off grid system

reckless1

Good afternoon people,

i have a friend whose home is consuming about 2000kwh per month during the summer and about 1500kwh out of summer months, what size system would he use to come off the grid, panels, batteries etc. Like me once the batteries deplete 50% the system would switch back to the grid. How do i calculate the amount of panels and batteries etc..

sincerely,

Robin

inetdog

Re: off grid system

reckless1 wrote: »

Good afternoon people,

i have a friend whose home is consuming about 2000kwh per month during the summer and about 1500kwh out of summer months, what size system would he use to come off the grid, panels, batteries etc. Like me once the batteries deplete 50% the system would switch back to the grid. How do i calculate the amount of panels and batteries etc..

sincerely,

Robin

Good afternoon Robin.
Could you start with an explanation of why he wants to come off the grid in the first place rather than using a grid tie inverter system? Maybe then we could give you better help with the calculation.
If the motivation is strictly economic, it does not make sense.

reckless1

Re: off grid system

Hello inetdog,

in jamaica the cost of electricity is .42 usd per kwh, and to go grid-tie he would have to get a special license from the power company who also limits the maximum residential grid tied system to 10kw.. so he would rather use an off grid system even if it doesnt take him off 100%. out here we use xantrex inverters and we use the LVD on the charge controller to switch a AC contactor which removes power from AC1 when the batteries are full and when it reaches 50% DOD the contactor puts power back to AC1 which takes power back from the grid..

robin

Cariboocoot

Re: off grid system

Let's be accurate here: 2 megawatt hours per month? As in 67 kW hours per day?

How much effort has been put in to conservation? Because even at $0.42 per kW hour grid-tie solar won't offer any savings and certainly off-grid won't. The capital expenditure involved can buy a lot of new appliances and insulation.

BB.

Re: off grid system

Some rule of thumb based math:

2,000 kWH per month / 30 days per month = 66.7 kWH = 66,700 WH per day (summer)

66,700 WH per day * 1/0.85 inverter eff * 1 day storage * 1/0.50 maximum discharge = 3,270 AH @ 48 volt battery bank

(recommend 5-13% rate of solar charging--Minimum of 10% for heavy cycled system recommended):

• 3,270 AH * 59 volts charging * 1/0.77 panel+controller eff * 0.10 rate of charge = 25,056 Watt array minimum (based on rate of charge)

And how many panels based on daily power usage and hours of sun per day:

Using Solar Eletricity Handbook:

Kingston
Average Solar Insolation figures

Measured in kWh/m²/day onto a solar panel set at a 72° angle (from vertical):
(For best year-round performance)

Jan
Feb
Mar
Apr
May
Jun


5.57
6.00
6.38
6.44
6.04
6.62


Jul
Aug
Sep
Oct
Nov
Dec


6.62
5.89
5.83
5.69
5.30
5.14



Pick 6.0 hours of sun nominal for "summer":

• 66,700 WH per day *1/0.52 system eff * 1/6.0 hours of sun per day = 21.378 Watt array minimum (based on loads+Sun).

So--There is a starting point... (excluding AC inverters, backup gensets/utility power). This large of system may look to higher voltage battery banks and inverters for this large of system (beyond my experience).

-Bill

Photowhit

Re: off grid system

I wonder if there would be a payback if you tried to not store energy? If with 2 10,Kw array's facing south east and south west, if you could run off grid 6 hours a day on average, (I think Bills numbers are for a fixed array might even get longer run time if the array was setup this way) and those being higher draw times of the day... figure 6Kws and hour, that would be 36 Kwhs a day perhaps at 42 cents a Kwh or 36 x 365 x .42 = $5500 a year. I think Sunelec stores panels for Caribbean distribution in Jamaica, had some UL Mono panels(Csun?) at 54 cents a watt there... Might employee the advantages of the LiFePo battery pack to deliver high amounts of current over short periods to run a very minimal battery bank.... Things to think about!

reckless1

Re: off grid system

Good morning all,

thanks for the responses, i will definately tell him to get an energy audit done and then proceed from there. BB thanks for your calculations..

robin

reckless1

Re: off grid system

BB can you show me actual calculation (formulaes)..

robin