Welcome to Electricity 101!
In doing a load analysis, we are primarily interested in POWER and TIME. This means:
Watts x Hours
If we are dealing with more power and/or time, we may use:
KiloWatts x Hours
3000 WattHours = 3 kWHours (3kWh)
Note that this is exactly the unit your electric bill is in (if you get one.) Also, it can be helpful to distinguish between POWER and ENERGY. This means:
Power x Time = Energy
For example, an engine can be very powerful but doesn't use much energy if it only runs for a minute. We capitalize the W for Watts because it is an abbreviation for James Watt, who developed the concept of horsepower. One horsepower equals approximately 746 Watts, by the way.
A quick and easy load analysis for a home on the grid can be done by looking at an electric bill. Usually you are billed for kWh per month. We suggest dividing that number by 30 and working with kWh per day. You may find energy usage from 5 kWh to 60 kWh per day, depending on the type of electrical appliances used. If you are interested in lowering your electric bill, it will pay to look into more efficient lights, insulation and appliances. After you have updated your home, you can size a solar system to offset any or all of your electrical consumption.
If you are looking to meet all of your needs with a solar electric system for an *off-grid* application, you will almost certainly have to eliminate huge loads like electric water heaters, ovens, air conditioners, electric clothes dryer, and space heaters. Tools and pumps are not generally a problem because they are not on for long periods of time. An average off-grid solar-powered home with "all the modern conveniences" is designed for about 4-10 kWh/ Day to keep system costs within budget.
Getting back to specific loads, it is possible to examine all your appliances and multiply the Amps on the nameplate times 120 (or 240 if a large appliance) Volts to get the Watts for each appliance. A saw rated at 12 Amps, for example, will draw 1440 Watts. Some items are rated in Watts already, like light bulbs, hair dryers or microwave ovens.
Volts x Amps = Watts
It is not necessary to tear apart the house to do this, but doing a few calculations like this will give you a better feel for what's going on as you use your system later. You can also buy meters, like the Kill-A-Watt meter, to plug loads into, which measure Watts and Watt-hours very accurately. Or, a good analysis can be done using values in the tables below.
|Appliance||Watts (V x A)||Appliance||Watts (V x A)|
|Central Air Conditioner||1000-1500 (running)
|CB Radio||10||Laptop Computer||60-120|
|CD Player*||35||LCD TV*||213|
|Ceiling Fan||100||LCD Monitor*||80-150|
|Clothes Washer||1,450||Plasma TV*||350|
|Coffee Machine||1,500||Portable Fan||100|
|Curling Iron||90||Popcorn Popper||250|
|Desktop Computer (Standard)||80-120||Sewing Machine||100|
|Desktop Computer (Gaming)||400-1,500||Sink Disposal||450|
|Dryer (gas)||300-400||Slow Cooker||200|
|DVD Player*||150||Standard TV*||190|
|Electric Clothes Dryer||3,400||Stereo*||10-30|
|Electric Blanket||200||Stock Tank Heater||100|
|Electric Clock*||4||Table Fan||10-25|
|Engine Block Heater||150-1,000||Toaster Oven||1,500|
|Espresso Machine*||360||Trash Compactor||1,500|
|Exhaust Fans (3)||144||Vacuum Cleaner||500|
|Fish Tank||5||Video Game Player*||195|
|Food Processor||400||Waterbed Heater||400|
|Furnace Blower||300-1,000||Water Heater||479|
|Garage Door Opener||350||Waterpik||100|
|Heater (portable)||1,500||Waffle Iron||1,200|
|Hot Plate||1,200||Well Pump (1/3-1 HP)||480-1,200|
|Tools||Watts (V x A)|
|Band Saw (14”)||1,100|
|Chain Saw (12”)||1,100|
|Circular Saw (7 ¼”)||900|
|Disc Sander (9”)||900|
|Lighting||Watts (V x A)|
|Compact Fluorescent (60W equivalent)||16|
|Compact Fluorescent (40W equivalent)||16|
|25 W. Compact Fluorescent Bulb||22|
|Refrigeration/ Freezing||Watt-Hours/ Day|
|20 cu. Ft. (AC)||1,411|
|16 cu. Ft. (AC)||1,200|
|15 cu. Ft Freezer||1,240|
*Phantom Loads: Many electrical devices draw power even when “off”. These are most often loads that constantly draw power. Some examples include: Television sets, appliances with digital clocks - microwave ovens, DVD players, or any item with a remote control. These loads may seem trivial though they are drawing power 24 hours a day, 7 days a week. That can add up to quite a lot! A simple method to avoid this power drain is to unplug these devices whenever possible or connect them to a power strip that can easily be switched on and off.
Incidentally the volts x amps = Watts works exactly the same for DC and AC. So we can use the same math for looking at loads, solar arrays, generators, or batteries. Although it’s possible to use DC loads for more efficiency, normally we use inverters to convert from DC solar panels and DC batteries to AC power, to run AC loads.
For off-grid, in addition to the kWh per day, we also need to consider the peak load, so the inverter will be big enough. To do that, add up the total loads (in Watts) that might be on at one time. Also, it is imperative to factor in the additional surge required to start large motors. The starting surge of a motor is usually about 4 times as much power as the running load. So for large motors, multiply the running Watts times 4. For Grid-Tie systems, the peak load is not usually a concern because the inverter is only producing energy (kWh) that is synchronized with the utility, and used or sold, but not powering the loads directly.
Next page: Solar Equipment Ratings