I'm planning on using solar power to supply electricity to a 12x24' building. This building will be located near our main house, but will have no connections to the house, electrical or otherwise. It won't be connected to our utility's power grid, so there aren't considerations for tying it in.

I've done as accurate an estimate as I can, and figure I'll have a peak load of 2524 watts. I plugged some data in to a solar contractor's system calculator and it gave me this info, based on our location in central Arkansas:

  • kWh per month: 288 kWh
  • Sun hours: 3.76 hours/day
  • System size: 3.06 kW
  • Wh per day: 9590

I'm looking for some help making sense of those numbers. (I have an EE degree, so I understand electricity and electronics, but some of the numbers related to solar power are foreign to me.) For example, I'm looking at a system that is an "800W 24V solar panel off grid system kit with 8 100W mono solar panels, a 1500W 24V-110V Inverter, 200Ah Battery, and 60A PWM charge controller".

Will that system power my building based on the needs I listed? Does the output of the inverter (1500W in this case) need to be greater than my projected peak load of 2524W? It seems to me it would need to be, but I'm new at this.

  • What's the reason for going completely off-grid with said building? Jan 31, 2021 at 19:41
  • If you are in the USA, NOAA/weather service has, buried somewhere in their websites, 30 year averages of climate data (including sun) for sites all over the country. Find the one, or ones, closest to your site and look at the version that breaks it down by month, at least, before making design decisions based on a year-round average. Year round average works fine for grid tie, but not for off-grid.
    – Ecnerwal
    Jan 31, 2021 at 19:44
  • @ThreePhaseEel mainly because I want to avoid the expense and hassle of connecting to the grid. Jan 31, 2021 at 20:11
  • Partly, you are asking whether an inverter plainly rated 1500W can drive 2524W of load, which seems unexpected from an EE. I gather that several places in here, you meant to say "watt-hours" and just mistyped. Please edit to correct as needed. Jan 31, 2021 at 20:16
  • 1
    @BobMathews -- perhaps you should ask a separate question about whether you have enough capacity left on your service? Include photos of your existing panels and the square footage of the house, as well as a photo of your air conditioner's nameplate Jan 31, 2021 at 21:01

1 Answer 1


There's no magic here. Plenty of engineering, though.

Yes, the inverter output needs to be greater than your peak load.

Not only that, your battery bank size needs to be adequate that the peak load is not exceeding the battery bank charge drawdown rate. Hint, yours is not - you're contemplating drawing over 100 amps at 24 volts off a 200AH battery, or C/2 - that needs to be about 10 times bigger for the usual C/20 that solar storage batteries are designed to tolerate.

That's harder with cheap but fussy lead acid batteries and easier with expensive but tolerant (except where they are not) lithium batteries. In both cases reducing your peak load is usually the most cost-effective option before any others.

Off-grid solar is expensive. In many locations, the "average full sun-hours per day" is a lovely figure that is essentially useless for the month you have to design for (in my area, November - gray and terrible from a solar point of view. If you can make it through November here, other months are easy, but enough solar input and batteries for a non-grid-tied November is daunting/expensive.) That also means that you have excess power (and no grid connection to sell it) most other months.

Those cheap but fussy lead acid batteries have limited lifetime and limited tolerance for being deeply discharged, which is one thing that makes the ongoing "cost per kilowatt hour" significantly higher than grid electricity, in most cases. The popularity of grid tie is in part because it's like having a really, really good battery 99+% of the time, and the maintenance is just paying for your connection fee, which is usually a lot less than paying for a few tons of batteries every few years (perhaps longer if you are very careful with them.)

I spent a while designing a solar system for my place, since the power company and neighbors were being uncooperative (the power line is not quite on the road, so an easment to get it to the road and then to my place was needed.) It was a heck of a lot of money. Fortunately the neighbor situation changed, and then the power company became cooperative, and it was still a heck of a lot of money to get the grid, but it was a lot less money than off-grid would have been, and lacked some of the limitations (I was going to have to have a generator to run for heavy loads that could not be reduced.)


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