Please Don’t Make This Rookie Mistake

If there is one mistake that is made over and over and over it is sizing your solar electric system too small. We all dream of living in the peace and quiet, off the grid, cooking over a rocket stove and washing our laundry on a wash board.

We have images of brewing our coffee in a glass coffee percolator on our propane range. Surely we won’t need that much electricity?

washboardWrong? Most of us have lived with the luxuries of modern living and adjusting to not having those luxuries is NOT easy.

It might sound romantic but when you have three or four children dirtying laundry faster than you can scrub it clean and hang it on the clothesline the romance is gone.

Be realistic. There are certain appliances you will probably not want to live without.

An electric coffee maker for example may sound like a huge energy pig but when you consider it is only operated for a few minutes at a time, it does not usually use that much power.

Ice boxes were a necessity a few generations ago but now that we have efficient refrigerators, it does not make sense to cut and store ice all winter for your ice box in the summer.

If you have a special someone in your life and/or children sit down and think about what you can live without and what you can’t live without. My wife requires a hair dryer while others may not. We use air conditioning in the summer as we have plenty of extra power in the summer but many live without A/C and don’t seem to miss it.

Make a list of every appliance you will want to have in your home or cottage

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. For example:

kill a watt meter

The Kill-a-Watt meter can measure an appliance’s electricity consumption instantaneously or cumulatively.

Hair Dryer
Coffee Maker
Laptop Computers
DVD Player
Alarm System
WiFi Internet Router
Clothes Washer etc.
Air Conditioning
Water Pump
Circulating Pumps
Heating Controls

The next step to figure out what each item consumes instantaneously, per hour, per day, per year, or per load (washing machine or dish washer) and then figure out how often you will use the appliance.

You can get this info from the manufacturer, the government or by using a meter such as a Kill-a-Watt meter.

energy ratings

Example #1: You purchase a refrigerator that consumes 300 kWh per year. Divide this by 365 days in a year and your fridge will use .82 kWh (820 watt hours) per day.

Example #2: You purchase a hair dryer that consumes 1200 watts (or 1500 watt hours per hour). You are going to operate the hair dryer for 15 minutes per day or 1/4 of an hour (15/60 mins). Multiply 1200 watt hours by 1/4 and your hair dryer will consume 300 watt hours per day.

Example #3: Your front loading washing machine consumes 140 watt hours (.14 kWh) per load and you will use your washer 9 times per week. First multiply 140 watt hours by 9 to get 1260 watt hours (1.26 kWh) per week. Divide this by 7 days and your washer will need 180 watt hours (.18 kWh) per day.

Example #4: Your WiFi router uses 23 watts and will be on 24 hours per day. Multiply 23 watts by 24 hours and your new router will take 552 watt hours (.552 kWh) per day.

digital readout AC kWH meterAdd all of your electrical needs to get a daily requirement and then double it!

You read that correctly double it!

Now use this figure when you design your battery bank and solar array system. Solar arrays do not make anywhere near their maximum rated power output and batteries will last far longer when not cycled deeply.

Almost everyone else calculates their power needs and they design the smallest solar array and battery bank that will operate it.

This never works! Don’t be like them!

small pvThink about it. You design your solar array to make just enough power to run your daily loads. Three days go by without sun. Now your battery bank is deeply discharged. The next day is a beautiful sunny day but your solar array just makes enough to operate your regular loads. There is nothing left to bring the batteries back up to a full state of charge. Now your battery bank is in a perpetual state of deep discharge, the power will go out regularly and you will be another person who tells folks to stay away from solar power as it does not work.

off grid home

Please be one of the successful off gridders and don’t believe what you are told unless you are getting info from someone who has lived off grid for many years.

Solar companies that do not use their own products are constantly making promises they can’t keep. It is not their fault sometimes. Sometimes they just don’t know any better.

Take Care,

{ 6 comments… read them below or add one }

Frank August 19, 2015 at 6:42 pm

Hi, Jody
Your blogs on batteries for newbies to solar power have been most helpful. My wife’s family has a small (20′ x 30′) off-grid cabin in Ontario in which I will be installing a 12V LED lighting system. There will be a total of 30 watts of lights, but doubtful all six 5 watt LED bulbs would be in use at the same time. But let’s say they are, from 7 pm to midnight, 5 hours, or 12.5 a/h max. I was planning to use two Trojan t-105’s and a 100 watt solar panel for recharging. Using 8 gauge zip wire in 4 circuits from the marine 12v fuse box in the cabin. Battery box to fuse box is 15′ of 4 gauge battery wire. What is your opinion of my plan? Too small? Frank


Jody Graham August 19, 2015 at 8:59 pm

Hi Frank,

Thanks for the kind words. Your wire sizing is plenty big. 8 gauge wire can handle up to 40 amps (480 watts roughly depending on your 12 volt battery bank’s voltage). The 15 feet of 4 gauge is also plenty big as it is good for up to 100 amps. When they used to install 100 amp entrances in homes they used 4 gauge wire from the pole to the circuit breaker panel. You are on the right track. Too big is good, too small is bad.

