Numbers might be a little off. I've ran 400 Solar panels through about a Dozen BT-boxes, and was able to power 10 Engines.
As I stated above, a single solar panel will power an blulectric engine just fine. The engine will simply convert whatever power it gets, no matter if that is incredibly little or a metric ton. The conversion rate is 1 kW = 1 MJ/t. And since a solar panel outputs 2 A at a maximum voltage of 100 V, then its maximum power output is 2 * 100 = 200 W, or about a fifth of a kW. And thus a fifth of 1 MJ/t. Real numbers will be less because voltage will always be below 100 V (if it was 100, then the panel would stop generating power).
The question you need to ask yourself is whether your power is actually arriving at the engine, because moving the power is the true challenge. If you had 400 solar panels and 10 engines, then each engine should be receiving 80 A. or at least, all 10 engiens together should be consuming a total of 800 A. If any less current than that is arriving, then you've made a grave mistake in your setup (because it means some panels aren't generating power at all).
Now, once you have confirmed that the amps are all arriving, check the voltage of the engines. You want that voltage to be as high as possible. If your engines are sitting at 70 V with their 80 A, you'd have a power throughput of 5.6 kW and thus each engine produces 5.6 MJ/t, for a total of 56 for your whole array of 400 solar panels. But, if the engines were at 90 V, then out of the same 80 A you would get 7.2 kW, meaning the same 400 solars are generating 16 MJ/t more.
How do you raise the voltage? By feeding the engines more amps. In other words, remove some engines, so that the power is being divided up between less of them. With only 5 engines, each will have to handle 160 A, and with 4 engines, A amps each. That's already approaching the maximum the engines can pull in practical application, and should result in a nice and high voltage. However, you then need to check again if all the amps are still arriving. Since you are raising the voltage in the engines, you're reducing the potential difference between the engines and the solar panels, meaning it becomes harder to move the current to the engine. At some point (where exactly, depends entirely on your setup), you will start seeing solar panels ceasing to produce power because they are hitting 100 V and have no means to transport their power anymore. When this happens, you either need to adjust your solar array, or place down another engine again.
As you can see, designing an efficient RP2 solar array is far more involved than just plopping down a field of blocks and hooking up a cable