Sidney Sizes his Solar Power System Sidney wants to size his van s solar power system. He s got a few things he d like to power in his van, and those items are where the design will begin. Step 1: Sidney sizes his batteries based on what he ll be running Sidney bought some items that will run off of his 12-volt batteries directly at 12 volts DC (direct current). Some of Sidney s fancier items had to be run off of an inverter because they require AC wall outlet power. Sidney s got a simple setup and he first lists his 12 volt DC items and their power consumption in watts: Sidney s Items 12V LED light Endless Breeze Fan (on average speed) Roof Vent Fan (on average speed) Watts Used 6 watts 24 watts 36 watts Your Notes:
Now, Sidney needs to know how many amps these devices take, since that will help him choose the right size batteries, which are measured in amphours. He remembers the formula from an old physics class: Watts = Volts x Amps so that must mean: Watts divided by Volts equals Amps or: Watts / Volts = Amps Since these items are all 12 volts, he will divide all of their wattages by 12 to get the amps they consume. Sidney s Items Watts Used Amps (divide watts by 12 volts) 12V LED light 6 watts.5 amps Endless Breeze Fan (on average speed) Roof Vent Fan (on average speed) 24 watts 2 amps 36 watts 3 amps Your notes:
Now that he s got the Amps that each item consumes, he s going to ponder how many hours a day each item will run to get amp-hours. Then he ll know how big his batteries should be! He brushes up on his formulas: Something that consumes 2 Amps and runs for 5 hours You multiply those figures together 2 Amps x 5 Hours = 10 Amp-hours Not too hard. So Sidney goes back to his list to calculate the amp-hours of each item. First, he ll estimate how many hours per day he s going to use each item: Sidney s Items Watts Used Amps (divide watts by 12 volts) Hours of use per day 12V LED light 6 watts.5 amps 5 Endless Breeze Fan (on average speed) Roof Vent Fan (on average speed) 24 watts 2 amps 4 36 watts 3 amps 10 Your notes:
Now he just multiplies the Amps and Hours to get the daily amp-hours used by each item: Sidney s Items Watts Used Amps (divide watts by 12 volts) Hours of use per day Amp-Hours per day 12V LED light 6 watts.5 amps 5 2.5 Ah Endless Breeze Fan (on average speed) Roof Vent Fan (on average speed) 24 watts 2 amps 4 8 Ah 36 watts 3 amps 10 30 Ah After all this math Sidney s ready for some answers. How big of a battery does he need? He adds up the daily amp-hours used by his devices: 2.5 + 8 + 30 = 40.5 Amp-hours He s finally ready to go get a battery. Maybe he ll be generous and round up to a 50 amp-hour battery. But then he remembers he doesn t want to run down his battery to 0% charge, as that will damage it. These things are expensive and he wants to make it last. He needs to run down his battery 50% or less on an average day. In that case, he will need to double the capacity of his battery. 100 Amp-hours of capacity sounds about right. Your Notes:
Now, with a 100 Ah battery, Sidney can run his devices all day long, and it will only use about 40% of the battery s capacity. Even though Sidney s chosen an AGM (absorbed glass mat) Deep Cycle battery, running it down only 40% each day will make it a lot easier on the battery, and it will last years longer. Step 2: Sidney sizes his solar panels based on his battery capacity In his haste to get the system together, Sidney forgets to account for his laptop running crazy cat videos all day off of the inverter. He lunges ahead and decides to size his solar panels. Now he knows that to keep his batteries healthy, he needs to make sure there s enough solar power on the roof to get his batteries back to 100% each day. If they don t fully recharge, basically the uncharged portion of the battery will begin to corrode, and become useless, during a process called sulphation. He knows this will turn his pricey battery into junk in no time. Sidney knows his battery is counting on him getting it 100% charged each day, and he knows another rule of thumb: At a minimum, you should have 1 watt of solar per 1 amp-hour of battery capacity to ensure daily recharge. In Sidney s case, this means a 100 watt solar panel to charge his 100 amp-hour battery. Your notes:
But Sidney knows this 1-to-1 ratio is a minimum. He knows that you may need up to 1.5 watts of solar per 1 amp-hour of batteries if you re in a climate that gets less insolation (hours of sun) each day. For Sidney, that would mean a 150 watt solar panel to charge his 100 amp-hour battery. Sidney knows he ll be hanging at the beach a lot, rocking out to his favorite 80 s hair bands, but he s also planning to head to the arctic north so he can pet the polar bears, one of his bucket-list items. The beach towns will provide plenty of sun, but when he drives north to visit the polar bears, he will be getting less sun hours, and he will need more solar panels on his van to get his battery recharged each day. He decides to err somewhere in the middle and opts for a 130-watt solar panel for his 100 amp-hour battery, a 1.3-to-1 ratio. Excited to finally have a system, Sidney wants to hook his panel up to his battery and start charging! But there s one more link in this chain: the Solar Charge Controller. Your notes:
Step 3: Sidney sizes his Solar Charge Controller Sidney knows that his panel puts out 20 volts in full sun, but that will damage his battery. After all, it is a 12 volt battery, hence the 12 volt. The battery will charge at 10 to 14 volts, depending on where its state of charge is. (10 volts when it s run down, 14 volts when it s charged up) He needs a smart device that can adjust down the voltage, on the fly. He sees these solar charge controllers do the trick, but dangit, there are two kinds. Too many decisions! He does some research, watches some videos from the Cargo Van Conversion Course, and surmises that the difference is this: PWM controllers are cheaper. They chop the voltage down to what the battery needs at that time, and you lose the rest of the power. If you ve got multiple panels on the roof, wire them in parallel to boost your amps and keep your volts lower (so there s less voltage to chop off the top!) MPPT controllers retain the most power coming in from your panels and deliver it to the battery. They run at up to 99% efficiency, but wow they are expensive! They drop the voltage to what the batteries need in real-time, but they magically boost the amps using some fancy algorithm. If you have multiple panels on the roof, you can run them in series to boost the voltage (amps stay the same as each panel) and have less wire resistance at that higher voltage. MPPT s can even boost voltage (by lowering amps) in lowlight situations. Sidney s wallet is getting tight, and he starts to go for the cheaper PWM charge controller. He just wants to hit the road! But then he remembers the polar bears.
He thinks about all of the low-light situations he might find himself in while visiting the arctic north, or if he heads to the beach in the winter. He wants to be sure his 130-watt panel recharges his 100 amp-hour battery each day, even if the weather is not-so-sunny. So he splurges for the MPPT controller to make sure he squeezes every drop of power out of his 130-watt panel. After clicking the Buy Now button on his new charge controller, Sidney gets an email from YouTube. His favorite crazy cat video channel has just put out another crazy cat video! He looks down at his laptop, realizing he s going to need these cat videos while living and traveling in the van. He could get a 12 volt converter to run his laptop right off the battery, but he has decided to run it off of an inverter, using higher voltage AC power. So he starts looking at inverters Your notes:
Step 4: Sidney Chooses an Inverter Sidney sees that there are these pure sine inverters that give smooth power and work on even your most delicate electronics. But he also sees that there are cheaper modified sine inverters that mimic the output of the pure sine models, but their power output is rougher and can damage some electronics. Sidney s wallet is really tight at this point, but he s worried that his laptop might not handle the rougher power coming out of the modified sine inverter. So he goes for the Pure Sine Inverter. He sees there are different sizes, and learns that inverters are more efficient when you use most of their capacity. So you don t want a 1000-watt inverter running only 50 watts of devices. Sidney s laptop takes around 90 watts, and he considers his 100 watt boombox that he plays his hair band jams on. That s 190 watts if he blasts hair band jams and watches cat videos simultaneously. Now that sounds like a good time. So he opts for a 300 watt pure sine inverter. Most of the time, he knows he ll just be using the laptop, so he s going to see how much battery capacity it will use on a daily basis
He plugs the laptop into a new table: Sidney s Items Laptop Watts Used 90 watts Now in Sidney s case, power coming out of the inverter will be at 110 volts AC (or 230 VAC depending on where he lives). But the inverter will be pulling power out of the 12-volt battery, so that s the voltage we ll use to calculate our amps: Sidney s Items Watts Used Amps (divide watts by 12 volts) Laptop 90 watts 7.5 amps Now Sidney needs to calculate how many hours a day he will use his laptop. He s been trying to cut back on his computer time, and decides he ll be on there 3 ours a day or less. Besides, this vandwelling thing is supposed to get him out into the world, not inside the van! So he logs 3 hours for the laptop Sidney s Items Watts Used Amps (divide watts by 110 volts) Hours of use per day Laptop 90 watts 7.5 amps 3
Now with some simple math, he ll know the amp-hours his laptop uses each day Sidney s Items 22.5 amp-hours is not bad for such an important item, but then Sidney remembers that his inverter is not 100% efficient. It s going to lose 10-15% of the electricity when it converts the voltage. Sidney is looking at a model that is 85% efficient. So he s going to take his 22.5 amp-hours, and divide by 85% to get the actual amp-hours these cat videos are going to take. He works the math Watts Used Amps (divide watts by 110 volts) 22.5 amp-hours /.85 = 26.5 amp-hours So the inverter needs 4 extra amp-hours a day just to convert the voltage for the laptop! Now Sidney s feeling smart for running a lot of stuff off of 12 volts, directly from his battery. Step 5: Sidney Gets to Recalculate! Hours of use per day So at this point Sidney wants to run his numbers again He s got the 40.5 amp-hours per day from his 12-volt devices and 26.5 amp-hours per day for his inverter devices. He adds it up to 67 amp-hours to run the whole operation! Amp-Hours per day Laptop 90 watts 7.5 amps 3 22.5 Ah
Now with his 100 amp-hour battery, he d be using 67% of its capacity each day, which will shorten the battery s life. He decides to beef up his system to 140 amp-hours of battery capacity, with 200 watts of solar on the roof, just to make sure he can power everything and his batteries get recharged each day. After all that math, Sidney s ready to roll out with his new van solar system for a few days of exploring!
Van Solar Demystified
Charge Solar Panel Controller Battery Inverter
Charge Controller Charges and Monitors Batteries Prevents Overcharge and Damage
Charge Controller
Battery Inverter Stores Power for later use Stable output voltage
Battery Inverter
Battery Inverter Converts battery power to wall outlet power Converts 12 volt DC battery power to AC power
Charge Solar Panel Controller Battery Inverter
100 watt panel Watts = Volts x Amps 100 watt solar panel = 20 volts x 5 amps
100 watt panel 100 watt solar panel = 20 volts x 5 amps
+ - + -
- + MC4 solar connectors (male & female)
- +
+ - + -
100 watts 5 amps x 20 volts 100 watts 5 amps x 20 volts
100 watts 5 amps x 20 volts 100 watts 5 amps x 20 volts
100 watts 5 amps x 20 volts 100 watts 5 amps x 20 volts + - 200 Watt Series Circuit Devices connected in a series or daisy-chained Add voltages together
100 watts 5 amps x 20 volts 100 watts 5 amps x 20 volts + - 200 watts 5 amps at 20 volts + 20 volts Add Voltages
100 watts 5 amps x 20 volts 100 watts 5 amps x 20 volts + - 200 watts 5 amps at 40 volts
100 watts 5 amps x 20 volts 100 watts 5 amps x 20 volts + -
+ 100 watts 5 amps x 20 volts - 100 watts 5 amps x 20 volts + -
+ 100 watts 5 amps x 20 volts - 100 watts 5 amps x 20 volts + -
100 watts 5 amps x 20 volts 100 watts 5 amps x 20 volts + -
100 watts 5 amps x 20 volts 100 watts 5 amps x 20 volts + - 200 Watt Parallel Circuit Add Amps together
100 watts 5 amps x 20 volts 100 watts 5 amps x 20 volts + - 200 Watt Parallel Circuit 5 amps + 5 amps at 20 volts
100 watts 5 amps x 20 volts 100 watts 5 amps x 20 volts + - 200 Watt Parallel Circuit 10 amps at 20 volts
100 watts 5 amps x 20 volts 100 watts 5 amps x 20 volts + -
100 watts 5 amps x 20 volts 100 watts 5 amps x 20 volts + -
+ - Charge Controller Charges and Monitors Batteries Prevents Overcharge and Damage + -
PWM + - MPPT Pulse-Width Modulation Maximum Power Point Tracking Lowest Price More Expensive Lower Efficiency Up to 30% More Efficient Monitors battery charge Monitors battery charge Chops voltage to what the and drops incoming voltage battery needs, lose the rest but boosts amps. + - Also better in low-light
PWM + - MPPT Pulse-Width Modulation Maximum Power Point Tracking Get Up and Running Cheaply Squeeze every last drop of power out of your panels + -
+ - + - 12V AGM Battery (Absorbed Glass Matt) No fumes, No Maintenance Store in any position Deep Cycle, can discharge more than starter batteries
+ - + - 12V AGM Battery Less Percentage Discharge = Longer Battery Life Battery Capacity Too Small = Over-Discharge/Short Life
+ - + - 12V AGM Battery Capacity Measured in Amp-Hours Symbol = Ah
+ - + - + - 100 + - Ah 12V AGM Battery 100 Ah Capacity Measured in Amp-Hours 200 Amp-Hour 12 Volt Battery Symbol =
+ - Inverter Converts DC battery voltage to AC wall outlet voltage ours tery For Running Household Appliances Use sparingly - loses ~10% of power to heat
+ - Pure Sine Inverter Modified Sine +V 0 -V Alternating Current (AC)
+ - Pure Sine Inverter Modified Sine More Expensive Less Expensive Smoother power is better for Rougher power works for delicate electronics robust electrical devices Cell Phones, Computers, etc. Power Drills, Blenders, etc.
+ - Pure Sine Inverter Modified Sine Recommended (powers everything)