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Determining your Needs for Electrical Power

Sizing and Design for Solar, Battery Bank, Inverter, etc.

Read This Disclaimer First:   I’ve written what I personally did, and my opinions. Don’t assume what I did was safe, and don’t assume it will work for you. Do more research, and make your own choices. I’m not responsible for your outcomes! :-)

 

If you haven’t read the Intro to Dry Camping I recommend that you click that link and start there.  It will help all this make more sense.

How Much Electricity do you Need?

To find that answer, you need to know how much electricity you will use. The way to accomplish this is to do an energy audit. On the other hand, you could just learn by trial and error. If you go camping and run out of power add more batteries and/or solar panels ;-)

Electrical capacity is expressed in Amp Hours. If you use a 2 amp appliance for 5 hours that’s 10 Amp Hours (make sense?). Batteries have an Amp Hour Rating, and your battery bank is the heart of your electrical system. So doing this energy audit is a really important step.

I know it may look complicated, but it’s not as bad as you think.

Doing an Energy Audit

Use a spreadsheet, or a legal pad and draw 4 columns.

Column 1: List every item that uses battery power in your RV. This includes not only DC lights, radio, etc, but also AC items that run off the inverter, like your Microwave, the TV, Desktop computer, etc.

Column 2: List the DC AMPS that this appliance draws. There are two ways to do this. You can actually measure the current draw, or you can use the rated current on the appliance label.  I’ll give you more information about test equipment on the next page

Using the rated current of an appliance will give you a big cushion in your calculations. That’s because ratings are always greater than the appliance actually uses on average. (Those doing actual current measurements will need to add in a 10% cushion).

Remember, we’re powering those AC appliances from 12v DC batteries through the inverter, so you’ll need to do a little math to convert AC amps to DC amps. It’s easy!

For an AC Appliance, if you know the AMPS multiply by 10. For example:  6 Amps AC = 60 amps DC.

For DC or AC Appliance, either one, if you know the WATTS divide by 12 and you’ve got AMPS. This works because “watts is watts”. Volts and amps change but Watts remain the same! For example:

60 watt AC bulb is 60/12 = 5 amps *

10 watt DC bulb is 10/12 = 0.83 amps

* 5 amps @ 12VDC – powering your AC bulb through an inverter

Item

DC Amps

Front TV (AC) 6.0
Rear TV (DC) 4.0
Stereo 1.5
DirecTV Receiver 1.5
DVD / VCR 2.0
Laptop Computer 4.8
Desktop Computer 8.5
Microwave 85.0
Fluorescent  Light 1.3
Reading Light 2.0
Water Pump 5.0
Etc…

 

Now that you have all that, it’s time for the next step:

Column 3: List how many hours in a 24 hour day you’ll use that item. This is different for everyone. Say 1 hour for the big TV, 2 for the little one, 1.5 for the stereo, etc. If you have more than one of an item multiply times that number, for example, you have 4 fluorescent lights and you’ll use them about 2 hours each, that’s 8 hours. See the table below.

Column 4: Multiply column 2 (Amps) x column 3 (Hours). Now you have Amp Hours

Item

DC Amps

Hours

Amp Hours (AH)

Front TV (AC) 6.0

2

12

Rear TV (DC) 4.0

2

8

Stereo 1.5

4

6

DirecTV Receiver 1.5

4

6

DVD / VCR 2.0

2

4

Laptop Computer 4.8

6

28.8

Desktop Computer 8.5

3

25.5

Microwave (small) 85.0

5

42.5

Fluorescent  Lights 1.3

    8 *

10.4

Reading Light 2.0

4

8

Water Pump 5.0

2

1.0

Etc…
TOTALS 152.2

* 4 lights, 2 hours each

Total up column 4, now you know how many Amp Hours (AH) you’ll probably use on an average day. Let’s round it to 150 for discussion.

Conclusions from Energy Audit

Now that we’ve done our energy audit let’s see what it tells us:

  • Battery Bank Size If we want to minimize generator use, we need a battery bank capable of giving us 150 AH each day.
    • Batteries should never be drained lower than 50% of full charge because that greatly shortens their life, so the minimum size battery bank for us is 300 AH (2x our daily usage).
    • If we don’t mind running the generator more, we could get by with less battery capacity. However, in this example four golf-cart size 6v batteries would be more than enough (420 to 440 AH).
    • What type of battery? There are several choices and it’s a fairly complex discussion.  Two pages later we’ll talk about Battery Types.
  • Charging Systems  We need to be able to replace 150AH of battery power each day. We can do this with Solar, with a Generator, or with a combination of the two.
    • Solar panels only produce their maximum output at noon, with the panel pointed directly at the sun. They also produce less output in the winter, and farther North. For this reason, we have to consider the time of year and whether panels will be tilted to figure typical output.
      • A 120 W panel will produce between 40 and 55 Amp Hours of power each day. Take the value of 50, and at minimum we need to install three 120W panels if we want solar to do all our charging.
      • Note that charging batteries is less than 100% efficient, and not all days are sunny, so it wouldn’t hurt to add a fourth panel.
      • Even with 3 x 120W panels we’ll still need to run the generator on cloudy days.
    • For generator charging a 3 stage charger rated at 50 amps should do.
    • If you need to buy a small, efficient generator a 2000 Watt generator will power your 50 Amp charger along with a few other small items; however, you’ll need to turn off the battery charger to run the microwave, hair dryer, etc..
  • Inverter Selection If your RV doesn’t have an inverter you’ll probably want to add one. To determine how large the inverter needs to be refer to your energy audit.
    • Decide which AC powered items you want to run with an inverter. Then, add up column 2, DC Amps, for each item and multiply by 12. This tells you the power requirement in Watts. Round this up to give you a cushion, and that’s the size inverter you’ll need if you want to run all the AC items at the same time.
    • You can see that the small microwave at 85 Amps will require 1000 Watts itself. All the other appliances together require about 300 Watts. So the minimum size inverter for our example is about 1500 Watts.
    • If you plan to run the microwave and other high power items from the generator instead of the inverter you can use a smaller inverter.

As you continue through the upcoming sections there will be a more thorough discussion of all these topics.

Links for more research:

Here’s a good resource for more advanced discussion of RV Electrical topics. Especially if you own an HDT and 5th Wheel or other truck/trailer combination. Jack & Danielle Mayer’s blog.

Here’ a link concerning shore power (electric service) in RV’s. Both 30amp and 50amp circuits are covered: www.myrv.us/electric/index.htm

Continue on to Electrical Test Equipment — or choose another topic from the top line menu under Technical and Projects.