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Originally Posted by CHEESE_WAGON
I can't seem to find the answer to the two main questions on my mind...... forgive me if this has been mentioned before, sometimes I'm lucky if I can remember what someone said five minutes ago.
Question 1)
If I were to have a 3,000 watt inverter with a 4,500 watt surge capacity..... at a given load of 70% capacity......
How do I calculate the current draw on the battery bank? Would it vary with the AC load on the inverter, or would the inverter have a steady load on the bank, no matter the load?
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The the amount the inverter will draw is equal to the output wattage plus any losses due to wire heating and converter efficiency, and varies with changes in the load. Figure using 11 amps at 12 volts DC for every one amp at 120 volts AC if the inverter is 90% efficient.
3kW x 70% = 2.1 kw load at 120 VAC. Divide watts by volts to get 17.5 amps AC. Multiply by 11 to figure roughly 192.5 amps at 12 volts DC. Since battery amp/hour capacities are usually rated at a 20 hour discharge rate, you should have a 3850 Ah 12-volt battery bank to run this much load in a steady state. In 10 hours you will reach 50% discharge. You can obviously run peak demand for short periods of time on a smaller bank if the battery wiring is big enough.
For this much power usage, it might be better to use a 24-volt battery bank and 24-volt inverter. The current would be down to 96.25 amps and the storage required would be 1925 Ah. Though the number of batteries in series-parallel would be the same, the lower current would mean less loss in heated wiring, and less fire danger from a corroding connection starting to get hot.
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Originally Posted by CHEESE_WAGON
Question 2)
If it does vary with load, with a 70% load, how do I calculate how many solar panels I would need to generate enough power to overcome that drain and provide a small charge?
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Calculate solar availability in watt-hours by multiplying array capacity times the standard hours of insolation for the season and the area. Compare this with your estimate of watt-hours to be drawn. Standard insolation hours figure in the fact that early morning and late afternoon are less productive than right around noon. Standard hours vary from about 4 in the north in winter, to I guess 6-10 in the south in summer.
Array wattage is specified at peak output, which is generally a higher voltage and lower current than you get connecting direct to the batteries. For 12-volt panels, the peak is usually around 17 volts. If you wire the array through an MPPT charge controller, which lets the panels run at their best output, multiply the specified wattage times the insolation. If you wire the array directly across the battery bank, and use a controller that disconnects the array when the batteries are full, then use the battery voltage times the short-circuit current (Isc) for the wattage and multiply this times the insolation. This is usually about 30% less.
I don't have a figure for how much extra capacity to add for losses in battery charging. You will not have 100% of the electricity you put into a battery available for withdrawal, so be sure to add more than the minimum.
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Originally Posted by CHEESE_WAGON
One other thing I thought of was to put certain portions of the load on its own bank and inverter (several small banks and inverters), I would think this would help in both troubleshooting and helping to keep one bad battery from taking out ten or twelve.
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There are two schools of thought on this - one big expensive inverter, or several small ones. If inverters are left in idle waiting for a load, instead of removing power from them, there are small amounts of idle current drawn. When you have multiple inverters, this begins to add up. Also, if you had one big load to power for a while, no one of the small ones may be capable of handling it.
On the other hand, with one big inverter, if it goes out you are in the dark. With multiple small ones, you could transfer a critical load to an inverter in place of a non-critical load, and keep on keepin' on. (Just like "Genghis Khan and his brother Don.") Multiple medium-sized battery wires would spread out the heat compared to one set of extra-large ones. With this option you would pretty much use 12-volts, as small 24-volt inverters are rarer.
A compromise is usually one big inverter, plus one or more smaller ones for critical loads. You might have a refrigerator or some medical equipment on its own inverter, so you could put the big one to sleep when you aren't up and active and still not lose the important item. You wouldn't lose the critical load if you tripped off the main inverter trying to run a circular saw or something, either. Or, you could install a small pure sine wave unit for delicate electronics, and make the big one a cheaper modified sine wave unit.
It sounds like you were thinking of multiple battery banks for multiple inverters. This could get overly complicated. With good battery maintenance, you should not need to do this. Just don't mix old batteries with new, or mix different sizes together. If one string has a battery that is weak and runs out first, the good batteries will run down trying to recharge their wounded comrade.
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