Electrical Load Evaluation Calculator and System Design Information

When designing an off-grid (not connected to utility power) solar or wind power system, it's very important to have an accurate estimate for how much energy you need. Off the grid systems must utilize batteries to store DC power from the solar panels and convert it to AC power with an inverter. If you are not storing enough energy, you will decrease the life of your batteries, possibly ruin your batteries and most importantly, your power will eventually go out. An electrical system does you no good if it can't provide you with the necessary energy to properly run your home, RV or boat. This is why we do a load evaluation. The calculator (found below) helps us determine how much energy your home will need. When filling in the calculator values, it's crucial that you do not leave anything out. If a device is drawing energy, no matter how small, it will have an impact on your solar system. The more accurately you fill in the calculator, the better your system design will be. Lets examine some electrical loads as an example.

Most homes and RV's have an outdoor light but it may not get used very often. Some people will either forget to list that light or decide that it's not important to include it in the design process. Lets also say you have a ceiling fan that rarely gets turned on and a garbage disposal that rarely gets used because you compost most of your scrap food. Many people think it would be ok to leave these items off of the calculation because they are rarely consuming power. However, lets now say that you have some family or friends visiting your home. They may arrive late at night which means you had to use the outdoor light for 15 or 20 minutes when they arrived. You prepare a large meal and ended up using the garbage disposal and then everyone relaxes under the overhead fan because it's a hot Arizona night in the desert. Normally these loads by themselves would probably not adversely affect your system, but if they all get used in the same day, it could have a large impact upon your stored energy. In the morning, you may use the microwave oven before the sun rises and because the outdoor light, garbage disposal and fan were not included in the system design, the microwave has dropped the battery storage below 50% capacity. This is when batteries start becoming damaged and it's irreversible. Damaged batteries will significantly reduce the overall performance and storage capacity of your system which will only exacerbate the problem. Batteries are far too expensive to allow them to be needlessly damaged or ruined. This is why it's so important to include all of your electrical loads on the calculator. We also add in a buffer amount of capacity to the design to account for inefficiencies and small phantom loads like a digital clock on your microwave and the idle power draw of the inverter.

Technical Note: Many people have a difficult time understanding how watts are calculated and how that relates to energy. Basically, the amount of power a device consumes is rated in watts. This is most easily seen with the old-fashioned incandescent light bulbs. Those have the wattage rating listed on them: 25W, 50W, 100W, 150W etc. The new compact fluorescent bulbs do not draw nearly as much power as their incandescent equivalent but they often list the incandescent equal in light output. So if you have a compact fluorescent bulb that puts out the same level of light as a 50 watt incandescent, that does not mean that the compact fluorescent consumes 50 watts of power. It probably uses about 20 or 25 watts compared to the 50 watts. The packaging will normally tell you exactly how much wattage the compact fluorescent actually consumes or it may even be listed on the base of the bulb. If you need to know the rated wattage of other electrical devices, such as your T.V., you can often find that information listed somewhere on the device. If it does not list the wattage but tells you the amperage and voltage, you can calculate the total watts. This is done by using a formula derived from Ohms Law. If you can't find the watts, voltage or amps listed on the device, we have included a basic wattage chart with the calculator for most devices and appliances. Your device may consume a little more or less than what is listed on the chart, but it will give you a pretty good estimate. For those that want a very accurate energy or power measurement for a device, we sell the kill-a-watt meter. If you plug your device into the kill-a-watt meter, it will tell you exactly how much power it consumes.

Here is how you can calculate the watts if you know the voltage and amperage of a device.

Watts = Volts x Amps

If your T.V. plugs into a standard 120 volt receptacle and uses 4 amps, the total watts is calculated by multiplying 120 volts x 4 amps. That equals 480 watts.

120 Volts x 4 Amps = 480 Watts

Watts is an instantaneous value. This is why the load worksheet has a column for how many hours the device is used. Knowing the wattage of a device is necessary, but it does us no good unless we know the length of time that the device is turned on. When you multiply time by watts, that is called energy and it is measured in watt-hours. To be clear, 1 watt is NOT equal to 1 watt-hour or 1 watt per hour. If you have a device that consumes 1 watt and use that device for 1 hour, that is equal to 1 watt-hour. If you powered that same 1 watt device for 2 hours, that would equal 2 watt-hours. It's simply watts multiplied by hours which calculates the energy needed. Once we know how much energy you use, we can then design you a system that will meet all of your electrical needs. Please don't be intimidated by the load worksheet. If you have any questions at all or need help filling out your worksheet, please give us a call and one of our design specialists will be more than happy to assist you. We are here to help!

We also offer the load evaluation worksheet in Microsoft Excel format. The spreadsheet will automatically calculate your daily watt-hours instead of doing it by hand. We still recommend reading the pdf version too because it contains some valuable information about designing a solar system.


Load Evaluation Worksheet
Load Evaluation Spreadsheet
Confusion of watts, watt-hours, and watts per hour