How Many Solar Panels? | Sizing your off grid system | Off Grid Permaculture

# How Many Solar Panels? | Sizing your off grid system

##### Daniel Mark Schwartz

One question that I often hear from people getting started with off grid solar is, “How many panels do I need?” While there is no universal answer, here are some common sizes, and the easy way to figure out exactly how many panel you need.

How many solar panel do you need? An average American home would need 60 ea, 100 W panels or 20 ea, 300 W panels to replace their current usage. However, no off grid home is average, and there is a lot to consider when sizing your off grid solar system.

While that figure above will give you a rough idea of how many panels you might need, the reality is that it depends a lot on your climate and usage. For the rest of this article, I’ll explain a few easy methods you can use to more accurately size your system, and potentially save yourself a lot of money.

## How to Size Your Off Grid Solar System

Getting a better estimate of exactly how many panel you need to go off grid with solar requires two lines of thinking. First we use data to estimate how much power a solar panel in your area can actually produce. Secondly, we add up how much power you will actually need. Then, we use these two numbers to find out the exact number of panels needed for your situation.

For the estimate above, I assumed an (average annual usage of 11,000 kWh)[https://www.eia.gov/tools/faqs/faq.php?id=97&t=3] with an average daily production coefficient of 3 – 5.

### Calculating Solar Potential

While a solar panel may be rated 100 W, that number only indicates a rough maximum output value. In real life use, a 100 W solar panel will produce much less than 100 W most of the day.

The best way to know exactly how much power you will get from a panel is to install one or two test panels on your property, and log their production for a year. For many of us, though, we just want to get a rough number to go by. The following map, produced by the National Renewable Energy Laboratory, gives you a good average indication of solar power potential in your area.

The number on the scale, which corresponds to the coloring of the map, gives you a number which you can multiply directly with the rating of your solar panel to get the energy production per day that you can expect on average throughout the year.

For example, a 100 W solar panel in an yellow area, having roughly a 5 on the scale, would produces 500 Wh per day. My own fridge uses about 950 Wh per day, so two panels would be enough to ideally run that fridge.

Be aware that power production will be much less than average in the winter, and much higher than average in the summer. This means you may have to size up your panels to account for lower production times, if your winter usage doesn’t also decrease as well.

While there are a lot of ways to measure how much energy your appliances are using, the simplest and fastest ways for plug in AC appliances is to get yourself a “Kill-a-Watt” measuring device. This is the fastest, easiest, and safest way I’ve found to directly measure your appliances power consumption. And, is will worth the investment considering the cost.

Measuring power usage of an appliance is super easy with the kill-a-watt. Just plug the kill-a-watt in to th wall, and plug your appliance in to it. Press the red button to show current power usage, and press it again to show logged energy consumption.

#### How to Measure Energy Use with a Kill-a-Watt

There are two ways to measure energy use with a kill-a-watt. The easiest, is to just leave the kill-a-watt plugged in to the appliance for a full 24 hours and read the kWh reading directly off the device. This is great because it handles cases where the appliance isn’t on all the time like a refrigerator or light. It also works great with appliances that have variable loads, like computers, who’s power usage depends on what you are doing with it at the time. Heavier computation requires more power.

The second way, and fastest way is to just measure the instantaneous power usage in W using the “power” setting, then multiply that by the number of hours it would be in use per day. Lets say you have a light on 24/7 that uses 3 W of power, then multiply that by 24 hours to get 72 W/h of energy per day.

If any of this talk of watts and watt hours is throwing you off, check out my introduction “Introduction to Off Grid Electrical Systems” article which goes over some of the basics.

#### How to Measure Energy Use of DC Appliances

If you plan on running and DC lights or appliances directly from the batteries, which is much more efficient, then you may want to test their power usage as well. The Kill-a-watt above only works for AC systems, so you would need a dedicated DC energy usage monitor. This model I find works very well and is extremely inexpensive. It is rated to handle up to 100 V and 100 A, which is plenty for pretty much any DC appliance.

The DC electricity usage monitor works about the same as the kill-a-watt, but it takes a little extra wiring. First disconnect the negative from the appliance and wire that to the shunt (provided metal bar with bolts). Use another wire to connect the shunt back to the appliance negative. Then run smaller wires from the energy monitor, one to each side of the shunt, and one to each of the DC input wires, positive and negative. See the diagram on the back of the device, which shows exactly what to do.

Once connected, the DC electricity usage monitor will show the instantaneous power usage, current, voltage, and total energy consumption. Just like the AC measurements above, you could just take the watts measurement and multiply that by how many hours of use a day the appliance gets. But, even better is to just leave the thing connected for 24 hours, and take the energy reading directly in kWh or Wh after that time.

### Considering Seasonal Variation

While the averages above would work to cover the yearly production of a grid connected home, off grid power requires that your system be able to produce enough to meet you needs every day of the year. Ultimately, this means you need to increase the size of your array depending on seasonal variations.

