Calculate for 3549 kWh per month

KM_RichardsKM_Richards Registered Users Posts: 2
looked on my electric company's website when paying my bill where it show my usage and the highest usage was 3549 kWh for August. All other months took less.

So how do I go about seeing what equipment I would need to produce 3549 kWh per month along with batteries to save juice?


  • BB.BB. Super Moderators, Administrators Posts: 33,327 admin
    Welcome to the forum KM,

    The short answer, a battery based hybrid inverter solar system at 3,500 kWH per month would be incredibly expensive. And would (usually) only make sense if you had terribly unreliable power or needed to run an off grid business (no power lines in region, etc.).

    There are two major types of solar power systems... One is a pure Grid Tied (utility interactive) system. Simply solar panels + GT Type inverters. These systems can be relatively cheap to install and save you money--And a big IF--If your local utility would allow a GT System to be installed in your place and connected to their grid. And not all utilities allow GT connections, and some have very "solar customer" unfriendly rate plans.

    Just to give you an idea of how the math works out for GT Solar... Guess you are around Austin Texas, and hot summers/lots of A/C.

    Fixed array facing south, fixed tilt, somewhere around Austin Texas:

    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 60° angle from vertical:
    (For best year-round performance)

    The above is "hours of sun" per day--Long term average. The basic formula for GT Solar would be:
    • 3,549,000 WH per August / 30 days per month = 118,300 WH per average August day
    • 118,300 WH per August day * 1/0.77 solar panel+GT inverter deratings * 1/5.72 hours of sun per day (Aug) = 26,860 Watt array "break even" August
    • 26,860 Watt array * 0.77 panel+inverter derating = 20,682 Watt = ~21 kWatt rated GT Inverter(s) (or total micro inverters)
    I have not kept up on costs of a commercial installation... Last I heard was around $5 per watt or roughly $134,300 for a "generic" turnkey install... (I am not in the business, and depending on location, requirements, inflation, etc... it could easily cost more--or even less)--Just trying to set expectations.

    If you wanted an "off grid" (or hybrid) solar system (hybrid means it does both off grid emergency power and when grid is up--feeds power back to the utility and "turning" your meter backwards).

    For an off grid system (just a quick rule of thumb design)... Battery bank for 2 days of storage and 50% maximum discharge--Or 4x your daily usage:
    • 118,300 WH per day * 1/0.85 AC inverter eff * 1/48 volt battery bank * 2 days storage * 1/0.50 max discharge = 11,598 AH @ 48 volt battery bank
    To calculate solar array--You need two calculations. First base on size of battery bank and second based on hours of sun per day and your daily loads:
    • 11,598 AH * 59 volts charging * 1/0.77 panel+charge controller deratings * 0.10 rate of charge = 88,868 Watt array @ 10% nominal rate of charge
    And based on hours of sun/location and daily loads:
    • 118,300 WH per day * 1/0.52 average off grid AC system eff * 1/5.72 hours of sun per day (Aug) = 39,773 Watt array "August" break even
    And for a full time of grid system, you need to account for cloudy weather at times, so you would need something larger than 39,773 Watt array...

    If the "emergency" off grid power was only during stormy weather and you can use much less power (no A/C, minimize electrical usage, etc.)--You could 'down size' the battery bank and solar array...

    Also, sizing the array/battery bank--Depends on how much (or how little) you want to use a genset--What is your "winter power usage" etc...

    With a pure Grid Tied system--You just add up your full 1 year energy usage, and how many kWH per day/month total power is generated--In California, you aim for power used = power generated... Although these days, it get more complex because of utility billing plans (time of use, peak/partial peak/off peak power pricing (and tiered pricing--Use more power, they charge more $$$ per kWH)....

    Anyway--You have some bracketing numbers that may or may not match your needs/desires.... Need more information on your energy needs (by month / seasons), And your expectations (save money with GT solar--Full or partial solar backup, or full time off grid).

    GT Solar--You can get down to $0.10 to $0.05 per kWH for solar generation costs. In California, we can offset summer peak pricing of $0.40 per kWH... I have read other utilties may only "pay" you $0.05 to $0.10 per kWH for solar generation.

    For pure off grid solar power--It can cost you typically around $1 to $2 per kWH (and a few people have gotten down to $0.50 per kWH with very careful energy usage).... It is not just the cost of hardware costs... It is also replacing batteries every 5-10 years, inverter/charge controllers every 10+ years, solar panels 20+ years, etc..). And also depends on you using "most" of the energy generated... I.e., any energy you do not use, goes into the battery bank--Which can, at best, only store ~2 days of "useful" energy... 

    Anyway--Not a detailed Q&A, and lots of variables that need to be discussed... But it gives you a basic idea of what a "large system" like this entails.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • KM_RichardsKM_Richards Registered Users Posts: 2
    edited November 2021 #3
    Wow, thanks!

    Lots of info to digest... thanks for taking the time to give me all this to chew on.

    It sounds like a whole house system off grid would be way more than I'd want to spend so I'll need to think about what I'd like to have in the event the power grid is down for a really long period of time which is what I'm wanting to plan on.right now

    We do have a water well on site that I'd like to have power for and I'd like to at least have AC in my garage... I currently have a big old window unit to cool the garage and I'm sure it's a power hog compared to a mini split unit which I hear is very efficient and some are even built to be used with solar power

    I'd imagine I'd need to switch to a min-split unit that not only cools but also heats so if the worst happens I'd at least be able to air condition the garage (average size 2 car garage).  Beyond the water well pump and air conditioning the garage, maybe a little extra power beyond that for lighting
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