge cellphones etc.

Just run the generator for a couple of hours at a time, maybe 3 times per day. Which would probably be enough to keep the fridge cold. And charge the 12V battery while the generator is running.

Those above linked systems probably have their storage capacity rated as 100% depth of discharge of the battery. You will quickly ruin the battery doing that. Calculate based on 50% DoD since you will be using it more as a backup solution.

Meaning that that Trademe unit has only 40AH. Or 0.48KW/Hr of useable storage. And you still need to buy some solar panels to go with it.

raytaylor

4017 posts

Uber Geek

Trusted

#2249390 31-May-2019 23:07

Assuming 2kw for the fridge each day

15 watts for a cellphone battery charger for approx 12 hours per day (4 devices?) is 180 watts

An extra 20% for inverter inefficiency in converting the voltage

==================

2,616 watts total load

2616 watts / 12 volts = 218 amp hours to 50% discharge of the battery, making the battery bank minimum size 436ah total per day.

If you are planning for a natural disaster, you could be without power for 4 days+

The solar panels would also need to be sized in case of a rainy day - in winter we plan for 15 days of cloud.

However you might be able to get by with a 50% load instead of 15% rainy day performance due to the shorter time your planning to be without power for.

So (2616 watts / 6 hours) / 50% = 872 watt panel array.

Google told me a domestic refrigerator uses 1-2kw per day so on a cloudy day you would need to not open the fridge very often to preserve the battery bank and lack of solar capture.

Its also a good idea to store water bottles in the fridge as that mass of cold doesnt fall out whenever you open the door, and water will stay colder for longer than air as it is more dense.

Ray Taylor

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