When it comes to the question of how many solar panels you need the answer depends on what are your goals and also what is the net metering policy of your utility in your state. If you look at your utility bill you should be able to calculate the total number of kilowatt hours (kWh's) of power that you use in the period of the bill and if you get all of your bills for the year you should be able to calculate the total number of kWh's you use over a year.
If you are in a state where there is 1 to 1 retail net metering then this makes it fairly easy to calculate the number of solar panels you need to cover your utility bill. The science as to how many kilowatt hours of power will be produced annually for each kilowatt of solar panels installed on your house is relatively well understood. This figure ranges from about 2.8 kWh per day per kilowatt in the least sunny parts of the country to 4.8 kWh per day per kilowatt in the sunniest part of the country. This data has been produced by the National Renewable Energy Laboratories for all weather station locations across the country and is remarkably accurate.
If you enter your details into the solar panel calculator and enter your address, your utility and the average amount of your monthly power bill the calculator will tell you how many solar panels do I need and it will also tell you how many kWh's of power such a solar system would produce in your location. It will also give you information about solar panels costs for installed solar power systems in your location and each of the solar rebates and tax credits that apply.
The upside of using this solar panels calculator is that it gives you an amazing amount of quite accurate information but be warned it also provides your details to up to three local installers so that they can send you a quote for a system based on your needs. However, this can also been seen as an advantage given that time is running out to get your solar panels installed before the 30% federal solar tax credit expires.
If you just want data about how many solar panels you need to produce a particular amount of power in a particular location you can just visit the NREL website. It's a bit more digging around than simply using this estimator, and will not give you the financial information, but it will avoid you getting contacted by installers if you are not yet ready to get quotes for solar panels.
How many solar panels do I need for my RV or boat?
Whether it is an RV, a boat, camper trailer or any other mobile application solar panels can create silent clean power when paired with a battery, battery charger, and inverter.
However, the number of solar panels you need to power these machines depends on what electrical loads each machine has. the sizing of systems for these types of applications is much more complex than deciding how many solar panels you need for a grid-connected home installation. With a grid-connected home PV installation you have grid power there to back you up should at any instant your demand for power be greater than the supply of power from your solar panels. In an RV, camper trailer or boat you don't have this luxury and so you need to make sure the system is sized such that it can meet all of your peak loads.
You really need to talk to an expert when it comes to all off-grid solar applications because of these complications, particularly where you are intending to rely on this power source in situations where no alternatives may be available.