NOTE: if you live a tropical climate without a heavily overcast rainy season, then seasonal variability probably won’t impact your design.

In most climates, production significantly decreases in the winter compared to the summer, and at the same time our use of lights and electronics tends to increase in the winter as well. For most of us, that means we need to size our system for winter time usage, therefor having excess capacity in the summer.

One exception to this rule is if you plan on running energy guzzling appliances only in the summer. This may be air conditioning or large scale refrigeration. For that case, summer output could be the limiting factor in your design, requiring again more panels.

For all designs, I recommend you run the numbers twice – once for winter energy production / consumption, and again for the summer. NREL has maps by month, so choose the months with the best / worst production as well as highest and lowest power usage. Compare the two numbers for summer and winter, and continue all calculations using the season with the biggest disparity.

### System Inefficiency

One last thing to consider when sizing your panels is overall system efficiency. Every stage of the power system looses a bit of energy due to heat — especially charge controllers, batteries, and inverters. For definitions of these terms, see:

Charge controllers and inverters typically have max efficiency point is the 90s of percent. Common batteries used in solar systems range from 80% – 90% efficient from charge to discharge. This means you might get about 90% of the panel power output when running a DC load during the day while the batteries are charging, but only about 76% when running from batteries. Likewise, an inverter for AC power adds another cost, giving roughly 80% of the power during the day and 68% of power when running from battery.

Almost manufactures of charge controllers and inverters advertise their products efficiency on the box on in the product data. However, this will always be “max efficiency” which means the average efficiency will be worse. Always allow extra room with marketing numbers when planning a design.

To see how component efficiency impacts your design, divide your power consumption by efficiency as a decimal. For example if you needed 5.5kWh per day, and estimated an 80% efficient system, then your true needs would be 5.5 / 0.80 = 6 kWh.

### Calculating How Many Solar Panels You Need

Once you know the amount of power you need per day and the amount of power a panel can produce in a day from the first section, just divide the first by the second. So if you needed 6kWh per day with a daily production value of 2.2, then you could produce 220 per 100 W panel. In that case you would need 6000 / 220 = 25 panels to produced the full 6k Wh.

## How Big of Panel Should I Use? 100W vs 200W vs 300W

Panels come in all sizes and are rated in the number of watts they produce under “standard conditions”, which are similar to a typical bright sunny day. The most import factor for most people when choosing a panel is the cost per watt or kilowatt. Right now, here are the approximate prices for panels of different size:

This makes smaller 100 W solar panels the cheapest for the amount of power you get. Check out my recommended solar panels to see which panels are currently the best on the market.

Additionally, smaller panels have benefits in that they are easier to handle, especially on a ladder or high places for a DIY installer without industrial lifts or scaffolding on hand. Also, having more, smaller panels gives you more flexibility with wiring and array voltage. Check out my complete solar guide for to see why this matters and how higher voltage arrays can save you money —

## How Much Do Solar Panels Cost?

Solar panels can be easily bought at about \$900 per 1 kW rated. This cost only include the panels themselves, and not necessarily with mounting hardware, wiring, control electronics.

In most off grid solar systems the cost of the batteries is about the same or more than the cost of the panels. Depending on where your panels are in relation to the system, wiring and mounting hardware can also be noticeable fraction of the total system cost.

For more details on how to budget for an off grid solar system:

## Poly-crystalline vs Mono-crystalline Solar Panels

Manufactures usually sell solar panels in both mono-crystalline and poly-crystalline varieties. While mono-panels are typically marketed as “more efficient” and higher tech, poly-crystalline panels are almost always the best choice for an off grid solar system.

The most important factor to consider when buying solar panels for off grid systems is the cost per watt of power output. In the end, a 100 W poly-crystalline panel produces the same amount of power as a 100 W mono-crystalline panel in the same conditions. Yet, a 100 W poly- panel only costs about \$80 while a 100 W mono- panel is about \$100. Given the difference in price alone, for the same performance, cheaper poly-crystalline solar panels are the way to go.

On paper, the biggest advantage for mono-crystalline is their size. Mono- panels are about 10% smaller than poly- panels for the same amount of power production. However, this advantage is only relevant if you are sizing a system for a very space constrained situation like putting solar panels on top of an RV or tiny home. And even then, it is often more cost effective to invest in more energy efficient appliances rather than spend the extra 25% on mono-crystalline solar panels.

Additionally, the more expensive mono-crystalline panels may actually perform worse on hot days than their cheaper poly- counterparts. One reason mono- panels can be smaller is the fact that their face is noticeably darker (a good way to tell the two types apart). However, their smaller size and darker face makes them more prone to over heating and more difficult to cool. Hot panels produce less power, and so the extra power you get by fitting in a few more panels can be pretty quietly lost due thermal efficiency redction

## What To Do If Your Solar System Doesn’t Produce Enough Energy

You may wonder, what do I do if my existing solar system doesn’t produce enough electricity to cover my needs. In that case you have two options — reduce your usage or increase production.

