Solar Load shedding/load managment of excess production

[thastinger]

If it's dark I'll just have to get romantic and light some candles....

I did some conservative calcs in post #9 which would make me believe it should work, cutting it close, but should work. If it rains I don't do washing so less load, it's cool so the freezer/fridge does not have to work so hard. Chest freezer can stay off for at least two days so a small gen can be used in a pinch. If need be I can use the grid, I'm not a tree hugger that wants to claim I'm off the grid. I want to be more self sufficient and try and save money/use it wisely along they way.

Also, a guy at work went off the grid for $25K but made no lifestyle changes/efficiency improvements. If he had made the necessary changes before he converted it would have cost him a lot less.

I have no shame in saying that I'm new to solar but just because everybody spends a small fortune on a PV system does not mean it has to be that way.

Ps. Average sun is about 7h, winter is about 6h I think, quite a bit more than in the states/some other parts of the world if I'm not mistaken.

7hrs and 6Hrs...so you're in Saudi Arabia? Again, you do understand that just because it is daylight doesn't mean you're panels are producing correct?

Yes, every watt saved is a watt you don't have to build capacity for.

What batteries do you plan to use? They will all eventually have to be replaced.

[Sunking]

Here is the Gotcha moment. God I love doing this as it is so much fun educating folks.

Any battery can only handle so much charge current without damage. For a Flooded Lead Acid Battery is C/8 where C = the battery Amp Hour capacity, and 8 = 8 hour charge rate.

So for example if the charge current is say 60 amps, the minimum battery capacity required is 60 amps x 8 hours = 480 AH.

[crystallattice]

Yes, every watt saved is a watt you don't have to build capacity for.

What batteries do you plan to use? They will all eventually have to be replaced.

And that's why I want to rather use the energy as it's being generated rather than storing it at a loss and then using it again at a loss = smaller battery bank + less cycling = longer life = smaller investment/replacement cost. Like I mentioned before the hybrid inverters does address this to some degree, but what it saves you in the cost of batteries it quickly makes up in the cost of the unit. So from a financial point of view it does not address the problem for me.

I bought a Eco-worthy MPPT 20A controller a while back to play around with as it was the cheapest non-fake MPPT I could find. I already have a 250W panel and have a set of golf cart batteries in the pipeline. Once I get my controller I will do some tests and see for myself what can be done in my location.

As I said before the current convention is to increase PV array + battery bank to meet demands. My idea is to decrease the battery and increase the management capabilities to meet my demands.

The same could be said for money: Some people earn 10x more money than me and yet they still don't have enough. They still have the same basic human needs as me but they have more luxuries and different strategies for managing their money.

Any battery can only handle so much charge current without damage. For a Flooded Lead Acid Battery is C/8 where C = the battery Amp Hour capacity, and 8 = 8 hour charge rate.

Yes, and unfortunately this is part of the (conventional) charge controller system which I will have to try and work around. I'm willing to try.

PS. A friend of mine supplied a lot of electric trolleys with FLA battery backup to a company. One of the guys installing it installed the battery reverse polarity in one of them, few months later there was some problem with the unit and he went to investigate, fixed the problem and noticed the battery and replaced it. Got back and installed the battery (still reverse charged) in to his car and used it like this for another 3 years! You know the saying about theory and practice....

[thastinger]

You might be able to get closer to your goals of almost no battery bank if you used LiFePo4 batteries, because of the C rates they can take and discharge at, but you must have a buffer between the CC and inverter.
Your inverter is 24 or 48v input?

[Mike90250]

the washing machine has a three phase motor

So, do you have a 3 phase inverter for it ? If not ...


Load shifting only really works on large, delayable loads. Starting surge of motors will require a larger inverter to power them, few inverters can produce more than 2x rated capacity, even for surges, the iron core of the transformer is the limiting factor, when it saturates, it cannot produce any more surge. Example. A 600w inverter can surge to 1200w for 10 sec, but shuts off at 1300w from core saturation. Add another $80 of iron and copper, and you get more surge, and a larger inverter. You can't extrapolate 1200w for 10 sec, 1400w for 6 sec and 2000w for 1 sec. The iron cannot support that. Any base loads running, detract from your starting surge

Solar Compatible.. My microwave is solar compatible. and my wife's hair dryer.

There is a reason I have a 6Kw inverter, and only about 250w base loads. Starting surges from well pump and 3 fridges. Being able to run the washer, dryer and pump is handy for the wife, if I want those chores done. If the inverter shuts off, I'm in big trouble.
And your batteries and DC wires have to be able to supply those surges without voltage droop that can shut down the inverter.

[crystallattice]

With three phase I meant they have a three phase motor/BLDC + controller inside and normal single phase connection. This means it won't have the normal startup surge which a normal single phase motor will have.

I was thinking along the lines of 2000W inverter to handle peak demands. Bank voltage has not been decided yet as the lower voltages limits the PV input to the MPPT but higher voltages needs more batteries.

I'm thinking along the lines of two 12V 100Ah betteries in parallel. Charge them up before midday. When full, start washing machine on PV, when done, run chest freezer for a bit. Then if sun is still shining, turn on water heater element on to absorb the amount of PV power still available.

Obviously if there is a cloud passing over the PV array, the battery will be used. But for the most part I want to use the PV directly. Night time use will be limited to a few LED lights/TV/DVD.

The MPPT is capable of working from 12-48v, if my plan works or not I will need PV panels, batteries I can always add more later if it doesn't work. Haven't seen any multi-input inverters though so will have to decide now or upgrade later.

Did some calcs and a small inverter + MPPT + PV System should pay itself back in 5-6years, if I get the batteries free...


Tristar MPPT looks best for my needs, whose the cheapest with these with international shipping?

[LucMan]

How about controlling your loads on the ac side?
There are load shedding controllers available for ac loads, they are completely programable for time and max loads as well as priority. I used to install these back in the late 70's in commercial buildings for load shedding to limit loads. They worked quite well using low voltage and communicating with the relays over existing electrical connections.
Honeywell made them as well as many others.

[crystallattice]

Yes, my plan is to use X10 to communicate over power line and control devices like that. I've also seen "priority relays" that will switch off one load to allow another to be used but not quite what I'm after.

[thastinger]

So you plan to use a 12V 2000W inverter and 2 marine batteries...let's review some rules.
1. batteries can only be discharged at a max rate of C8...that is 25A you can pull from your batteries so that means that you can draw a max of 300W from that battery bank (25Ax12V=300W). You don't need a 2000W inverter or the welding cables to hook it to the inverter as you'll immediately go into voltage sag and shut the inverter off. I hope you're at least starting to see why functional and useful off-grid systems are pretty much the same, there are no revelations to be had here.