First solar system inquiry

Re: First solar system inquiry

Regarding your loads--If you have a Watt*Hour meter (like a Kill-a-Watt meter)--you can measure your Watt*Hour loads and better plan your power needs.

Guessing your average loads over 4 hours:

• 90 watts * 4 hours = 360 WH per day == 20" TV with cable box (90w)
• 100 watts * 1/6? * 24 hours = 400 WH == Bar fridge (100w 6:1 duty cycle with 400w compressor surge)
• 55 watts * 4 hours = 220 WH == Laptop (55w)
• 45 watts * 4 hours = 180 WH == 3 15w compact fluorescent lights (45w)
• =====================
• 1,160 Watt*Hours per day average load

Using PV Watts program, Toronto Canada, assuming 1kW worth of solar panels (even number to estimate output), 0.52 Derating Factor (off grid inverter and flooded cell battery bank):

"Station Identification"
"City:","Toronto"
"State:","ON"
"Lat (deg N):", 43.67
"Long (deg W):", 79.63
"Elev (m): ", 173
"Weather Data:","CWEC"

"PV System Specifications"
"DC Rating:"," 1.0 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 0.5 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 43.7"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 0.1 Can$/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value (Can$)"
1, 2.69, 44, 0.04
2, 3.51, 52, 0.04
3, 3.98, 63, 0.05
4, 4.68, 69, 0.06
5, 5.29, 78, 0.07
6, 5.48, 75, 0.06
7, 5.51, 76, 0.07
8, 5.23, 73, 0.06
9, 4.96, 69, 0.06
10, 3.61, 55, 0.05
11, 1.95, 27, 0.02
12, 1.97, 32, 0.03
"Year", 4.07, 714, 0.62

Lets assume you will run the system 9 months of the year (toss out the lowest 3 months)--We get 52 kWHrs per month per 1,000 watts of solar panels (average February). Or:

• 52 kWH/month * 1/30 days per month = 1,733 Watt*Hour per day per 1,000 watts of panels minimum 9 months of the year

You use 1,160 Watt*Hours per day:

• 1,160 WH per day / 1,733 WH per day per 1,000 watts of panels = 669 Watts of panels minimum for 9 months of year

Regarding your inverter--Your fridge may take more than 400 watts to start (most what/amp meters are not quick enough to "see" the starting surge). For a full sized frost-free refrigerator, you should be looking at a minimum of 1,200-1,500 watts of inverter. (note, I did not include the inverter stand-by losses--some are pretty high and need to be included in the original power calculations).

Now, for the array Vmp (maximum power point voltage)--To properly charge a 12 volt battery bank, the rough Vmp minimum needs to be Vmp>2volts+Vbatt-charging... or:

• Vmp>2 volts + 15 volts = ~17 volts minimum panel rating

Note that Vmp falls with high ambient temperatures, and if this was in a very hot location, you may need even higher than 17 volts for properly battery charging.

Now, there are two major types of solar charge controllers. PWM (pulse width modulation) and MPPT (maximum power point tracking).

PWM are cheaper controllers but become less efficient the higher Vmp is with respect to Vbatt (i.e., a 34 Vmp panel connected to a 12 volt battery bank will only be 50% efficient)--so for PWM, you want your Vmp in the 17-19 volt range for efficient operation.

MPPT are more expensive but are actual power converters. They will efficiently down convert from High Voltage / Low Current of a solar array to Low Voltage / High current to charge the battery bank.

All About Charge Controllers
Read this page about power tracking controllers

Your choice with controller type you will want to use. MPPT controllers are certainly more efficient and flexible for use with larger arrays (roughly larger than 200-400 watts). PWM are much less expensive.

-Bill

Re: First solar system inquiry

Mark,

Yep--that will be the number you want to aim for... Note though, you may want to cycle the bar fridge through your 4 hour cycle--if the contents (beer/etc.) are pre-chilled, then it should be close to your estimate. If the fridge needs to draw down the temperature--it could be easily running 100 watts for 4 hours.

You can adjust your needs---I have all of the basic assumptions there. Say, this is only a "summer time" use--then obviously, you can pick another "minimum month" to build out your array for.

By the way, make sure your DYI solar array is not over any flammable stuff (roof, ground cover, etc.)... You may be using wood/plastic in the construction and it is not unknown for these types of panels to catch fire during normal operation.

Also, you should fuse (or breaker/combiner box when you have more than two solar arrays in parallel) each string with a series fuse roughly 2x the Isc (short circuit current) to limit the string current(s) in the case of an internal short circuit).

-Bill

Re: First solar system inquiry

Well, PV Watts uses 0.77 derating for solar panels (roughly 80% derating, and ~95% derating for grid tied inverter), add 80% for flooded cell batteries (or 90% efficiency for AGM type), and ~85% for the inverter:

• 0.77 * 0.80 battery * 0.85 inverter = 0.52

-Bill

Re: First solar system inquiry

I should add, that this is a very rough rule of thumb and is intended to be somewhat conservative...

There is a Derating Help link in PV watts that shows how they take certain values into account (I don't agree with all of them--but they do average out to be pretty accurate for an average season of power collection). The PV Watts program is based on average weather (included rain, clouds, smog, etc.) over some 20 years or so of measurements. Coastal/Marine areas are many times hit by humidity/fog/marine layers.

Of course, your panels need to be mounted in full sun (or at least 9am-3pm or so)... If you are under trees, on the shady side of a mountain valley, etc.--it can badly hurt your output. Solar panel need full sun to operate efficiently. Any shading during the middle of the day can dramatically reduce solar PV panel output.

There are lots of variables... Solar panels on MPPT charge controllers lose output power as they get hot--so mounting them on roofs with 5-6 inches of air space underneath or on rack mounts above the roof, above ground, or on poles all help to keep panels cooler. In very cold climates (below freezing), the panels can output 10-25% more power than the above estimate would show (including snow/water reflections, etc.).

Flooded cell batteries tend to be less efficient as they age--new FL Cell batteries may start out 90% efficient and AGM's may start out in the 95-98% efficiency range. Also, how you operate the batteries (keeping them near 100% full, running lots of equalization, etc.) can also reduce efficiencies. And some batteries designed for very deep/tough cycling--like forklift batteries--will be on the less efficient side of the scale.

Idling inverters can take from 6-20+ watts (depending on design and operating mode used).

MSW (Modified Square Wave) type inverters can lose upwards of 20% of their output energy running inductive loads (like electric motors--fridge, etc. motors run hot) vs TSW (True Sine Wave) types.

You can have lower losses by running large gauge wire (1-2% vs 3% maximum recommended).

In the end, we try to give you estimates that will layout a system that will pretty much meet your operating needs. And in any case, most people tend to grow their loads over time and designing a system to be 2x larger than your current needs is not a bad thing to do either.

-Bill