# Double checking my math - When does it make \$\$\$

Solar Expert Posts: 65 ✭✭✭✭
Double checking my work before I got and sprout the wrong information on my
wife's urban farming blog. Trying to do the math to see if it makes sense financially
for people to install their own solar system.

~ The 2 panel system is a .25Kw off-grid system ~
Cost of solar panels: \$600
Cost of charge controllers: \$120 . . . .(not needed for grid-tied)
Cost of Batteries: \$250 . . . . . . . . . . . (not needed for grid-tied)
Cost of Inverter: \$65
Total Cost of off-grid system: ~\$1,035

PG & E currently bills us at \$0.14 per Kwh.

In order to break even and have “free power” there after:
(\$1,035 system cost) / (\$.14 Cost of electric car) = 7,393 Peak light hours needed to break even.
(7,393 daylight hours needed) / (4 useable peak hours per day) = 1,848 Days until break even (= ~ 5 years)

I was doing a lot of rounding but just trying to check this for accuracy. 8)

Re: Double checking my math - When does it make \$\$\$

Typically, 250 watt panels are not Vmp=17.5 or 35 volts (required voltage to efficiently charge a 12/24 volt battery bank with PWM charge controller). (rereading your post, I see that you are probably looking at 2x 125 watt panels--usually a bit easier to find Vmp=17.5 volt panels).

Typically, a 30-60+ amp MPPT charge controller is going to be in the \$325 to \$600+ range.

Next, what are you planning on using the AC inverter for--If it is for charging small electronics, AA battery chargers, running wall wart transformers, etc... Then you should really look at a TSW (True Sine Wave) inverter. Much more reliable, and overall, usually more efficient than a MSW (although, TSW are much more expensive).

Next, you need to derate your system... Typically, from solar panels through charge controller to battery bank to AC inverter is around 52% efficient (more or less). So, your total kWH produced needs to be reduced by about 1/2 if using AC loads.

And, you are mixing Grid Tied (or not grid tied) in there--Of course if you do grid tie solar, you will need (in this case) micro inverters (you need to match the right panels to the correct inverter).

And you should also include copper wire, fuses/breakers, switches, etc. too.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 65 ✭✭✭✭
Re: Double checking my math - When does it make \$\$\$

Bill, are you kidding me? a 52% efficiency rate?
that's terrible.

So my usage is about 1/2 Kwh?
I know there are only 4 good hours but there's an hour on either side of those 4 blocks and that's got to amount for something.
• Solar Expert Posts: 630 ✭✭
Re: Double checking my math - When does it make \$\$\$

The 52% derate would be for battery-based system. A grid-tie system (using microinverters matched for your panels) achieves much higher efficiency, could be in the 90s. Grid-tie usually requires a permit process, and inspection according to your county's local utility, a challenge for any DIY project.

Gloves wrote: »
Bill, are you kidding me? a 52% efficiency rate?
that's terrible.

So my usage is about 1/2 Kwh?
I know there are only 4 good hours but there's an hour on either side of those 4 blocks and that's got to amount for something.
• Banned Posts: 17,615 ✭✭
Re: Double checking my math - When does it make \$\$\$

It would be more accurate to figure the amount of kW hours the off-grid system will produce over its lifetime and divide the cost by that to get a figure to compare to grid power.

Something like:
250 Watts * 4 hours / 2 = 500 Watt hours per day. The hours before and after the sun hits the panels don't really contribute much of anything; it all adds in to the "4 hour of equivalent good sun".

500 Watt hours * 365 days = 182.5 kW hours per year. This assumes you always have that 4 hours sunshine average. That is probably not a safe assumption.

Multiply by 5 years minimum battery life. You might sneak more out of them, but five years should be viable. That's 912.5 kW hours. Cost per kW hour: \$1.13. Break-even point: never.

I recently checked the cost of reproducing my system at today's costs and found I could spend \$2,000 less and get more power. The over-all cost per kW hour went from \$1.50 for the original system to \$0.75 for the "new" version. BC Hydro is upping their rates to \$0.13 per kW hour. The only reason my off-grid system is viable is because it is miles from the nearest electric lines.

When you've got grid, off-grid doesn't make economic sense.
• Solar Expert Posts: 1,973 ✭✭✭
Re: Double checking my math - When does it make \$\$\$
It would be more accurate to figure the amount of kW hours the off-grid system will produce over its lifetime and divide the cost by that to get a figure to compare to grid power.

Something like:
250 Watts * 4 hours / 2 = 500 Watt hours per day. The hours before and after the sun hits the panels don't really contribute much of anything; it all adds in to the "4 hour of equivalent good sun".

500 Watt hours * 365 days = 182.5 kW hours per year. This assumes you always have that 4 hours sunshine average. That is probably not a safe assumption.

Multiply by 5 years minimum battery life. You might sneak more out of them, but five years should be viable. That's 912.5 kW hours. Cost per kW hour: \$1.13. Break-even point: never.

I recently checked the cost of reproducing my system at today's costs and found I could spend \$2,000 less and get more power. The over-all cost per kW hour went from \$1.50 for the original system to \$0.75 for the "new" version. BC Hydro is upping their rates to \$0.13 per kW hour. The only reason my off-grid system is viable is because it is miles from the nearest electric lines.

When you've got grid, off-grid doesn't make economic sense.
Use PVWatts to predict the output of the PV system, but you are correct in that it is virtually never viable from a purely economic standpoint to go off-grid if the grid is available. It's not even remotely close.

In my first job in solar, my employer sent me to trade shows to generate prospects. When I would ask the tire-kickers who came by our booth what they wanted to accomplish with a PV system, their answer was overwhelmingly, "I want to get off the grid." My reply (paraphrased for brevity) was, "No, you don't". Perhaps that contributed to my not being with that company any more. ;^)