Electrical Load Calculation -How Much Power Does Your
Renewable System Need?
http://www.absak.com/design/load.html
Steve Spence, CEO, Green-Trust
Our family does a lot of camping. We pull a popup camper with a minivan. We find that many campsites that have great nature views, also have limited facilities. Since we are ecologically minded, we are attempting to find alternatives to power, water, sanitation, etc.
I will use this space to address the practical methods that we have employed, as well as discuss some of the theoretical methods we might be able to use.
Transportation
The Ford Windstar minivan we currently use as a tow vehicle is scheduled to be replaced with a F350 diesel quad cab pickup. This vehicle will be converted to run on waste vegetable oil. A Lance Truck Camper may be in our future.
The camper is a 10' Jayco pop-up (and out). King size bed on one side, full size on the other. Dinette drops down to be a bed as well. Kitchenette with sink, gas stove, and 3 way fridge. Looking to replace this unit with a larger hard shell pop-out.
Electricity
The flat roof of our camper is a good spot to mount solar panels. We currently have a single 75 watt Siemens panel, and will add 3 more. Although not ideally pointed at the sun, we do not have to consider which direction the camper is pointed. Very little damage to the panels is probable due to flat mounting them as they will not catch on trees or be buffeted by the wind.
The panels will charge a bank of four 100 amp hour 12v batteries mounted on the tongue of the camper. This keeps the fumes and gases out of the camper. The batteries can also be charged by the vehicle. A Morningstar PS-30M charge controller will protect the batteries from overcharge.
The batteries will feed a 350 watt modified square wave inverter from APC. We paid $80 for this unit, and obtained it from www.cdw.com, part number 387672. This unit powers my Dell Laptop, Nokia Cellphone, a 13" TV/VCR combo, a Microsoft XBOX/DVD, Compact Fluorescent lighting, and a rechargeable flashlight.
12volt lighting is also available from the battery pack, as is a car radio/cd, and a domestic water pump.
A 110vac battery charger is available if and when 110vac is available.
Determining load, capacity, and generation:
Determine how much electricity your equipment uses. Most equipment has a small plate or label that says it requirements, in volts and amps.
For example, a Dell Inspiron 8200 Laptop uses a maximum of 90 watts (1.3 amps @ 120VAC, or 7.5 amps @ 12vdc).Determine how often you use the equipment, and how long you need to run off the batteries. This determines how big the batteries will need to be.
For example, let's assume 2 days @ 8 hours per day averaging 150 watts. This is 2 x 8hrs x 150 watts which equals a 2400 watt-hour battery bank (or 200 amp-hours @ 12V). We don't recommend draining a lead acid battery beyond 50% so that would make it 4800 watt-hour battery bank (or 400 amp-hours @ 12V).
Once you know how big the batteries will be, then you need to know how much sunlight you are likely to get at your location at the time of year you want to use this equipment. Detailed solar insolation maps can be found at WindSun, but in most places in the USA you can figure an average of anywhere from 2 to 7 'sun-hours' depending on the time of year and your location. Then you need to know the energy efficiency of your batteries. Most batteries require anywhere approximately 125% of the energy going into them vs. what you will get out of them. Using the 'sun-hours', battery size and efficiency you can calculate how large your panel needs to be.
For example, if you get 5 'sun-hours' at your location, and you want to recharge an 50% flat battery in two sunny days and you need 125% going into the battery then it's 2400 watt-hours x 1.25 / 10 hours = 300 watts. This would equal four 75watt panels.
Water
A 20 gallon fresh water tank is installed and can capture water from the roof when it rains. Campground water hookup is also available. We don't drink from either without filtering.
Sanitation
We currently use a small porta-potty, but are planning on replacing it with a Biolet Standard Composting Toilet (250 watt surge, 1320 watt hours / day). Backwoods Home magazine has an article in the Jan/Feb 2003 issue that covers Solar-powered mobile toilets.
Heat
Heating is currently propane catalytic (Olympian Wave)
Cooling
No air conditioning is currently installed. Looking for a propane unit.
Cooking
Propane cooktop is currently used. It can be removed and used outside as well. A microwave is installed for occasions when 110 vac is available, but current inverter is too small to power.
Trash
Much care is taken when purchasing products for trip to ensure a minimal amount of packaging, especially non-biodegradeable, is taken along. Paper and cardboard become part of the night's campfire. Plastics, glass, and metal get packed out for eventual recycling.
Further Reading:
