When you’re building a cabin off-grid, or adding electricity to one, solar power is likely to be something you’ll consider.
I like solar power as an option for off-grid cabin electricity. It’s clean. I don’t need to haul up fuel. And staying off-grid is not only freeing, but from a cost an practicality standpoint, it’s often a necessity. But then the question arises, how many solar panels do you actually need to power your cabin? The answer obviously isn’t one size fits all. But this article is meant to give you the tools you need to figure out how big of a solar system you need for your cabin.
Figure out how much power you use per day
Before we can figure out how many solar panels you need, we need to figure out how much power you actually consume at your cabin.
The best way to do this would be to add up all of the average usage of the electric devices and appliances in your cabin. Appliances should state in their owner’s manual and often on a label somewhere how much power they use on average. If the number they give is yearly kWh, divide that by 365 to get daily usage in kWh. If the number is just in Watts, then multiply that number by how many hours per day it’s in use then divide that by 1,000 to get the number of kWh per day.
For example, a 400 Watt appliance that runs constantly will use 400 Watts times 24 hours divided by 1,000 which is 9.6 kWh per day. Many appliances are only used occasionally, like a toaster. That might get used 30 minutes per day. So a 500 Watt toaster uses 500 Watts times 0.5 hours divided by 1,000 which is 0.25 kWh per day.
How many lightbulbs are there in your cabin? What’s their output? A 60-watt bulb uses 60 Watts. Multiple 60 watts times the number of bulbs to get total Wattage for all the lights in your cabin. Now multiply that times the approximate number of hours per day that each light is on. Divide by 1,000 and voila! You have kWh per day.
So again, if you have 20 bulbs throughout your cabin and each one is only on for 3 hours per day on average, then your lights are using 3.6 kWh per day. If they’re LEDs then they’re probably using more like 8 Watts so by replacing those 20 bulbs with LEDs you’ll only use about 0.5 kWh per day for lighting.
You can see that adding up all the electrical devices in your cabin can take time. You should account for kitchen appliances, lighting, furnace if you have one, A/C if you have one, computer, TV, phones and tablets you charge, even the vacuum. This can be tedious. But some of these items are small, so just think of what you can and add a little extra to be safe.
The easier option is to find another house or cabin that’s similar in size and has similar appliances and use and find out how much energy they use. If they’re on the grid, their power bill will tell them. You can even use your house for comparison. If your primary home uses 1,000 kWh per month, then your cabin that has smaller appliances, less lights, and no air conditioning may only be using 400 kWh per month. This sort of comparison will get you in the ballpark.
For your reference, the average home in the US uses about 900 kWh per month but can range from 200 up to 2,000 or more. 900 kWh per month equates to about 30 kWh per day.
Consider how long you’ll stay at the cabin
Now that we have a good idea of how much power we’ll use when we’re at the cabin, we need to think about how long we generally stay there. If you use the cabin an average of 2 days per week, then you don’t need as much solar capacity as you would if you use it 6 days per week. That is, assuming you have sufficient power storage. So now we need to figure out how much power is going to be used while we’re not there.
There may be some things you want to keep running while you’re away. Maybe you go to the cabin weekly and like to store some food in the refrigerator. Maybe you like to keep the furnace on at a low temperature setting to keep the water pipes from freezing. Whatever it is, we need to figure out how much power we use when we’re not there.
Go through the same process that we outlined above. Add up the usage for all of the things you want to keep running while you’re away.
Now let’s add up our power usage per week. If we’re there 2 days and away 5, then we’ll take 2 times our usage while at the cabin plus 5 times our usage while away. That gives us weekly power usage.
For example, if we use 20 kWh per day while there and 5 kWh per day while away, we take (20 x 2) + (5 x 5) = 40 + 25 = 65 kWh per week. Well, that’s an average of just under 10 per day. So as long as I have enough battery storage to get me through the 2 days per week I’m there (40 kWh) then I can actually install a system that puts out more like 10kWh per day.
Add a buffer
Now that you at least have an idea of how much power you’re probably using, add some buffer.
You don’t want to end up in a situation where you have less power than you need. If you’re on a solar system, the sun stops shining, and your batteries run out then your electrical appliances and devices will simply turn off. Power outage!
Most people will tell you to add a 25% buffer to whatever you calculated. That’s not a bad idea. So if the usage we calculated per week is 65 kWh, we’ll take 65 x 1.25 = 81.25. That’s 11.6 kWh per day we need our system to provide so we never run out of power while we’re at the cabin.
Consider power storage
Now let’s talk about power storage. For this part especially, you’ll want to contact a local solar power company. You can save a bunch of money by installing your own panels and batteries if you’re comfortable doing it, but you’ll want some help getting the right battery setup for your cabin.
Here’s the thing, if you have enough power storage to get you through your normal cabin stay, then you can get by with less solar panels. But storage isn’t cheap. And you may find that it’s better to have more panels and less batteries.
If you can store 40 kWh of power and never use your cabin for more than 2 days at a time, then you should be fine with panels that generate about 10 kWh per day. But if you don’t have that much storage, then you’ll need enough panels to keep up with your 20 kWh per day demand the whole time you’re there. Again, these numbers are for example purposes only. Your actual needs will depend on your cabin.
Calculate your solar panel needs
Solar panels are usually rated to put out 150 to 370 Watts. And that output can vary a lot by size and type of panel. Plus, that’s the output you can expect with direct sunlight. So to figure out how much each panel can generate per day, you’ll have to take a few factors into account.
- Where your cabin is. Some areas of the world get more direct sunlight than others. The Renewable Resource Data Center has some really good maps and information to help you figure out about how many hours of peak sunlight you can expect in your location.
- How shaded your cabin is. If you’re in a wooded area, you likely get less hours of peak sunlight than they state.
- The angle of your roof (or wherever your panels are mounted). If you don’t have a nice pitch on the south side of your cabin, then you won’t get as many hours of direct sunlight.
If you determine that you’ll get about 4 hours of direct sunlight and you’re using 250 Watt panels, then a single panel will generate around 4 x 250 = 1,000 Watt hours or 1 kWh per day. So if you need 10 kWh per day, then you need 10 panels.
Optimize your panels during daylight hours
There is a way to decrease the number of panels you need.
When you’re at your cabin, you can optimize the panels for maximum direct sunlight.
If you mount them on the roof of your cabin then they’ll just point wherever they point and they won’t get maximum sunlight. But if you can change their angle throughout the day, then you can get more hours of direct sunlight.
By mounting our panels in two axes, we are able to adjust them to various angles throughout the day to get better use of our panels. That may not be something you want to worry about while you’re relaxing at the cabin. But it can be a simple way to save on the cost of solar panels, and allow you to squeeze a little more power out of the system that you’ve got. Save Money by Going Big!
One way to cut down a little on the cost is by using larger panels. You can generally get more Watts per dollar if you go with larger panels. So getting a higher output 375 Watt panel will cost more than a 250 Watt panel, but the cost per Watt will be less. That will allow you to use less panels in the end and save some hard earned money in the process.