I currently have four 100W 12 volt Siemens solar panels. They each put out about 17VDC.
That voltage will vary according to the amount of solar energy hitting the panels (called solar irradiance) and the load on the panel. An MPPT charge controller finds the optimum load for the solar array in order to transfer the maximum energy to the battery/load.
Running a fridge for a while during the day would be an extreme high-end load for this or maybe the microwave in the camper.
You will require a battery to store enough energy to run a small fridge or a microwave oven. Always look at the inverter specifications to see what inductive load it can handle during a motor starting scenario to make sure it won't burn out a motor or trip the inverter. Then remember that a fridge will take energy all day long so you need to look at your total energy requirements and make certain you can convert and store sufficient solar energy to supply to the inverter when needed.
Is it even possible to run this 400W solar array directly to an inverter and skip batteries altogether, understanding it will only work in full sun
Yes but then the total load will need to be far less than 400 watts as this is what it will likely put out on most sunny of days near noon time. Then you have losses in the charger controller/regulator. You will also quickly become aware of cloud cover and other weather issues as your solar system will drop out due to more energy being used than it can supply.
If I have to use batteries, can I use a couple of plain old SLA 12v batteries, maybe 100 Ah each, just because they're cheap(er)?
Just about any lead acid 12 volt battery will work on these systems. It is only a question of cost, lifetime and their ability to store and deliver the desired energy. The charge controller you reference appears to specify SLA batteries so do your research.
How many batteries would make the most use of the 400 watts available or is that a function of the inverter output?
It is time to start thinking in terms of energy (typically kw-hr in the US) instead of power. You need to consider how much energy your panel will generate on the average day. It will be less on rainy or partly cloudy days and it could be more on a sunny day. If you panel has 3 perfectly sunny hours in a particular day, you may generate 1.2 kw-hr (400 watts * 3 hours) but you will store less due to the inefficiency of the charge controller and the battery.
To calculate the average annual energy generation, you
start with a map to show you the average solar irradiance for your area. You can then use this to calculate your overall infrastructure requirements. Monthly versions should then be checked to make sure it will meet your needs in winter when there is less solar irradiance:
Sengupta, M., Y. Xie, A. Lopez, A. Habte, G. Maclaurin, and J. Shelby. 2018. "The National Solar Radiation Data Base (NSRDB)."
Renewable and Sustainable Energy Reviews 89 (June): 51-60. Is 2000 watts just a ridiculous expectation for my 400 watts of solar? Or does that just mean I need more batteries to make the most of the 2000W? i.e., fewer batteries would discharge quicker with that kind of load?
That is the general idea but again start thinking in terms of energy both from a charging and discharging perspective. Your energy into the batteries must exceed the energy drawn from the batteries in order to overcome inefficiencies in the battery charging and the converting of battery power to AC.
The battery also needs to be able to deliver the peak power needed for the inverter and the load. A 2000 watt load translates to 95 to 100 amps at 24 volts out of the battery. Just like starting a car, that type of load will quickly drain a battery. You may only get 166 watt-hr of AC energy if the battery can only do this for 5 minutes.
If I wire the panels as a 24V array, 2 in series and 2 in series and then the two pairs in parallel, do I use that current rating as the input for the charge controller? Wiring as 24V means I can get by with smaller wire, between and from the panels to the charge controller? 10ga? 4?[
It generally works out best to put the panels in series up to the allowable voltage of the charge controller. The controller you reference appears to support 100 volt strings so you could put all four of your panels in series. Then check the amperage. If your entire string is in series, it will source less than 10 amps. Unless you have an unusually long run from the panels to the charge controller, 14 gauge wire would be more than sufficient. Use 12 gauge if your budget can support it. Check that the terminals on the charge controller can take the gauge your choose. Respect the potential voltage levels and wire it as if it were normal house wiring.
Don't forget to do your research on the RFI generated by the charge controller and the inverter if you have any hopes of using radios in the vicinity of the installation.
- Glenn W9IQ