Follow Our Progress

Electrical

My friend Alan has helped me to demystify the electrical requirements for our bus while we are disconnected from the grid.  He has given me several really good books on the subject of DC electrical systems and I wouldn't have known where to start without his help.  In the spirit of giving I thought that it might be helpful to other's following along to take a look at my notes as I read and figure out how our DC electrical system will keep us powered up and off the grid as long as possible.

Determining Electrical Needs

The first logical step to determining how much electricity we need is to determine how much electricity we use on any given day.  That sounds simple enough, but turned out to be a bit more complicated once I set out to do it.  There is not a nice little sticker on every electrical appliance that states the number of DC Amps that it uses.  Sometimes you only have the number of Watts or the Amps based on 120V AC.

Here are two basic formulas that I used to convert the information on the appliance stickers to watts or Amps (whichever was easier).

• Amps = watts / volts

•  watts = Amps * volts

I started out by collecting whatever information I could find from each electrical appliance we intend to use.  Once I had that information I went to work converting everything to something that can be easily added up.  The goal of this step was to have all of our electrical needs put into DC Amp Hours so I can move on to the determining what size battery and charging systems we will need. 

I broke the list of appliances into 2 groups.  The DC appliances that can pull power directly from the batteries and AC appliances that will need to have AC power from an inverter.  I didn't want to wire the bus with 2 sets of plugs, so almost all of our appliances are designed to be plugged into AC outlets. The only appliance I could think of that made direct use of DC was the water pump. A short list...

I found it a lot easier to calculate the wattage of AC Appliances and then convert the total to Amps after I had the total watt-hours.  Many of the appliances (like the laptop, printers and various rechargeable items) are actually DC devices, but the voltage ranges from 19.5 to 2.5, so I will have to do some work to build or acquire chargers that I can plug in to the few DC outlets that I have installed in the bus.

If my understanding of inverters is correct, most inverters are around 90% efficient.  Since all of the AC appliances will be drawing power from the batteries through the inverter, I need to count both the total of all of the appliances and add 10% to that total to account for power consumed by the inverter.

Now that I have the total amp-hours for the DC and AC appliances I can add them together and have a somewhat accurate estimate of our total daily power needs.

So on average, we will consume 257 Amp-Hours of power on a daily basis if our usage estimates are accurate.  We can reduce this total by using more efficient DC appliances as time goes on.

Just for fun I compared the number of Watt-Hours (2,097 or 2.097 Kwh) to our current electric bill at our house...  We consumed over 25 Kwh on average per day this last month!  Why?  Lots of appliances in the house are not going to be used in the bus.  Here's a list of the major power hogs:

• Large side/side refrigerator (816 watts per hour)
• Electric clothes drier (4,386 watts per hour)
• Dish washer (1080 watts per hour)
• Garage door opener (660 watts per hour)
• Large (900 watt output) microwave oven (1548 watts per hour including fan, light and turntable)
• 65 gallon freshwater aquarium (275 watts per hour - average for heater, lighting & pumps)

In addition to those easy to find ones, there are many other reasons for the huge difference...  Every light switch in the house turns on no less than three 40 watt bulbs.  The lighting above the mirror in the master bathroom has twelve 40 watt bulbs (480 watts) and is used as the main light for lighting the bedroom at our house.  Even though we have natural gas heater, stove, fireplace insert, water heater and BBQ, the fireplace and heater for the house still take quite a bit of electricity for the blower motors.  It's no big surprise that our "energy efficient" home uses about 9 times what we calculate the bus will use.

Sizing Inverter

Now that I know what our total power needs are, I also need to know what our peak power need might be so that I can get an inverter that can handle the maximum wattage that we might demand from it at any one time.

By looking at the larger watt appliances I can see that there is some potential for quite a big draw on the system.  Especially in the evening with most of the lights on.  Appliances like the microwave, vacuum cleaner, forced air heater and computer have to be taken into account.  While it is not likely that we would use them all at once (or even on a daily basis) it's important for me to figure out what is likely...  The total for all of our appliances is right around 1750 watts.  A draw of 1750 watts would destroy a battery bank in very short order.  Batteries last longer and return electrical power more efficiently when you draw from them slowly.  Most of the power hungry appliances like the microwave and vacuum cleaner will only be used for a few minutes at a time.

For now I am going to pick 2000 watts as our peak usage.  It looks to me like our general power load will be around 200 watts at any given time.  Looking at any item that is used for 4 or more hours a day as "always on" and a base line should give me a fairly accurate picture of our base electrical load.  I figure most of us will be home at the same time and during the evening and that is when we use the most power from all of the 4+ hour appliances.

Now that I have a base power of 200 watts, I need to look at the high demand appliances (ignoring the ones included in the base power calculation of course) to see what our peak power needs might be.

It looks to me like somewhere around 1000 watts would account for simultaneous running the microwave, vacuum cleaner, laptop and heater...   I doubt that I could work on the laptop while someone was vacuuming, but it doesn't hurt to make sure that if it did happen we wouldn't overload the inverter.