The T105s in series are perfect for what you want to do and the 100 watt module will be plenty to operate your lights and keep the batteries full. What charge controller are you using? You will need one to keep the T105s from over charging.

My only concern about your system would be the marine 12 volt fuse box. Although it may have a CSA/UL rating, it is for mobile use (which is a much lower standard), not residential use. Will you have any trouble? No. Unless you have an electrician or inspector snooping around you will be fine. Just wanted to give you heads up about the fuse box. You could get away with 14 gauge wiring for your lighting circuits but my concern would be that someone who is unfamiliar with DC might put 100 watt DC bulbs in your fixtures and overload the wire’s capability at some point in the future. You are doing great. Keep up the great work…Jody


Frank August 19, 2015 at 9:23 pm


What are your thoughts on the two T-105’s for the 12.5 maximum nightly ampere/hours and the 100w panel to recharge? My reasoning was to use half the available battery A/H for the calculation. Figured I can add to the battery bank and panels each summer as I get my wife’s family accustomed to the luxury of off-grid lights. Prior to this they had used propane. I have a $1,200 budget for the first year.



Jody Graham August 19, 2015 at 11:01 pm

Hi Frank,

You are completely on the right track. The T105s will have an amp hour rating of 110 amps hours at 12 volts if you use 50% of their capacity. If you leave all of your lights on for five hours you will pull 12.5 amp hours out each evening. The 100 watt module will likely produce about 6 amps in full sun if you are using a PWM charge controller and closer to 8 amps if you are using an MPPT charge controller. That means the solar module will need about 2 sun hours per day (2 hours X 6 amps = 12 amp hours) to meet your maximum load of 12.5 amp hours. You can always add on as you can afford it. I am sure as your wife’s family gets used to having some electricity they will want more. Everyone does! Keep in mind you will NEED switches that are rated for DC to operate your lights. Even at 12 volts DC, a standard 120 volt AC switch will be destroyed in a matter of days by arcing and could catch fire. Read Even though it is about fuses the same rules apply.

I just want to say you are doing a really good job of designing this system. I don’t often speak to folks with such a good undertanding in solar system design.
Keep the comments/questions coming…Jody


Frank August 20, 2015 at 1:14 pm

Hi, Jody

OK, based on your suggestions I’m rethinking parts of my design. I will have 3 circuits, a total of six LED bulbs (each 5 watts). I want to use wall switches for the lights. One circuit for the two bedrooms (one switched light in each), one circuit for the kitchen and dining area (two switched lights); one circuit for the great room (two wall lights on one switch). Total amperage is only 2.5 amps for the entire system. I was going to use the marine fuse panel to fuse each of the 3 circuits. Questions:
1. Since I have a max of 2.5 amps if every light is lit, can I use just one 5 amp fuse for all three circuits? Thinking one 5 amp fuse between the positive load lead of the controller box and the line into the cabin. Line into the cabin will go to a terminal block from which the 3 circuits will begin. Thinking 5 amp fuse because it is double the max 2.5 amp load. Or should I go 10 amps?
2. I have not found a source for off-grid 12v switch boxes like a house AC electrical system’s one and two gang electrical boxes. I want boxes to attach to the cabin wall 2×4’s to contain the 12 V switches, similar to a normal house switch. Do such boxes exist for off grid 12v switches? Or is there a 12V switch that will attach to the common 120v boxes? Or do I need to MacGyver a plate to hold the 12v switches for the AC boxes?
Jody, I really appreciate your help! (BTW, thanks for the kudos on the design. Being a ham radio operator does give me some working knowledge of electrical theory.)



Jody Graham August 20, 2015 at 9:13 pm

Hi Frank,

To answer the fuse question, one 5 amp will be perfect and then you can use a busbar or terminal block to spit off into three circuits. I remember years ago being able to buy “normal” light switches that were ok for up to 48 volts. Let me think on that and get back to to you as soon as I can. If you want a 5 amp dc breaker they are about $12. Google MNPV5. I have lots of them here. They fit in boxes made by Midnite Solar called Baby Boxes or Big Baby Boxes. That is if you want your breaker/fuse to be UL. You could also use a USM1 fuse holder with a 5 amp fuse. They are rated up to 600 volts DC and UL. I keep lots of that stuff here too.

Your other non UL option for the switches is to buy a cover (single or double gang) with no holes and drill a hole for an automotive type switch. But let me think about the DC switches and I’ll get back to you asap. Before inverters became efficient/affordable and made a clean sine wave we did dc lighting all the time. Talk to you soon…Jody

Found a UL switch for dc that fits in a regular box. Not cheap though…

Try this by clicking on the photo…


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