Hi Veggie,
I always enjoy your interesting and well done projects!
I do concur with CS6_owner.
Without more info on the system, it is hard to see the value in reducing the draw on the 24v battery to 800 vs 1000W via dropping resistor since roughly half the savings are wasted as heat. But do drop me a note if you'd like to discuss it in more detail.
If the specific model inverter is doing MPPT, adding a dropping resistor may do the job, but as CS6 points out, you then have added an expensive power eating and heat generating element so that battery draw will still be perhaps 900W while the inverter is stuffing 800W into the grid. You may also achieve the desired result by adding resistance in the AC output feed, again, depending on the inverter design; it will see line voltage getting too high and may back off- or for some designs may just shut down, thus thwarting your plan. The specifics of the inverter design matter and it can be hard to find such detailed design info.
Calculating resistance is as simple as ohms law and there are plenty of online calculators. The problem is that the specific characteristics of your inverter's mppt circuit or output voltage regulation scheme matter. You will be at something under 40 amps on the 24v side or under 4 amps if on the 230VAC side. If the MPPT circuitry would cooperate appropriately with a drop of 4V, the R=V/I or 4/40 or 0.1 ohm and the needed wattage is 4x40 or 160W minimum (better 250 to 300W).
Here's and adjustable one ($33 US):
https://www.digikey.com/en/products/detail/vishay-huntington-electric-inc/AVE030020ER100KE/269995Since there is no guarantee of how this may work out with your inverter I would consider testing with a coil or length of steel fence wire before popping for a power resistor. Here's a calculator (select iron):
https://www.omnicalculator.com/physics/wire-resistanceA battery jumper cable to make the steel fence wire connection will allow adjustment. Plated wire will change the value somewhat. A milliohm meter would help. I use both 3/16 mild steel rods and galvanized steel strip resistors as adjustable current limiter for my 36/24V battery powered DC stick and scratch TIG welder. I used the 3/16 rods for initial testing, good data is available online for mild steel wire resistance. SInce your 24VDC side is at only 40 amps, not the 160 amps of my stick welder, you should be fine with roughly 7 foot of 1/16th inch steel wire for brief testing. One useful trick is to measure the resistance of a longer length, minus the meter lead resistance, so you can get a reasonable estimate for your needed roughly 0.1 ohms. 1/10th of the length that measures 1 ohm, for example.
If I was trying the resistance on the 230V AC side (roughly 4 amps), and was shooting for say a maximum of 15V rise, that would put my target resistance at 15V/4A or 3.75 ohms at 60 W (better at least 100W). Adjustables are handy for this and not hideously expensive.
https://www.digikey.com/en/products/detail/ohmite/D100K4R0E/4298783As CS6_owner points out, choosing an inverter with a programmable set point may be the easier and cheaper solution unless you have the spare time to experiment. There are microinverters that are designed for 350-400W, 24V nominal solar panels. Perhaps a couple of those in parallel...
Best Wishes,
Bruce M
