Montana Homestead: Solar Power Part One

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I live miles away from the nearest electrical grid and generate all my own power, either via solar or with a gasoline generator. I’m transitioning from my small, temporary solar power system to a newer, more powerful system and I thought I’d post the details about both systems.

I’ll edit this thread as I go along to correct typos, fix mistakes, and to re-phrase things for clarity. When I’m done I’ll put it all together in a downloadable PDF file.

I should note that I am not a “Solar PV Expert”. I’m also not an electrician, although I was an electrician in the military once upon a time. I usually try to adhere to the National Electrical Code, but there are tons of new codes to go along with the PV industry and I honestly don’t know all the codes, and the copy of the NEC Handbook that I own is old enough that I don’t think it even mentions PV systems.

Comments, questions and criticisms are welcome!

PART I

The goal is to size a solar power PV system that we can use to provide almost all our electrical power and minimize generator usage. The steps I use are:

1. Determine Electrical Requirements
2. Size the Inverter & Determine the System Voltage
3. Size the Battery Bank
4. Size the PV Panel Array

The wrong way to build a PV system is to buy a bunch of solar panels and then figure out what to do with them. The batteries are the heart of the system. So first I’ll figure out how much energy I need to store in and pull from the batteries, then size the battery bank, then calculate how many PV panels I need to charge the batteries.

Our current system is relatively small:

1. 300 watts in solar panels
2. 30 amp PWM charge controller
3. (2) 6 volt 380 amp-hour L16 batteries wired in parallel to make a 12 volt battery bank
4. 600/1000 watt Chinese pure sine wave inverter.

The panels:

Charge controller:

Batteries:


http://usbattery.com/products/6-volt…es/us-l16-xc2/

Inverter:

First I should say that I’ve been running the original system 24/7 continuously for 2 years and all the equipment worked as it was designed and I would recommend any of the components for use as a small system. I feel like I got my money’s worth from it.

But there are a couple problems with the 300 watt system: it is too small for our needs, and 300 watts in panels is not enough to properly charge the batteries.

The 600 watt inverter is enough to handle some of our needs and some of the kitchen appliances. It will not handle a microwave oven or a toaster, but it will handle the new refrigerator we got. I’m not sure if it will handle the fridge and freezer at the same time, but even if it can, I’m sure that if there are any additional loads the inverter will be pushed past capacity.

During the day when the sun shines, the system handles the fridge without any problems. But with the other loads on the battery bank, by the time I go to bed the batteries don’t have enough charge to last through the night while powering the fridge, so I can’t leave the fridge on all night.

A 380 Amp-Hour battery bank needs to be charged with about 38 Amps. The panels on my solar array are capable of providing 5 Amps each. So at most, I have 15 Amps with my 300 Watts in solar panels which would be enough to charge a 150 Amp-Hour battery bank. The Chinese PWM charge controller does not make the most efficient use of the power I do have. I should have at least 600 watts in solar panels which would provide 30 Amps. Still not enough, but it would be a lot better. The PWM charge controller is rated at 30 Amps so I couldn’t use more than 600 Watts in any case.

As a result, we run the generator a lot. Every day. I want to get away from spending a lot of money on gasoline, but more importantly, I want to be independent of the generator and of the gas.

I will keep the old system and use it to run the inside water pump and some indoor 12VDC lighting. I may also put critical items on it, like the fridge and freezer, but that will be determined later.

I’ll outline how I calculated my electrical requirements, how I used that information to determine what I need for a new system, and the details of the new system.

The first thing to do is find our electrical requirements for a new solar power system, then choose a battery bank, inverter, and charge controller that can handle my needs.

1. DETERMINE ELECTRICAL REQUIREMENTS
Our electrical requirements are puny compared to people who have a grid connection, but some people might see us as truly wasteful. At this time there are three people living at the cabin. While some of the appliances we use (and the frequency with which we use them) may seem extravagant, I did have to convince my wife that we could have a near-normal lifestyle living off-grid, as motivation for the idea. So we use a lot of electricity.

A few months ago we bought a small freezer and we recently saved enough money to buy a new fridge, dishwasher and propane range, and. We bought everything at Lowes because we like the prices and the Lowes Protection Plan. I left the buying decisions entirely up to my wife. Sometimes it’s just easier that way… She was careful to buy appliances with the lowest Energy Guide ratings she could find, while also looking for features she wanted.

