Considerations for Solar Electricity

Solar Electricity at your home requires some decisions

The experiments I did with the small solar electric system have yielded a wealth of knowledge about how to design and build a system the right way. I have also done a great deal of research into Solar Electric systems. Backwoods Solar Electric Systems has some good write ups in their "Find an Article" button on their home page. Home Power Magazine is a good resource for information. I'm going to take this knowledge to the next step and power my entire home on sunshine alone. It will require more experimenting to get there, and that's the fun part for me.

Decision # 1: on-grid or off-grid?

For me that's easy: OFF-Grid.

• I want independence from the power company.
• I want control of my electricity source.
• When the Grid power fails and goes OFF, I want mine to be up and running normally.

There are many good reasons for choosing either type of solar power system for your home. Either type will serve you well, and the planet Earth, for many years.

Being an experimenter at heart, the independent system will allow me to try many different things that I would not be allowed to do with a Grid-tied system. To do anything with your Grid-tied system you need to have permits, hire a licensed electrician, hire a licensed installer, hire a local fire and electrical inspector if all you wanted to do is move one solar panel or the wiring just a little bit.

That's not for me. I want the freedom to change things and experiment. For example, I want to know why, everywhere I read about off-Grid systems they ALWAYS include a propane generator. Does it have to be? My plans do NOT include one of those for my off-Grid system. I want to see if I can do it without one.

How big is it going to be? But wait, there are RULES! What?

First we need to figure out what the system is going to run.

There are some general rules for designing an "off grid" battery bank solar electric system. Rules? Well, yes, and they come from a long history of solar electric systems built and operated successfully.

1. You cannot use an electric Range or Oven.
2. You cannot use an electric Clothes Dryer.
3. You cannot use electric Resistance Heating.
4. You cannot use an electric Water Heater.

Two factors dictate why these things are a no-no:

• Economics

It would cost a fortune to build a system large enough to handle any ONE of these things. If you want the system to run TWO of them, well that's TWO fortunes. I think you get the point.

• Efficiency

Converting electricity into heat with "resistance elements" is not efficient at all. See why here. The takeaway item in that article is this: Converting electricity into heat is NOT efficient.

Before we continue, a few words about Grid-tied solar...

With this type of solar power your home is still connected to the Local Power Company, the main power Grid. Solar panels in your back yard or on the roof are sending power to the Local Power Company, not to any batteries. Your Local Power Company essentially becomes your battery. Your solar panels "charge up" your Local Power Company. They in turn will return "money" to you on your monthly bill statement. If the weather isn't so nice, not much sunshine say, then your Local Power Company isn't "fully charged up" for the month. When that happens the Local Power Company collects "money" from you on your monthly bill statement.

The bottom line with a Grid-tied solar electric system is that you CAN USE all of the items in the list above. You can use them all at the same time if you like! The Grid is a very powerful source of electricity.

Now let's continue...

Let's compare Natural Gas to Electricity for heating water

How much of a load is it going to be on the system?
I use a natural gas water heater.

• I consume about 10 therms per month during the winter months.
• According to the article 10 therms = One Million BTU in One Month.

If I had an electric water heater.

• It would consume 293 kilowatt hours per month = One Million BTU.

Let's go through the numbers for a 12 volt battery bank solar system.

• A normal electric water heater has TWO 5000 watt heating elements which operate at 220 VAC.
• Two times 5000 watts is 10,000 watts when they are both operating, that's 10 kilowatts.

I tried, but could NOT find a 12 volt inverter on the market with capabilities needed for this system. We'll abandon the thought of building a 12 volt system to operate the electric hot water heater.


Let's go onward with a 48 volt battery bank solar system.

The Inverter

The Load requires 10 kilowatts at 220 VAC. I found one that will work in our system.

• Magnum Energy 4448, 4400 watt sine wave inverter, $2159.20 each.

This inverter has the capability of operating four of them in parallel. We need enough Watts from the inverter to operate our load, 10 kilowatts.

• 10 kilowatts divided by 4400 watts = 2.27 inverters, round up to 3.
• Total bill for inverters, $6,477.60.

The Batteries

Batteries have to be sized to handle a 10 kilowatt load.

• Current draw would be Watts divided by Volts. 10,000 / 48 = 209 amps.

