Converting DC to AC is necessary if you want to run household electrical appliances with a solar power system. But what size inverter do I need for a 100 watt solar panel?
You are probably thinking that a 100 watt solar panel hooked up to an inverter means I can run AC appliances. Like a fridge, a washing machine, power tools or even a cooker. But unfortunately these types of appliances require large amounts of power to run them.
A much larger solar array is required to provide enough re-charging power to a battery to keep the inverter going. The reality is that the inverter needs to be sized to the appliances you want to run. But let’s look at what size inverter a 100 watt solar panel is suitable to be combined with.
Quick Jump Menu
What Size Inverter Do I Need For A 100 Watt Solar Panel?
“ A Pure Sine Wave Inverter can be combined with a 100 watt solar panel but it doesn’t matter what size. This is because the inverter draws it’s power for conversion from DC to AC from the battery. This means the solar panels need to be sized for the battery not the inverter. The Inverter needs to be sized by the AC appliance wattage you intend to run.”
Hang on a minute Steve, “some other websites say that you can use a 100Ah battery with a 100 watt solar panel. I am sure this combo will run a decent sized inverter to power AC appliances.” Well you can but if you use a 100w panel with a 100Ah battery discharged by 50%. It would take around 10 hours of full sunlight to recharge, in reality that is more than 24hours.
What Watts and Amps Are Required To Run An AC Appliance (Load)?
It’s not just about the size of the battery it is about the number of watts and amps that are required to run the appliance. Let’s use a wi-fi router as an example. Mine uses 7 watts and over a 24 hour period it will consume 168 watts.
To calculate the number of amps the Wi-Fi router is using. The watts (7) need to be divided by the number of volts (12) assuming a 12v system this equals 0.58A. This means that over a 24 hour period it will consume 13.92A. Because of course a WI-Fi router needs to be switched on 24/7 and 365 days a year.
Imagine you live in the north of the United States, say Seattle. The average sun hours per day across the year is around 5.5. Bearing in mind that July has the best sun hours 9.8 per day and December has the worst with 2.
The seasons will also play a factor because the sun is weaker between late fall and early spring. So to compensate for sun intensity and available sunlight across the year. We reduce the sun hours by around 30% to 4hrs.
The average 100w solar panel pushes out around 5.5 amps per hour in full sunlight. To find out by how much the battery will be recharged over 4 hours we simply multiply 5.5A by 4hrs. This equals 22A and seems to satisfy the use of a 50AH or 100Ah battery with a 100w solar panel.
Adding A Laptop To The System
Let’s say we have a 15 inch laptop that uses 65 watts per hour and it is used 8 hours a day. This means it will consume 520 watts per day (65 x 8). Converting it to amps is 5.42 per hour equalling 43.36 per day.
Combined with the Wi-Fi it equals a total amp usage of 57.28A per day (43.36 + 13.92). As you can see a 100 watt solar panel isn’t enough to recharge a 100Ah battery. A larger laptop depending on it’s components will use even more amps, possibly 8.33 to 15 per hour.
The Inverter Size To Use Should Not Be Calculated By Solar Panel Size
The solar panel size has nothing to do with the size of the inverter you should use for your system. You need to calculate this from the wattage of the AC appliances you want to power. Then work backwards battery size, solar array and finally solar charge controller.
With this in mind let’s go back to the example of a WI-Fi and a 15 inch laptop setup..
The Wi-Fi has a rating of 7w and the laptop has a rating of 65w. Taking into account inefficiencies in the system. For example the loss of energy transferring power between the battery and inverter and inverter and appliance. Power fluctuations and surge required to initially power the load.
It might seem like overkill but it is best to over estimate by 3 times the total appliance wattage (load). In this case 72 watts x 3 equals 216 so a 200 watt inverter would be best. You can multiply it by 2 if you wish. But by multiplying it by 3 you will take all power inefficiencies and fluctuations into consideration.
We have already established that a 50Ah capacity battery would not be large enough. Because the total amp-hours per day this system would use would be 57.28A. You could use either 2 x 12V 100Ah deep cycle lead-acid batteries or 1 x 12V 200Ah.
If you don’t already know you can only discharge a lead-acid battery to 50% of its capacity. This is because even a few discharges below that will damage it.
But Lithium Iron Phosphate (LiFePO4) can be safely discharged to 90% capacity. So a single 12V 100Ah would do the job very nicely.
Solar Panel Array
The number of solar panels required for this system need to be based on the battery amp-hours. The best way to work this out is to:
Take the battery amp hours (Ah) and multiply by volts (V) equals Watt Hours (Wh). Then watt hours (Wh) divided by sun hours (H) equals solar panel wattage. Using our 4 sun hour Seattle example;
100Ah x 12V = 1200Wh
1200Wh divided by 4H = 300 Watts
So 300 watts of solar panels either 3 x 100 watt or 2 x 150 watt would do the job.
If you were fortunate enough to live in a sun healthy city like Dallas Texas the average sunlight is 7.8 hours per day. Taking the seasons and other environmental factors into consideration. It would be right to reduce that figure by around 30% to 5.5hrs
Using the equation above this means that in Dallas you would need around 200 watts of solar panels.
Solar Charge Controller
The easiest way to work this out is to divide total watts by volts and add an extra 25% for environmental factors.
Solar panel watts 300 ÷ 12V = 25 x 25% = 6.25 + 25 = 31.25. Then round to the nearest whole 10 number = 30A solar charge controller.
Solar panel watts 200 ÷ 12V = 16.6 x 25% = 4.16 + 16.6 = 20.32 rounded to the nearest 10 = 20A solar charge controller.
What Type Of AC Appliances Can I Run With An Inverter?
Any type of AC appliance can be powered by an inverter. The inverters used in off-grid settings come in lots of different sizes from small 500 watt. These are meant for in-car use to larger 3000 + watt capable of powering small ovens.
Appliances an AC inverter can power;
|Electric Lights||Microwave||Power Tools|
|Smartphone||CD Player||Satellite Dish|
|Refrigerator||Vacuum Cleaner||Coffee Maker|
|Blender||Hair Dryer||Washing Machine|
I have refrained from adding wattages for each appliance. This is because the manufacturer’s specifications are always the most accurate. So you should always refer to them before sizing an inverter.
Some Things To Remember When Using An Inverter
- Make sure that the appliance surge at start up isn’t higher than the inverter surge rating. If it is, the appliance might not start.
- Make sure you do not overload the inverter. The total wattage of appliances you have connected should not exceed the inverters continuous watts.
- When using an inverter with appliances that use heat for example a hair dryer. Again make sure the inverter can handle both the surge and continuous power output.
- Remember the more and higher rated appliances you have running the faster your battery will discharge.
My Closing Thoughts
It now seems obvious that a 100 watt solar panel is probably not the best size of solar panel to use with an inverter. They are ok to use with a portable solar power station for small applications. But for providing charging power for a battery and inverter running larger appliances. A much larger solar panel or solar array is required.
Also when sizing an inverter it is always best to work from the AC appliance wattages you want power. Then work backwards to determine the size of the rest of the solar system.
If you would like to know more about inverters follow this link;
I hope you enjoyed this post and have found it helpful. If you have any questions about sizing an inverter, batteries, solar panels and charge controller. Or want to leave your own personal review, please feel free to leave a comment below.