Off grid system

momo7June 30, 2011

Hi, I was wondering if I posted a system that I'm looking at for our new house if anybody could give me some advice as to whether they thought it would be adequate. We are a family of 6 with three more home on regular visits, We'll have a propane stove, dryer, and hot water heater as well as the usual household appliances and a well pump. Well, here goes

Magnum Inverter/Charger MS4024PAe

Magnum E panel with 15 amp ac breaker, BTS

60 amp DC breaker for solar


Remote LCD display

12 Surrette S-530 6 V Batteries

9 Kyocera 235 W solar panels

Xantrex Solar controller

that's basically it as well as various cables, and a propane generator.

I don't really understand solar even though I have been reading about it for years. I thought the battery bank seemed a bit small but the solar installer said something like that is what they recommend now. So maybe my information is a bit dated.

If anybody could take a look at this and tell me what you think, I would really appreciate it. I know we could increase the size later if it's not big enough but I would rather do it now.

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Looks like a 1.7kw system. It is probably too small unless your daily usage is ~ 3 to 4 kwh and you have alternative electrical generator(s) to cope with periods of low sunlight or excessive usage.

    Bookmark   June 30, 2011 at 11:26PM
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Really? The fridge and the freezer are each supposed to use about 1 kWh/day and the dw about .8 kWh, so I think it's safe to assume more than 3 - 4 kW/day. We will have a generator but I wouldn't want to have to use it everyday.
Where does the number 1.7 kw come from? If you multiply the number of solar panels by their watts, you end up with 2.1 and then I thought you multiplied that number by the hours of sunlight, which would vary of course. Is that different in real life? My information just comes from books - no real life experience. Are you off grid?
Btw, thanks for taking the time to look at this and help me out.

    Bookmark   July 1, 2011 at 12:21PM
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It should be 1.8kw. The figure you get by multiplying the number of panels by the output (optimal) gives the DC output. Typically, solar installers take into account the inverter losses and take a further discount off the output.

There will also be some losses in charging the battery bank (which is based off battery technology from the WW2 submarine service). The batteries should never be run down to 0 charge.

Each day, you will obtain ~ 3 hours of peak generation capacity.

PV output is also affected by the ambient temperature, so if you have an asphalt roof, the output will be impacted since the roof temperature could exceed 120 F on a late spring day.

No, I'm not off grid, though I'm a net producer of electricity from PV. What I've noticed is that the instantaneous power draw during the day could exceed the PV
generating capacity. The meter spins forwards for a short period of time before spinning backwards again.

For a grid attached PV system
Depending on the quality of the solar panels (and inverter), a 1.8 Kw system should be able to achieve ~ 16 Kwh (AC) under optimal conditions. On a heavily overcast day, the production could plummet to ~ 2 Kwh.

You need to account for air conditioning, lighting, entertainment systems, computers, food processors, microwaves, etc as part of the total electrical consumption.

To be off grid, I envisage the need to ensure that the existing electrical systems continue functioning without any brown outs while the batteries get charged during the day.

    Bookmark   July 1, 2011 at 2:28PM
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Thank you for clearing that up, now I understand.
Personally, do you think this is big enough. I promise I won't come back and blame you. Between 2 and 16kWh, it's hard to say if that would be adequate. Obviously, we would use more than 2 but a lot less than 16. We won't have an airconditioner or a microwave. I think we'll have a seperate system for the well. Except for lights, everything else would probably use less than 2, so maybe this would be fine.

    Bookmark   July 1, 2011 at 3:10PM
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I still think it is a little low for 6 people, especially during the winter. The max output is typically achieved during a cool sunny day. If the temp starts climbing > 29 Celsius, expect the output to drop. The extent of the drop depends on the solar panels as well as the increase in temp.

It would be good to figure out how many lights (and appliances) are used, time, length and the frequency of usage to get a rough feel for the daily usage first.

Since any PV system is a significant investment especially for people going off grid, it would be prudent to factor in additional capacity for possible needs in the near future (You may find that perhaps additional storage and/ or additional panels are required.

Typically the installer will size the inverter to closely match the output of the panels. Adding capacity later could well mean changing the inverter or the entire system (due to physical constraints on where the panels can be located, ...).

It is also likely that the installer will dictate the brand of panels used. For example - RecSolar and SunPower have installers who specialize in the respective brands.

Get a good installer - one who has done similar off grid systems, will walk you through the sizing and other technical details (such as - how would I expand the system and what would it cost).

In my case, I had to choose between an installer who just wanted to put in a > 2kW system and be done vs another who was willing to size out the system, walk through the details and tolerate a deluge of questions and what-if scenarios.

    Bookmark   July 1, 2011 at 5:09PM
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You've given me lots of good points to bring up with my installer. She has the exact same system as this but it is just her and her husband but she says she has more power than she can use. It shuts down or off or whatever at 10 in the morning. One book that I've poured over, The Renewable Energy Handbook, is written by somebody around here and it has several examples in it. This system is bigger than his but again it is just the two of them. But they do have everything - actually more electric and less propane than we would and he says his solar system supplies 80% of their electrical needs. He also has a wind generator which supplies about 15%.
I'll think about this and talk to her. I do like her, and it's a good thing because there's not too many (only her in this area) that do off grid systems. Everybody does the grid tied systems.
Thank you again for your help, I really appreciate it.

    Bookmark   July 2, 2011 at 9:22PM
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Have you consulted with fowler solar?

Here is a link that might be useful: fowler solar

    Bookmark   July 10, 2011 at 1:22AM
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Impressed by low power usage expectations! I have a 10Kwh on grid system and it does not meet household usage--but I have A/C, some electric appliances and household members who insist on leaving lights on... I would love to go off grid, but the size of the required battery packs is prohibitive. Without a massive wall of batteries, system would be running on backup generator too much. There is also a loss in storage and recovery of energy from battery packs. If utility service is available, better for both sides to hook up. They benefit from your surplus peak production in the middle of the day and you benefit from their filler power when their demand is down.

The further north you are, the greater the seasonality of power production. Seasonality is also effected by the slope of solar panels. In Pennsylvania, with a low slope roof, I have roughly triple the output in peak summer months as December. Being further north it is likely you will have strong seasonality. The inherent power in sunlight at noon in December is much lower than the output at noon in July. Length of day differences work in the same direction. Furthermore, unless roof is steep enough to shed snow, there will be periods with no power output due to snow cover. Batteries can not reasonably compensate for this. I would expect to run a propane generator a lot in your situation.

    Bookmark   July 24, 2011 at 12:55PM
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