The general rule for a home power solar battery bank is this:
DO NOT draw more than 1/10th marked battery capacity.

• So, 209 amps times 10 = 2,090 amp hours to just MEET that general rule.

NOTE: If you actually designed a system this close to the maximum load, your batteries may last 2 or 3 years before needing complete replacement. This would be a very BAD design.

• L16 style 6 volt batteries are marked 464 amp hours.

I chose these because they're big but manageable for a homeowner and readily available.

• Eight times 6 volts = 48 volts, so we need EIGHT just to get to 48 volts at 464 amp hours.
• 2090 amp hours divided by 464 amp hours = about 5 times EIGHT L16 batteries, 40 of them.
• 40 times $825.65 each. Total bill for batteries would be $33,026.00.

The Solar Panels

• 293 kilowatt hours / month divided by 30 days is 9.77 kilowatt hours per day.
• Let's use 4 hours Prime Solar window for our calculations here, your location may be different.
• 9.77 kilowatt hours divided by 4 hours = 2.45 kilowatts of panels.

Choose panels in the higher wattage range so we can use fewer of them. This way we won't take up the whole back yard with solar panels, and maybe part of the neighbors back yard too.

• Alt200-24p 200 watt 24 volt solar panel, $279.00 each.
• We need to charge 48 volt batteries, so that's 2 panels, but we add the power here = 400 watts.
• So, 2.45 kilowatts needed divided by 0.4 kilowatts per panel pair = 6.125 pairs, round up to 7 pairs.
• We need 7 pairs times $279.00. Total bill for solar panels would be $3,906.00.

Solar Charge Controller

We need to choose one that can handle at least 2800 watts of solar panels and charge a 48 volt battery.

• Morningstar TSMPPT-60 will do 3200 watts at 48 volts, price $519.00.

Let's sum up what we've got so far

Yes, so far. We haven't covered the balance of system, commonly referred to as BOS in the industry, which includes the following: (just a few of the bigger items)

1. Solar panel mounting hardware.
2. Solar panel wiring.
3. Solar panel combiner box.
4. Solar panel disconnect safety switch.
5. Vented battery box.
6. Battery wiring.
7. Battery safety fuses.
8. System wiring box with circuit breakers.

Cost, so far:
Total bill for inverters, $6,477.60.
Total bill for batteries would be $33,026.00.
Total bill for solar panels would be $3,906.00.
Total bill for Charge Controller $519.00.

Total 4 items -----  $43,928.60

That's a lot of money just to marginally operate a normal 50 gallon electric hot water heater and NOTHING ELSE! No TV, no lights, no refrigerator, no clothes washer, just the hot water heater. We have a marginal Battery Bank in this design with a 2 year life span. At $33,000.00 per life cycle, divided by 2 years and we get...

That's $15,000.00 per year for hot water. Yikes!

If the cost of batteries every two years doesn't look appealing then just double the size of the battery bank and get about 5 years of service out of them. But look at what that's done to the cost of your whole system:

Double Battery Bank, 5 year life:

• Double size battery bank = $66,000.00 for batteries.
• Solar panels to support the batteries = $7,812.00.
• Charge Controller to support solar panels = 1,038.00.
• Total system upgrade = $78,050.00.

That's $15,610.00 per year for hot water.

Triple the size and you may get 8 or 10 years of service out of them.


Triple Battery Bank, 10 year life:

• Triple size battery bank = $99,000.00 for batteries.
• Solar panels to support the batteries = $11,718.00.
• Charge Controller to support solar panels = $1,557.00.
• Total system upgrade = $112,275.00.

That's $11,227.50 per year for hot water.

Remember, this is just to operate a 50 gallon hot water heater using off-Grid solar electricity. The general rule appears to be true. The numbers look pretty convincing to me.

My natural gas bill is about $20.00 per month for 10 therms, One Million BTU. As we've seen with this exercise, the economics don't work for using an off-Grid solar electric system to heat water. However a solar thermal system for heating water works very well, and is truly a success story.

In summary, off-Grid solar electric power works, but you have to consider the best ways of using it. We're so used to being connected to the Grid, a seemingly endless source of electric power, that we've become unaware of the true value of electric power. Until you start making electricity in your own back yard, you just can't imagine how precious every single kilowatt hour is.


Gregg Scholfield    6-16-2013