To approximately measure the efficiency and solar power output of your grid-tied installation you can use one of the available online solar calculators. These are good for having a rough estimate of your possibilites.
1. PVWatts is the standard and most accepted one inside the solar community. A tool developed and hosted by the National Renewable Energy Laboratory.
You can use the following link: PVWatts map viewer
There you can either pick your location by using the map or directly entering your ZIP code.
When found, you should click the "Send to PVWatts" button.
Then you will be sent to the PVWatts Calculator page where you will be asked to fill in some details corresponding to your hypothetical PV array setup. The application is well documented and provides help for every field.
2. Solar Estimator is a very good application from solar-estimate.org that has a wider variety of options such as solar hot water systems and solar spa/pool heating. You can test it by clicking this link. The approach is different from PVWatts since it returns the necessary setup to provide a determined percentage of your electrical consumption. The information output is complete, showing charts and lots of useful data.
3. Find Solar Calculator is an application similar to the Solar Estimator with much less output information but with interesting data like carbon emission savings. You can test it by clicking this link.
The rest. Most contractors website have their own solar calculators. However we strongly recommend using one of the listed above as contractors calculators may present information in a tendentious manner for you to decide it is profitable to go Solar.