Now I'll add the base 200 watts to the worst case 1000 watts and I come up with 1200 watts as my guess for peak power consumption for short periods of time.  That's over 100 Amps being pulled from the batteries at one time <gulp>.

Determining Battery Needs

Now that we know what our average and peak power needs will be I can start making some decisions on the type and size of the batteries we will need.

It looks like we will be consuming around 257 Amp-hours of electricity on any given day.  According to the books I am reading on the subject of batteries, there are many different types of batteries, but almost all of them deteriorate rather quickly if they are regularly depleted much more than about 50%.  Apparently some of the very large deep cycle batteries can be drained up to 70%, but not frequently.

The sweet spot (in terms of power and charging rate) for a battery is between 50%-80% of the battery's capacity.  In other words, you get good power draw and don't damage the battery by only pulling the battery down to 50% of it's capacity and you can fairly rapidly return the charge to around 80% before you have to reduce the charge rate which increases the charging time.  That means that we can effectively use around 30% of our total battery bank for daily power without hurting the batteries or wasting a lot of time charging them all the way up to 100% daily...

I am going to assume that we will need around 800 Amp-hour total capacity and a battery bank that can deliver 100 Amps of power for short periods of time when necessary.  Keeping the batteries at or above 50% of their capacity will make it so the batteries can handle the high amperage draw from time to time.

I'm reading up on all of the different types of batteries to determine the best choice so far the very expensive gel (sealed) deep cycle batteries seem to have the right mix of storage and delivery.  The wet style deep cycle 6V batteries look like a good 2nd choice because of their capacity, but they are not designed to deliver the high number of Amps that the inverter will need...  I'm still reading... 

The batteries that seem to be the best compromise of durability, capability and price are the wet cell deep cycle batteries.  I can pick up a set of four 200 AH (Amp-Hour) 6V batteries that will make a 400 AH 12V battery bank for around $250 dollars.  Two 400 AH banks give us the 800 AH capacity we need and by making two banks that can be separated, I can charge them efficiently with a smaller charger or even the limited output of a solar array.

Determining Charging Needs

How do you care for 800 Amp-hours worth of batteries?

We have a few options for charging the batteries:

• The bus's alternator
• Battery charger connected to the grid
• A gas or diesel generator
• Solar panels
• Wind generator

The bus's generator produces a lot of power for the batteries while it is running, but it does not give all of the power to the batteries that they could normally handle.  On long trips charging the batteries with the bus's generator is the obvious best choice since we will be disconnected from grid power and the length of time the bus will run will do the job of recharging the whole bank.

A battery charger connected to the grid will charge the batteries efficiently, but we want to avoid being forced to rely on grid power for our daily needs.

Solar panels and wind generators are my personal favorite, but the weather up here in Oregon makes solar panels pretty inefficient for most of the year.  Wind generators have become a real alternative, but require some maintenance and are not a reliable (constant or predictable) source for all our power needs.  

I would love to be able to power the entire bus without the need to ever connect to shore power, so I will most likely find some combination of alternatives to meet our needs.

I don't want a gas or diesel generator.  Engines need attention, make noise, smell and require fuel that needs to be re-filled.  I also don't have the luxury of adequate storage space for a generator or it's fuel, so it's not a very attractive option to me in the long run.

Solar power is clean, silent and somewhat reliable when the sky is clear.  I would like to make solar the main source for our power needs.  We plan to travel quite a bit in areas where the sun shines more often than not.  I have lots of surface area on the top of the bus that is not otherwise being used, so having even a large number of solar panels up there is not a problem.

It is a pretty large investment to get enough solar panels to replenish the batteries that we will be using.  My friend Alan has been making use of a solar power system for the past few years with quite a bit of success (even here in Eugene Oregon).  That has added to the attractiveness of solar power as our main source of electrical power.  I did some calculating and with twelve 80W solar panels, we could easily meet our calculated daily power needs.  On those days where we can't generate enough power to recharge the batteries, we can use the bus's alternator to get us by.  We will also install a large battery charger so that we can make use of grid power for charging the batteries when it is available.

Here's how I figured out the number of panels we will need:

Each panel generates 80W in perfect circumstances.  We will not likely ever have "Perfect" circumstances where it is sunny and clear, the air is cold and we're close to the equator at noon...  From what I have read, we will only get about 50W of power from an 80W panel, and only for about 5 hours or so of the day.  That adds up to 250W (or about 20 AH) of power reliably from each panel on a good day.  We calculated that we need 257 AH of power a day for all of our electrical needs, so 257 divided by 20 AH comes out to almost 13.  Since there is some current generated outside of the 5 hour reliable range, the additional amp hours needed to make up the 15 or so AH will not be all that much of a problem I think.  We always have the bus's alternator and grid power to give us a boost if necessary.  Of course we can always take a day off from our power hungry life style and "rough it" without the laptop, TV or microwave too <grin>.