We had a 500 gallon propane tank installed and I installed black iron pipe under the house for an on-demand water heater and for the cook range. The water heater uses zero electricity, so it won’t be included in this section.

I haven’t put the Kill-A-Watt meter on the new items (except for the freezer and the propane range) so I’ll be using the Energy Guide ratings (the big yellow tag that comes with most new appliances) to determine the energy usage. The Energy Guide ratings are based on average use and are specified in KW-Hours used per year. One can simply take the anual KWH number and divide it by 365 to get the KWH used per day.

I put all Watt-Hour numbers into a spreadsheet, and at the end of this thread I’ll post a link to the spreadsheet.

Refrigerator: The fridge is a Whirlpool 17.7 cubic foot freezer-less model. The Energy Guide says it uses 326 KWH per year, so that is 893 Watt-Hours per day.

http://www.lowes.com/ProductDisplay?…reId=10151&N=0

Freezer: The freezer is an Idylis 5-cu ft Chest Freezer which uses 592 Watt-Hours per day. We keep it outside on the north porch, so in the winter it will not need power at all.

http://www.lowes.com/ProductDisplay?…reId=10151&N=0

Dishwasher: Maybe it doesn’t make sense to have a dishwasher while living off-grid, but on the other hand, we don’t have to use it if we don’t want to or if we don’t have the power, or we can use it during peak sunlight activity or when the generator is running. The dishwasher is a Bosch Ascenta and according to the Energy Star rating uses 737 Watt-Hours per day.

http://www.lowes.com/ProductDisplay?…reId=10151&N=0

Range: The range is a Samsung 5-Burner Freestanding 5.8-cu ft Convection Gas Range. I did put the Kill-A-Watt meter on the range. It uses no measurable electricity while in standby mode, and I haven’t checked to see if it uses electricity while a burner is going (but I think it won’t be much, if any). But it does use 400 Watts while the oven is operating due to the presence of a heating element in place as a safety measure to make sure if the flame goes out it will be relit. Or something like that. I figure if we use the oven for an hour per day, that will be 400 Watt-Hours.

http://www.lowes.com/ProductDisplay?…reId=10151&N=0

Washing Machine: The washing machine is at least ten years old, and bought before we ever thought we would live off grid. The model is a Whirlpool Duet, and I’ve used the Kill-A-Watt meter to determine it uses 300 Watt-Hours per load. (It also uses about 10 gallons of water per load – it is a front loader). At one load per day, that is 300 Watt-Hours per day.

Television: We have satellite TV and two LCD TVs (the satellite box uses 15 watts and each TV uses 100-200 watts). While I type this I have Fox News on the main TV and my wife is in bed watching the other TV. Sometimes at least one TV may be on for most of a day. This uses a lot of power.

Lighting: All lighting is comprised of LED bulbs, and about half the lighting in the house is 12 Volt DC LED bulbs, with the remaining lighting consisting of 120 Volt LED bulbs. As a result, Watt-Hours used per day for lighting is so small that it will be negligible in terms of the overall system. But I did add some numbers to the spreadsheet.

Water: The well pump is used every few days to fill the 275 gallon storage tank in the attic, and a 12 Volt DC indoor pump is used to provide water to the fixtures. The well pump is 240 Volt AC and draws 10 Amps according to the documentation that came with the pump. I don’t run the well pump every day, and based on my usage I’ve estimated I will use 377 Watt-Hours per day, on average. I can adjust my filling times depending on how much energy is available.

Miscellaneous: We also have cordless drill battery chargers, laptop computers, cell phone chargers and dozens of other typical devices. We often run heavy duty tools (planer, table saw, etc) but will probably run the generator for these occasions.

Based on our usage, I estimate we will need 6,663 Watt-Hours per day (that’s both AC Watt-Hours and DC Watt-Hours used to run the inverter).

So I’ve figured out what we need for the new system and shopped around for parts. I’ve already ordered and received the new charge controller and inverter, and I have a good idea of where I’m going to get the solar panels and batteries. I will outline the details of those in the following posts.

2 thoughts on “Montana Homestead: Solar Power Part One

  1. Ok, I am only pointing this out because you asked for input and I can be a bit of a grammar nazi. The correct term is kilowatt. There I said it, I got it off my chest. I really enjoy your articles and am seriously impressed with your accomplishments. Keep up the good work!

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