Evaluation - The most important phase of System Design

Although we don't mind it, we here the same question a dozen times a day "I have a 2000 square foot home, what size solar system do I need ?" or I have an average sized home with average appliances, what size system do I need ?" We've been designing solar systems for a long time and one thing that we've learned though the years is that the size of the home has absolutely no bearing on the size of the system.

We've seen customers with 1200 sq foot homes that consume 3000 kWh per month and then we've seen customers with 2800 sq ft home that consume 1000 kWh per month. The size of the system boils down to, how much electricity do you consume in kilowatt hours and much much of that consumption would you like to eliminate.

Most of the loads an "average" person wants to power are AC. The grid is AC, therefore common tools and appliances are AC. Many gadgets are actually running on DC, but come with a wall mounted transformer to allow use of household AC.

Solar electric panels produce DC. Batteries store only DC. Thus for an off-grid system we need to get from solar DC, to battery DC, to household AC. Also we may want to use generator AC or grid AC if we are connected to the grid. Sometimes we take this AC and store it in the batteries so this is another conversion, this time from AC to DC. Usually this is done with an inverter operating in reverse as an AC charger, or it can be done with a separate charger. This is why some inverters are actually correctly called "inverter/chargers".

If you are setting up a solar system please be aware that an electrician may be unfamiliar with some components and techniques necessary for high current DC systems. This is simply because these things are not a concern in normal residential or even commercial work. With the right components and attitude, this will not be a problem.

DC-rated disconnects, cable, circuit breakers, etc, are not always sitting on the shelves at the local supply house. In some cases standard items will work, and I will refer to those items when possible.

Note that a high-current DC system requires a greater attention to detail than a typical residential AC-only system. This is because:

1. DC electricity is harder on connections and has a greater tendency to arc than AC.
2. A series of DC connections must be perfect to avoid overall loss of power. There is no Megawatt grid pushing power through the system if there is a marginal connection.
3. DC circuits are typically operating at lower voltages than household AC, again putting a premium on correct connectors and adequate wire size.
4. DC circuits often operate at higher currents than are commonly found in residential wiring, putting a premium on safety
 

In doing a load analysis, we are primarily interested in POWER, and TIME. Which equals Watts X Hours. If we are dealing with more power and/or time, we may use KiloWatts X Hours. 3000 WattHours = 3 kWHours. 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. 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.

A quick and easy load analysis can be done by looking at an electric bill. You may find energy usage from 5 kWHour to 30kWHour per day, depending on the type of electric appliances used. If you are interested in lowering your electric bill, it will pay to look into more efficient lights and appliances. If you are looking to meet all or most of your needs with a solar electric system, you will almost certainly have to eliminate huge loads like electric water heaters and ovens. Tools and pumps are not a problem because they are not on for long periods of time.

An "average" solar-powered home with "all the modern conveniences" is designed for about 4kWHour / Day to keep system costs within budget. The average Grid-connected American home uses 20-30kWHour / Day. So it isn't generally practically to take an average home and convert it to off-grid. Electric heating devices, forced air blowers, inefficient refrigerators, and phantom loads just add up to too much daily energy loss. But, Grid-intertie solar electric systems on normal homes can still be very practical due to the backup function of the grid. With current government tax credits, any size of system makes sense - large or small.
You may require far less or far more power than "average". It is essential to do (or have done) your own detailed load analysis. It is possible to examine all your appliances and multiply the Amps on the nameplate times 120 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.

It's 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.

Contact us by Phone (907)561-7941, Fax (907)561-7946, Email or U.S. Postal Mail 145 West Dimond, Anchorage, AK, 99515

30% of your total system cost may be eligible for federal tax credit!