When designing an electrical system for your home or business, you need to do a budget. That is, how much energy you will need and when you need it. Don't get scared off—this doesn't need to be any more complicated that doing a grocery budget.

While the focus of this article is for off-grid systems, you really need this even if you are connected to the grid. This is normally taken care of by the builder or, in places such as the US, government regulations. In both cases, the system is usually over-designed for typical usage patterns and changes in use patterns of individual houses will not have a significant impact on the electric grid.

When you are building an independent system, unless you are willing to way over-design the system, you are forced to do your homework. While over-design is the alternative, it is likely to be very expensive. In fact, doing an off-grid design where your primary energy source is renewable and meets 100% of your demand all the time is not likely to be cost-effective as well.

When determining demand, there are two major considerations:

1. Total daily usage which we will measure in watt hours or kilowatt hours.
2. Peak demand which we will measure in watts.

If you are going to have more than one voltage available (for example, 12 volts DC and 120 volts AC, you need to keep track of peak demand at each voltage. The best way to do this is to build a table with the information. Here is an example for a system that includes 12VDC and 120VAC.

Item	When	Hours	W@12VDC	W@120VAC	WH
Coffee Maker	7AM	0.2		1500	300
Lights	6-11PM	3	20		60
Computer	7-9PM	2	30		60
Refrigerator	24/7	6	75		450
Well pump	unimportant	0.5		300	150

While your house may have lots more items, this should be enough to explain the technique. First, let's look at the columns.

Item

The name of what you are going to run.

WhenWhen is the device operated. This is not to imply that it is operated continuously during this time.

Hours

The number of hours during the day that the device is in operation

W@12VDC

Power consumption in watts for a unit running on 12 volts DC.

W@120VAC

Power consumption in watts for a unit running on 120 volts AC.

WH

Total energy consumption per day in watt hours. (You just multiply the hours column times the sum of the two watts columns.)

In the Lights line, while there is a five hour range given, the hours column had a three in it. That's because not all the lights will be in use all the time.

In the refrigerator line, you see that while it is in connected all the time, (24 hours a day) I said it uses power only six. Realistic as the compressor on a refrigerator is not running all the time.

Finally, for the well pump, in the When column, I put unimportant. That is because while the power will need to be consumed some time during the day, we can adjust the time should there be a peak power issue. For example, when you are running the coffee maker, you most likely would not want to also be running the well pump (or, probably more important, a microwave oven if you had one.)

Clearly, the right way to do this is using a spreadsheet which is how I did it. But, for the example, this will do. Let's see what the table tells us.

If we add up the WH column we come up with 1020 watt hours. That figure is your total energy consumption per day. Just a bit over a kilowatt hour.

The peak power consumption at 12 volts DC is 125 watts and the peak power consumption at 120 volts AC is 1800 watts but as the well pump can be run any time, we actually only need to play for 1500 watts.

If your battery system is 12 volts than handling the 12 volt consumption should require nothing special. If, however, your batteries are at some different voltage (mine are 24 volts) then this will help you determine the capacity needed for a DC-DC converter to change your battery voltage into what is needed.

DC-DC converters are rated based on their current capacity rather than power. Thus, you will need to convert the peak watts number to amperes. You do this by dividing the watts by the voltage. In this example, you get 10.4 amperes. (Note that the nominal output voltage of most DC-DC converters that produce what is called 12 volts is actually 13.8 volts. Assuming that the devices connected will not consume more power at a higher voltage (a reasonable assumption for some such as a laptop computer but possibly not for others) then the actual current will be somewhat less than 10 amperes.

On the AC side, you will need an inverter to convert DC from your batteries to 120 volts AC. An inverter that can deliver 1500 watts should be sufficient. (Note that if a significant part of your load requires starting a motor, you will need to consider the peak demand for starting the motor when sizing the Inverter.)

That's enough to determine your energy budget. The next step is to decide how you will produce this energy which is subject of a separate article.