Before investing on more power input, it is always prudent to investigate whether an upgrade of appliances, more efficient lighting, or completely removing an unnecessary power consumer would solve the problem. For many off grid homes, saving power is much cheaper than increasing production.

However, you may find yourself in the situation where you need to increase your overall power production. The first thing to do is to make sure you are getting the most out of your existing panels (see the next section). After that, it is time to consider adding additional solar panels to your system.

To add more solar panels, if your system is relatively new, maybe a year or less, then the simplest thing to do is to buy more of the identical solar panels that you already have, and add additional parallel strings. Check out my solar free designers guide if you are not clear on series and parallel solar panel wiring —

The second option you have to increase the size of your solar panel array is to to install a second, completely separate array and charge controller. This option is preferable if your solar panels are older, or you plan on buying a different brand or size of solar panel for your system.

The reason for a second charge controller, is that every solar panel has a operating voltage where you get the maximum amount of power. This is called the “maximum power point,” and many solar charge controllers (MPPT controllers) dynamically adjust the array voltage to stay at this point. By mixing and matching brands, sizes, and ages of solar panels in a single array, you are likely to be reducing the overall power output of the array, because the maximum power point of all the panels is unlikely to be the same, and therefore some of the panels will be running at reduced power output.

Another benefit of having two or more charge controllers is that you have a backup in case on of the charge controller breaks.

## How to Get the Most Out of Fewer Solar Panels

If you are just getting started with solar, it can be easy to make a simple mistake in designing and installing your solar system that could be causing your solar system to under perform. Here are a few things to double check to make sure your solar panels are operating at peak efficiency.

While it is common sense that you should put solar panels in as much direct sun as possible, many people don’t realize just how bad partial shading can be for their solar power output.

Even a partial shade of one of the squares on the face of a panel can reduce the output of that panel by 1/3rd. If you have multiple panels in series, which is a good idea to reduce wiring costs, then one panel at 66% production means the whole string is at 66% efficiency.

Just a tiny, hand sized spot of shade can severely impact the output of your solar array.

• Be very careful to avoid any self shading of the panels
• Watch out for shadows from chimneys, vents, and poles
• Wire panels in parallel rather than series (generally lowering array voltage)
• Place panels so shade hits only one string of series panels at a time.
• Avoid midday shadows rather than morning/evening if you have to make a choice

The very best way to reduce shading is to place the panels in a completely sunny place, and remove any possible obstructions. Allowing some panels to cast shade on other panels is a particularly common mistake to make and could be detrimental.

If some shade is unavoidable it try to limit the effect by placing the panels so only on series string is effected. Likewise, solar panels have internal strings of series cells (see above diagram). If you can, orient the panel so that only one or two of the series strings is shaded as the shadows approach, rather than all three.

### Ensure Good Cooling

Solar panels all have a negative thermal coefficient, which means that they produce less power the warmer they get. Most panels are tested at 25 C (77 F), but in the summer, they can very easily get much hotter, and thus produce less power than they are rated for.

Also, higher temperatures decrease the overall lifespan of the panels. Not providing proper cooling to your panels will take years off of the usable lifespan of your system.

Solar panels must always have 6–8 inches of space between them and the roof or any backing material to allow for free airflow behind the panels. It is best practice to install panels in places that are naturally cooler. This means roofs are not always the best choice compared to a ground based mounting, since a roof in July is usually much too hot.

### Clean Off Dust and Pollen

This may seem obvious, but it is always worth planning for how you will clean off your solar panels. Too many systems suffer from reduced power output because the dust and pollen is never cleaned off.

Yet another reason why I recommend not installing solar panels on the roof, having them closer to the ground where they can be easily seen and hosed off on occasion can mean a lot more power in the long run.

## What Effects Solar Panel Power Output

How much power your panel produces in a day, and over a year, depends on how many hours daylight you get, and how bright that light is. The farther north you go, the less power you will get from a solar panel, because it is darker for longer, and the sun spends more time lower in the sky.

Also, if you live, like me, in a climate that is frequently cloudy, then you will produce much less power per panel, than someone who lives in mostly clear areas. For example, a solar panel installation in San Francisco would produce a lot less power over the year than an installation in Colorado, where it tends to be sunny most of the time.

Other preventable conditions impact solar panel output as well, including whether the panels are overheated or dirty. Also, the use of a MPPT charge controller will often produce more power from the same panels compared to a PWM charge controller.

### How much solar do you need for off grid?

To replace the average households usage, you would need a 6kW – 10 kW system depending on climate. However, there are many opportunities for energy efficiency and minimal living that would drastically reduce this number.

### Can you go off grid with solar panels?

Yes. Living off grid with solar panels is both possible and relatively affordable with current technology. By many estimates, solar power is actually cheaper in the long run than buying power from the grid