Working out your power needs for an off-grid solar power system
Before doing anything else to design a solar power system you must first work out your power needs. Do the following steps.
Part A - Fill out a power needs table
Get out a sheet of paper, a pencil and a calculator, or open up a spreadsheet in a spreadsheet program like Excel and fill in a table like the one below. A spreadsheet with a blank table is availble for download below.
Each column in the table is described below.
| Appliances/Loads | (A) Power (watts) |
Power used at one time |
(B) Hours used in a day |
(C) Days per week |
Watt-hours per week (A)x(B)x(C) |
|---|---|---|---|---|---|
| Fridge | 58 | 131* | 24 | 7 | 9744.00 |
| Office light | 15 | 15 | 3.5 | 7 | 367.50 |
| Kitchen lights (2) | 26 | 26 | 2 | 7 | 364.00 |
| Living room light | 20 | 5 | 7 | 700.00 | |
| Bathroom light | 15 | 0.75 | 7 | 78.75 | |
| Bedroom lights (2) | 120 | 0 | 7 | 0.00 | |
| Hallway light | 60 | 0 | 7 | 0.00 | |
| Computer & monitor | 118 | 118 | 4 | 7 | 3304.00 |
| Modem | 10 | 10 | 3 | 7 | 210.00 |
| TV | 103 | 103 | 2 | 5 | 1030.00 |
| Radio | 1 | 1 | 1 | 5 | 5.00 |
| Kitchen fan | 16 | 16 | 5 | 7 | 560.00 |
| Bedroom fan | 0 | 11 | 3 | 0.00 | |
| Alarm clock | 0 | 0 | 24 | 7 | 0.00 |
| Answering machine | 0 | 0 | 24 | 7 | 0.00 |
| Highest power used at one time: | 420 | Total power per week: | 16363.25 | ||
* See the section about the Power used at one time column below.
Here's the downloadable spreadsheet with a blank table for you to fill
in:
power_needs_spreadsheet.xls - 11KB, Microsoft Excel 97/2000/XP format
Some important tips:
- Electric heaters - Do not use solar power to run electric heaters for long periods of time. Electric heaters use up a huge amount of power (1200 watts for example) and a solar system to power them would cost too much. Use propane, natural gas, wood, solar air heating or solar hot water instead. Things like toasters and coffer makers use electric heating elements but are okay since they aren't turned on for very long. An electric stove or oven is not okay since they are on for long. Consider gas, wood or solar cooking instead.
- Air conditioners - Like electric heaters, air conditioners use a large amount of power and are left on for long periods of time. You'll find these prohibitively expensive to power with solar too.
Part B - Add 25% for total system inefficiencies
Multiply the Total power per week which you got at the bottom right of the table above by 1.25.
Example: 16363.25 x 1.25 = 20454 Wh/w
Common industry practice is to simple add 25% to your needs right at the beginning and then not worry about calculating inefficiencies and losses for every single part.
Part C - Convert to watt hours needed per day
Divide the amount from step 2 by 7, since there are 7 days in a week.
Example: 20454 / 7 days per week = 2922 Wh/day
The end result for step 1
You now have what you need to move on to sizing the rest of your system, such as your batteries and your solar panels. Record the result from Part C above, i.e. the Wh/day (watt hours per day), and the Highest power used at one time which you got as a total at the bottom left of the above table, and move on to the next steps by clicking on the icons at the top or bottom of this page.
Column descriptions for the power needs table
Applicances/Loads - In this column put each thing that you want to power with your off-grid system.
(A) Power (watts) - Put here how much power each of the appliances you put in the first column uses while it's turned on. For light bulbs it's easy since the amount of power is written right on the light bulb (e.g. 13 watts or 13W).
Some items are listed in the table below. These may be wrong for your appliance as things differ a lot but they'll help if you're stuck.
For a fridge it's more difficult. The way I find out is to use a watt meter (or kilowatt meter or kill-a-watt meter). If you're not familiar with these then click on the link in the previous sentence for a page all about them. You plug your fridge, or whatever, into the watt meter and then plug the watt meter in the wall. The watt meter will tell you over time how much power was used in kWh (kilowatt hours.) Leave it like this for 48 hours (for example.) At the end of that time, divide the power used (e.g. 2.784 kWh) by 48 and then multiply by 1000. That will give you the average power used per hour which is what's needed for column (A). For example:
2.784 kWh / 48 hours = 0.058 kW
0.058 kW x 1000 = 58 watts
Some things demand a lot of power briefly when you first turn them on, known as a surge, and then use less power after that. For this table ignore the surge. We're concerned only with the normal power usage once it's up and running.
Power used at one time - One of the parts you'll need in your system is called an inverter. The inverter will be feeding power to everything that's turned on so it has to be able to handle your power needs during that time of day when you are using the most power. For me, that time is when I'm cooking supper. During that time I'll have the kitchen light on as well as the kitchen fan. But I might also be doing some work in my office room while things are cooking so I'll have the office light on as well as my computer, monitor, modem, TV and so on. Plus, some things I have no control over, like my fridge. So for each thing I might have on during that time I copy its value from column (A) to the Power used at one time column. When that column is totalled up I'll know what the biggest power requirement for my inverter will be and so I'll know which inverter to buy.
Note that in the Power used at one time column, the value for the fridge, 131 watts, is different than the value for the fridge in column (A), 58 watts. For the fridge column (A) contains the avergage powered used per hour whereas the Power used at one time column contains the actual power that the fridge uses when it's running. On a watt meter this value is given as watts.
(B) Hours used in a day - Put how many hours each appliance is used per day.
(C) Days per week - Put how many days per week the appliance is used. If you use it every day then put 7. If it's something like a vacuum cleaner that gets used maybe 1 day per week then put 1. If the location is a cottage and you'll be there only on weekends then put 2.
Watt-hours per week (A) x (B) x (C) - Multiply each value in columns (A), (B) and (C) together and put the result here.
Finally, total up two of the columns. The Power used at one time column adds up to give you the Highest power used at one time which is used to help select an inverter. The Total power per week is your power needs that you'll use to select pretty much everything else and is the first thing you'll put in the sizing worksheet.
Table of power usage for different appliances
The following is a table of power usage for different appliances for use in column (A) and the Power used at one time column above. The values are a last resort and it's preferable if you can find out the actual values for your specific appliance instead.
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