Always the best first question!
Quar, make a list of your loads that you want to be able to run, then write down how much wattage they take while running, and also write down how often and how long each day you use them. The wattage will be on the nameplate that is
usually somewhere near where the power goes into the device. Some times it wont say wattage, but will tell you the input voltage and input amperage, which if you multiply the two together, you get wattage.
For example:
Code:
1 Table Lamp 120V (Volts) 100W (Watts) (Bulb rating) 2 hours (1 hour, twice a day for 2 hours total)
2 Radio 120V .6A (Amps) 72W 3 hours a day
3 Toaster Oven 120V 1,200 Watts .25 hours a day (Two 7.5 minute periods)
4 Mattress Heater120V 65 Watts 4 Hours a night (auto cutoff)
Here comes the first part of the math needed, to convert all of that to WattHours (WH) per day:
1 Table Lamp, 100W * 2 Hours = 200WH
2 Radio, 72W * 3H = 216WH
3 Toaster Oven, 1200W *
.25H = 300WH
4 Mattress Heater 65W * 4H = 260WH
Total = 976WH per day needed to run the loads listed.
Which means that you need to not only "create" at least that much power per day with solar panels (assuming no cloudy days), but also store at LEAST that amount in your battery bank. Most batteries should not be drawn down to full "nameplate" capacity if you can help it. Most don't want to be drawn down past 50%, some no more than 20% before it starts really eating into their "charge cycles". Deep Cycle batteries, like trolling motor batteries and golf cart batteries really should be kept in the 80-100% SOC (State of Charge) range for best lifespan and highest number of Charge Cycles. This means that the "1000WH" (rounded) list of loads above would really need to have at least 5000WH of battery pack/bank capacity to keep it in the 80% or better range. If you bought good Deep Cycle batteries, or AGMs batteries that were designed for 50%, you would only need 2000WH or battery pack/bank. And that still does not cover cloudy days. If you have a cloudy day every two sunny ones, multiply that 5000WH or 2000WH pack size by 1.33. A cloudy day every other day, multiply by 1.5.
While 2,000WH to 7,500WH of battery bank size seems rather large, figure that an average golf cart battery (6V * 210AH) has around 1,260WH of power in it and can be bought for around $100-$125 each (buy them in pairs since you need at least 12V battery packs to make chargers and inverters work). $200-$250 will get you 2 EGC-2 golf cart batteries if you shop around, which will give you 2,520WH of NAMEPLATE rated wattage. At 50% SOC, that gets you back to 1260WH of usable power per two EGC-2 batteries. At 80% SOC, that only gives you 504WH of usable power per two EGC-2 batteries. So you can scale your system capacity that way.
The batteries are where you end up spending a good part of the money of a solar PV system. It is also where you can do your math wrong (like using nameplate ratings on everything, or not factoring in cloudy days) and make a nightmare of a system that never gets a full days power to your loads and ends up killing batteries right and left. Yes, the batteries can get expensive. But wait, I'm not finished with the calcs. There are other options than trying to make it work for 2000-7500WH of battery bank design and 1000WH of actual power needed from the system...
First, let's look at the solar needed to run that load level...
Normally, most areas of the US get between 5 and 8 hours of
USABLE sun per day. This does NOT usually include "Dawns frist light" at 7AM, or "Just before Dusk" at 6PM. Think more like 9:30-10AM till maybe 4-5PM. Let's play with the 6 hours a day theme, just for simplicity's sake. If you have a single 100W panel, and can keep it pointed at the sun for those 6 hours, you can assume
close to 600WH of power from the panel. Again, nameplate ratings are just that, manufacturers specs that the manufacturer states to give you a ballpark of the capabilities of the panels in IDEAL conditions, not necessarily everyday use. I used to always derate the ideal vs. everyday conditions by knocking around 20% efficiency off the panels, especially if you don't have a way to "track" the sun as it "moves" across the sky. With this, a 120W panel gets thrown into the equations as a 100W panel, and a 100 watt panel gets treated as an 80W panel. There are other "losses" in the system as well, but for now, that is the one we need to concern ourselves over since we are talking about the panels themselves. So we need to be able to develop at least 1000WH of power a day (plus any cloudy day factoring).
Again, let's pick on 100W panels, which we derate to 80W of usable power coming out...
Code:
1 panel 80W * 6H = 480WH (not enough)
2 panels 160W * 6H = 960WH (getting there)
3 panels 240W * 6H = 1440WH (Good for one day, plus some overhead)
4 panels 320W * 6H = 1920WH (almost enough to recover for 2 days)
5 panels 400W * 6H = 2400WH (good for 2 days plus a little overhead)
6 panels 480W * 6H = 2880WH (good for almost 3 days of power, almost)
7 panels 560W * 6H = 3360WH (good for a solid 3 days of power)
Again, the extra days of capacity will work out IF your battery bank is sized big enough to COVER that load with no input and not go below the 50% or 80%SOC.
Here is one of the biggest secrets in Solar PV system sizing: Concentrate on reducing your power needs BEFORE figuring out what size PV system you want. So let's run through the list of loads again, this time swapping out the loads with more efficient ones:
Code:
1 Table LED Lamp 120V (Volts) 13W (Watts) (Bulb rating) 2 hours (1 hour, twice a day for 2 hours total)
2 PortableRadio 120V 15W 3 hours a day
3 Microwave Oven 120V 1,200 Watts .1 hours a day (Two 3 minute periods)
4 Mattress Heater 120V 65 Watts 1/2 Hours a night (just before ya hop in)
Which now drops the WattHours (WH) per day way down:
1 Table LED Lamp 13W (13W rated to develop 100W worth of light), 13W * 2 Hours = 26WH
2 PortableRadio, 15W * 3H = 45WH
3 Microwave Oven, 1200W *
.1H = 120WH
4 Mattress Heater 65W *
.5H = 33WH
And all of a sudden you have the same capabilities, for only 224WH per day instead of 976WH per day.
Now, 3 panels will get you 1440WH per day, enough for several cloudy days, and two EGC-2 batteries will hold (to 50%SOC) 1,260WH of power, enough for at least 3 days of clouds. The system is MUCH cheaper if you pick and size the loads right first.