Q:

How large a system do I need?

A:

There are two answers to this question.

If you are not connected to the grid and are expecting to supply all of your power from the solar PV system then the following applies. - Sizing a PV system is a very specific task. Rules of thumb are not used. Each electricity consuming device is accounted for - the number of lights, TV's, radios, phones, toasters(!), refrigerators(!) micowave ovens, pumps, etc., etc... along with their wattages and allowed hours of operation are all calculated to determine exactly how much power is going to be required, and what the peak consumption is going to be. Why such a detailed list and determination of use? Because solar electric systems are expensive. Each installed peak watt will cost a minimum of $4 to install (not including batteries!). Put another way, your average toaster will require $6,000 worth of solar PV equipment to operate!

PV systems are not built to provide power to heating appliances (see the toaster above). They are best used for electronics, lights and pumps/motors.

If you are attached to the grid, then it's not "what you need", it's "what can you afford".

Solar PV systems, without batteries, for residential applications (under 10 Kilowatts), cost about $5,000 for the first Kilowatt of installed capacity. Additional Kilowatts will cost less. A ground mounted rack will add to the cost. The power that is generated is all delivered to the grid - possibly none of it will be used by your own home, so the PV system is really an income generating investment. In theory, if you consider the return on the investment to be a good one, you would put in the largest system you could afford. In the MircoFit program you are limited to a maximum of 10 Kilowatts - and you may only own one system.

With net-metered PV systems, where you push PV power back into the grid and run your electricity meter backwards, it does not make sense to have a PV system capable of generating more power than you use on an annual basis. The reason is that you do not get credit for power delivered in excess of you annual power consumption.