EDIT: on hold while I check conditions
@Harper pointed to the fact that my battery is rated to draw 165A continuously for an hour, which should cover the needs of my inverter when my vacuum starts up. I do sometime have to switch over to a backup battery, and I need to verify this hasn't somehow interfered with the measurements. In the worst case, it might mean that the measurements were made using a 136Ah AGM battery from Sunwind with a rating of 67Ah with C=1 – about half that of the Concorde.
Still, the points on wire thickness might be relevant … I usually employ really thick (35mm2 – 70mm2) jumper cable wires and the length between the inverter and battery is less than a metre, but in this case I have just relied on the cables supplied with the inverter, which are far from thick: maybe 2,5mm2.
I have a single 12V battery (a 305Ah Concorde Sun Xtender 2580L) feeding the power needs of my off-grid cabin. This has worked well, but I recently got a 2000W inverter which introduced a new problem: power surge and continuous high-power delivery.
I had figured out that my setup would be fine, since I only needed this inverter for ten minutes or so during a weekend (powering a 220V 1400 Watt vacuum cleaner), but this failed to work: as soon as the vacuum powers up, the shown voltage on the inverter drops from 12.3V to 9.3V. I take this as a sign that a single battery is unable to sustain such a high load. What I'd like to know is this:
How can I know how "big" a battery bank I need to sustain X Watts in continuous power draw? Is there some indication in the battery specs that will help me in finding the numbers, or are people just using some heuristic, like "max power of a battery bank is approx 2x its numeric rating in Ah"?
P.S. Feel free to talk to me in Volts, Ampere hours and Watts. I can do the math 🙂
Best Answer
I spent way too long thinking I was going off-grid and then didn't, as the system cost to do what I needed and the grid connection cost crossed paths when the power company had a slow-down in new connections and got suddenly reasonable...
305 Ah battery is going to have a C/20 rate of 15 amps, for roughly 183 watts on a 12V system. C/20 is the discharge rate over a 20 hour period and is typically how the "305 Ah" is rated in the first place (you get more Ah capacity discharging slower and less discharging faster - and the Ah tends to drop from rated capacity as the system ages.)
A good "rule of thumb" on (lead/acid) battery bank size to support a certain draw is to stick at or below the C/20 rate. If your inverter supports multiple input voltages, I would strongly suggest going to a higher voltage (24, 36 or 48V) since maintaining a battery bank charged in parallel is much more prone to problems than a series string.
Or, get a different vacuum, or a non-electric sweeper. You don't provide details about your vacuum cleaner, but many are quite high Amperage draw at 120V (or 240V), which will be more than 10 times (or 20 times) as much draw at 12V (inverters are not 100% efficient.) That will result in a very significant increase in the size of the battery bank you need. Alternatively, you fire up a generator for 15 minutes once a week to meet your 10 minutes once a week of high power draw, and haul fuel for it, and maintain it.