OK, it looks like you don't really understand volts, amps, and power - it's quite common.
Lets start with volts, possibly the easiest bit. Volts is like "electrical pressure".
Imagine you have a bicycle pump, you stick your finger over the hole in the end and push on the plunger - the harder you push, the higher the pressure. No slightly release your finger so some air leaks out - you keep pushing on the plunger and until you run out of travel on the plunger then the pressure is still determined by how hard you push.
Now, while maintaining this constant pressure, you can vary how much air you let out. Within limits, it doesn't matter how fast you let the air out, the pressure is still determined by how hard you are pushing on the plunger - but it does determine how fast the plunger runs out of travel. This air flow is analogous to amps - the "electrical flow".
But you can also see that if you leave your finger off the hole, then you won't be able to create much pressure in the pump as the air escapes too fast. In effect, you are drawing more air than the pump can supply at a given pressure.
Now, a couple of things to note :
As long as you stay within the flow capacity of the pump, then the pressure is determined solely by how hard you push on the plunger. If you have a tiny orifice (finger fairly tight on the hole) then the flow is low; if you have a larger orifice (finger not so well over the hole) then you have a higher flow rate - but the pressure stays the same.
For a given pressure (voltage), the flow rate is determined by the load - not by the supply.
Now back to your bits of kit. You have a 2.8kW immersion heater - except that you don't really ! What you have is a resistor (around 20 Ω (ohms) -
lookup ohms law) that when connected across a 240V mains supply will draw around 11.7A (lets call it 12A to make life simple). 240V x 12A = 2880W (not far off your stated 2.8kW). If you connected that same immersion heater across a 120V supply, the "pressure" (volts) is half, the "flow" (amps) is half, and so the power is 1/4 (1/2 * 1/2 = 1/4) or around 700W. However, we normally talk about loads like this in terms of the power taken when connected to a "standard" supply voltage - hence saying it's a "2.8kW immersion heater". Note that many loads (computer power supplies for example) behave differently but we'll ignore those for now.
A typical house supply will be easily capable of supplying 20, 30, even 60 or 80A if there are loads to draw that - but all the supply does is provide a constant(-ish) 240V of "pressure" (volts). So if you connected 2 of your 2.8kW heaters, you'd pull a total of 24A from the mains and so on.
Now, lets introduce your inverter. It doesn't create any energy, only converts it. So say we were going to connect the 2.8kW heater to it, it will need to pull in 2.8kW to convert for the output load plus a load extra that it converts into heat (it's not 100% efficient). Lets say the losses are another 200W, making 3kw total. With an input voltage of 12V, that means we need to draw 250A (12 x 250 = 3000).
Your 100W panel is capable of producing around 8.3A at 12V (8.3 x 12 = 100). Yet we're going to ask it to supply 250A.
Now lets return to the bicycle pump. Remember what happens if you let too much air out (ie don't cover the hole at all) ? Well the plunger just goes straight in with little resistance and we cannot create much pressure at all. We've overloaded it and it just doesn't make any pressure. The same thing will happen with your solar panel - the output voltage will just collapse and you'll get very little out of it. Due to the nature of the panel, I don't think it will break it, but in general if you overload an electrical supply like that then something will "give". Rather than cause things to collapse and break (melt, "blow up", whatever), we fuse electrical supplies so that if you tried to overload something that badly you would simply blow the fuse (or trip the breaker).
Now, if you were to buy another 29 panels (30 in total) then you'd just have enough power, on a sunny day, to run your load. In practice, when the sun wasn't quite sunny enough, the output voltage would drop with the (now too big) load, and the inverter would trip off. If it automatically restarts, it would start, overload the panels, trip, rinse and repeat. Because there is a problem balancing load and supply, anyone using solar panels "off grid" connects batteries to them.
When there is bright sunlight and little load, the panels charge the batteries; when there isn't enough sunlight to supply the loads, the batteries make up the difference. Note that the batteries don't create energy, they just store it - losing some during the charge/discharge cycle. All you then require is that the average output from the panels is reasonably above the average load, and that you have enough battery capacity to cover the gaps.
So say you wanted to do this to run your heater from one 100W panel. And lets say that you need the heater to be on for 2 hours. So we'll draw 250A for 2 hours from the batteries, and then the panels will need to recharge the batteries by supplying 8.3A for over 60 hours ! At 12 hours/day, that's 5 days (actually more due to the losses during discharge and recharge) of bright sunshine for one tank of hot water - plus a lot of batteries.
You could in fact just connect the panels directly to the immersion heater - supplying it with DC from the panels.
This would work better, as the load would vary with voltage, and the voltage would vary with the amount of sunlight. In effect, how hard you press on the pump plunger is analogous to how much sunlight there is, and how much current flows through the heater is analogous to how much air flows through a fixed size orifice on the end of the pump - more sunlight = higher pressure (volts) = more flow (current) through the fixed load.
But those 30 panels will cost you a lot of money. You'd be better converting the incident solar energy to heat - more efficient and much cheaper. You may still want a panel to charge some small batteries for things like lights (pick LEDs for minimum load) - and ideally use equipment that runs direct from 12V (or 24V) so as to avoid the losses from running an inverter.
