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Could Powerwall help me go off grid? Looks way too small for my needs.

Discussion in 'Tesla Energy' started by MileHighMotoring, Jun 9, 2015.

  1. MileHighMotoring

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    I have a large home, but with careful energy auditing (lower power computer, TV, all LED bulbs, etc.) I've gotten my energy usage down to about 1500 kWh per month on average. My local utility doesn't provide any rebates, tax breaks, incentives, etc., so I'd get the 30% federal credit and if I wanted to do net metering, they're paying half what they charge for electricity.

    Anyway - I've wondered about installed a solar array large enough to go off-grid. Something around 12-13 kW solar should do it. But, we'd need a battery system for night time use. Given we use 1500 kwH/month, that's 50 kWh per day. So the 7 kWh Tesla Powerwall would only provide, at best, 5 hours of power. It seems that I'd really want 3 times that much battery if I wanted to go off-grid. Am I doing that math correctly? Maybe more to account for cloudy days, bad weather, etc. If I am thinking about this correctly it would never make sense for me to go off-grid.

    Thanks for your thoughts.
     
  2. cpa

    cpa Member

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    Mile High,

    I am NOT an EE. So, my only comment is my actual results with a puny 4.4 kW solar array. Your climate is likely diametrically different from ours, so your output will not scale linearly with ours. Our original installation was 2,600 watts and we generated between 4,000 and 4,250 kWh per year. We added another 1,900 watts when we bought our Tesla. Coming up on its first anniversary, and I believe that we will wind up with about 3,000 kWh +/-.

    We do have some trees that eclipse the sunlight in the late afternoons, and especially so come late September through late November when they finally fall to the ground. Our summers are likely much longer and hotter than yours, and efficiency of PV panels drops as temperatures increase. (It is not uncommon for our overnight "low" to be 73-79 degrees.) Normal winters for us (i.e., not in drought conditions) have a lot of high clouds or tule fog that really reduces output from late December through mid-February. Days with dense tule fog have resulted in less than 1kWh generated for a full 10-11 hour day.

    Good luck, and I hope this helps you understand a little better!

    We try to mind our usage, but still wind up with an annual balance due under the Net Metering Program of $250-$300 per year. This is approximately 1,800 to 2,000 kWh purchased from our good buddies at Pacific Gas & Electric.
     
  3. miimura

    miimura Active Member

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    If you already have a grid connection, you should keep it. With a system that is designed for being off grid, you can prevent the solar from feeding back into the grid, but only go into the batteries and powering loads inside your house. Then, when the batteries get too low due to higher energy usage or weather-induced poor solar production, the system maintains the minimum battery level from the grid. This way, you never give away your solar generation at a low feed-in price, but you don't have to go crazy big on the batteries or solar to make it through low production times. The main drawback is that you will have to pay the minimum monthly fee for the grid connection even when you're not using it.
     
  4. MileHighMotoring

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    Exactly - and they're upping it to $50 a month soon. F-ing ridiculous.
     
  5. Cosmacelf

    Cosmacelf Active Member

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    Going off grid is very expensive. You have to have a very large battery array to handle many sequential winter cloudy days. When you figure out the cost of the system, it'll almost always cost significantly more in terms of $ per kWh than what you are paying for grid power. The only people that do off grid either live in very remote places where grid power is not available at any kind of reasonable price, or survivalist kind of people who reduce energy usage way below what you are using.
     
  6. Jeffgtx

    Jeffgtx Member

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    Hi. how is such a system designed? we have a ground mount system in teh backyard. i am trying to determine the best way to go to battery for energy backup. how do i configure the batteries and transfer switch to do what you outline above? thanks.
     
  7. omgwtfbyobbq

    omgwtfbyobbq Member

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    If you can push enough of your consumption into the daylight, off-grid may be the same price or less expensive than even grid-tied solar. With the 30% tax credit, the levelized cost of solar can be really low, around 6-8c/kWh ($3.30/W installed, or less if you can DIY) over a 30 year lifespan (Good chance you'll need to replace an inverter, but the panels will likely be going strong). The downside is that storage adds a substantial amount, something like 12+c/kWh with a PowerWall, so every kWh you store and then use is around 20c/kWh.

    Having said that, setting up your own DSM (demand side management) system and tweaking your panel set-up can be fairly involved. If your schedule can accommodate it, you might be able to orient your panels either east or west and spend as much time during the day charging your S. Is heating another big energy draw? A passive solar heating system with a storage tank could help a bunch in that case.

    Besides that, moving all the other energy consumers into the day time is just a matter of purchasing automated plugs/receptacles and setting everything up to maximize/even out your daily loads. Ideally, if you inverter is wifi enabled, you can set up some service to monitor your daily energy production, pull the expected forecast, and activate/deactivate stuff around the house in an optimal fashion. If you utility has records of your use, you really optimize things. A generator would probably be required too.

    If you can push ~80% of your energy use evenly into the daytime, keep your battery use limited to ~15% of your consumption, and your generator use limited to ~5% of consumption, you'll be able to be off-grid at around 12c/kWh.
     
  8. electracity

    electracity Active Member

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    Off grid is 4/5 days worth of battery storage and a generator. $50/month is a much better value.
     
  9. Owner

    Owner Active Member

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    You've really got to look at the winter / summer difference in solar generation. It's huge in the northern hemisphere. I wrote up a bunch of historical data from my panels here in California:

    Powerwall A Middling Product | TESLA OWNER
     
  10. MileHighMotoring

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    Sure seems like you're right on the money. I think I'm going to abandon the idea for now. Thanks for the input everyone.
     
  11. nwdiver

    nwdiver Active Member

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    #11 nwdiver, Jun 16, 2015
    Last edited: Jun 16, 2015
    This is the part of the utility BS that REALLY angers me; Unless you live like a hermit >1 mile from any other human or transmission line grid-tie is ALWAYS cheaper than off-grid... the utilities just MAKE grid-tie more expensive to discourage distributed generation. $50/mo just to be connected wether you use it or not is absolutely absurd. That's what XCEL has managed to do in NM. I can be grid tied and pay ~$50/mo even if I NEVER use the grid OR get a small gen-set and go off-grid. It ACTUALLY IS cheaper to go off-grid....

    I think what I'll try is bypassing the production meter and steal from myself... paying $0.036/kWh PRODUCED is absurd. XCEL should be ashamed. If they cut me off THEN I'll go off-grid...

    We need a federal law banning fees for capacity or production... charge for peak usage (kW) that's fine. Charge for energy use (kWh) that's fine. Pay me NOTHING for exports.... heck CHARGE ME for exports kinda messed up... but ok.

    DO NOT CHARGE ME A FEE FOR THE ENERGY THAT NEVER SEES YOUR GRID!!
     
  12. miimura

    miimura Active Member

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    If you want to use the system as off-grid, but have the grid present for support during low solar production, then you would basically move all your loads from your main panel to sub-panels that are fed by the off-grid inverters. The solar would go straight into the batteries through charge controllers and the utility panel would then only feed pure utility loads (if any) and the grid connection of the inverter. Inverters like the Outback Radian can be configured so that one grid connection is only a battery charger and it can be configured so that it only charges when the batteries hit the low voltage trigger. This type of connection scheme would also prevent you from back-feeding the grid with your solar, in case that is not allowed by your utility.

    This is basically the same setup as wk057 did in the famous thread Plan: Off grid solar with a Model S battery pack at the heart but you obviously don't have to go as crazy-big as he did.
     

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