Solar System Sizing

How to determine the size and
specifics of your PV solar system.


Solar system sizing methods.

There are two basic ways that people go about determining the size and specifics of the solar system they will need (solar system sizing).

Method #1: 

The "Make Some Now, Add Some Later" Method

Some people handle solar pv system sizing by going ahead and building a good standard sized solar system first (like one of the systems we show you how to build on this site in Solar Panel Wiring), implementing it onto their house and then using whatever solar power they get out of it in conjunction with electricity from the power company.

These people can also add more solar panels to their system in the future and increase their solar power production gradually as their funds allow. They generally build less power than they will need and sort of "learn along the way" (through actually using it) how much more power they will be requiring. This solar pv system sizing method is kind of like "playing it by ear".

Over time, they can build their systems up to provide all the power they need and even eventually use no power from their utility company at all.

This is a very common approach (to solar system sizing) for the do-it-yourselfer as it allows them to get their foot in the "solar door" and start benefiting from solar power, quickly, for the least expense and without too much tedious planning.

Method #2:

The "Make Enough For All Your Needs Now" Method

The other way that people determine the size of the solar system they will need (solar pv system sizing) is by actually figuring out exactly how much power their home consumes and then building a PV system that can handle that load.

Please note that even if you do decide to start small and build over time you should still figure out what size system your household will require for all your energy needs, so you have a good general idea of what size system to eventually strive for.

Determining this calculation requires you to do some investigation in and around your own home. More specifically, you will need to check the kilowatt usage on your electric bill and measure the available sunlight in your area.

From these calculations, you can determine how many watts the solar system you build will need to have to accommodate all of your home's energy needs.

Click here to learn the standard procedure for solar system sizing - by determining the watts your system needs to produce to accommodate all your home's energy needs (doing an energy audit).

 

Understanding how many watts, volts and amps you'll need for your appliances.

Pv system sizing tutorial.Regardless of whether you decide to make a PV system big enough to accommodate all or just some of your energy needs, you are still going to need to at least understand how many watts, volts and amps you'll be producing and whether it will be enough for all (or some) of your specific appliances and power storage capacity needs.

This is an important part of the solar system sizing process, especially if you are going to be adding solar power as you go (over time).

You will need to ask yourself some basic questions related to solar pv system sizing like:

  • How many watts will I need for my specific power use?
  • How many volts should my system produce for my specific appliances?
  • How many amps do I need in order to be able to produce solar energy fast enough for my usage needs?

I will explain these questions below, along with how watts, volts and amps work. You can learn even more about electrical fundamentals by clicking here.

Solar sizing - watts.Question 1:

How many watts will I need for my specific power use?

Watts represent the amount of power produced or used. Think of it sort of like your "power reserve".

When it comes to pv system sizing, you need to make sure you have enough watts to power all of your specific appliances.

Sometimes the watts required for certain appliances are more than you may have directly available or stored. Eg. Trying to power a refrigerator with a PV system that produces very little power (watts) per hour or with a battery bank that has very little power (watts) stored .

Increasing or decreasing the watts your system can produce and store is accomplished by adding more solar panels and batteries to your PV system. Add more panels to make more power. Add more batteries to store more power.

So let's say you want to power a laptop computer with your solar system.

You need to check your laptop's watt rating (check sticker on the back of computer and multiply the volts x amps to get the watts).

If your laptop is rated at 72 watts, then this means it needs 72 watts of power per hour to run. So your solar system must also be able to either produce or provide from the battery bank up to 72 watts or more per hour in order to have enough juice to power the lap top computer.

Determining your daily, weekly or monthly watt usage

So how do you determine your watt usage for the whole month, or week or day?

The answer is: You have to calculate the watt hours.

Watt hours / Kilowatt hours

Watt / Kilowatt hours is the measurement used by your electric company to charge you on your bill. It represents the number of watts consumed multiplied by the number of hours you consume it for. One watt hour is equal to consuming one watt of power per one hour.

Watt hours = # of watts consumed  x  # of hours

A kilowatt is equal to 1000 watts. It's just another way of saying 1000 watts, only it appears neater and is less bulky looking on your utility bill. So one kilowatt hour is equal to consuming 1000 watts of power for one hour.

To calculate the amount of watts/kilowatts a specific appliance consumes (and therefore will need your solar system to produce) you need to find out two pieces of information.

  1. The watt rating of the appliances you will be using.
  2. And how long you use each appliance.

Watts

So, if for the time period of 1 day, you used your 72 watt laptop for 4 hours, you would have used 72w x 4hrs = 288 watt hours (that's not even a kilowatt) therefore the amount of watts that would have to be readily available from your solar system battery bank would be 288 watts for the whole day.

To calculate the total amount of watts you consume for all your appliances or a specific group of appliances, you would have to go around to all of those appliances, get the watt rating off each and multiply each by the number of hours you would normally use that appliance for.

Then add up all the totals and you will know approximately how many watts/kilowatts of power you need your solar system to be able to produce to accommodate those appliances for the time period that you specify (month/week/day).

As you can see, you're daily solar power potential greatly depends on how many watts you are able to capture and store during the daylight hours.

If your solar system is rated at 300 watts total, this means that the most your system can produce / store is 300 watts of power for each hour your solar panels are in optimal sunlight conditions, but this number may be much less in non-optimal sunlight conditions.

Depending on the size of your system and how many hours of sunlight you have available during the day, you can produce and store energy in your battery bank all day long and use it as you need it.

With our 300 watt system example above, if you had 6 hours of optimal sunlight per day, you could potentially store 300w x 6 = 1800 watts per day. That's way more than enough juice to power your laptop which only requires 288 watts for 4 hours use (or 72 watts per hour).

Always check your own electronics or appliances for the right watt rating but just to give you an idea of what to expect, here are some common wattages for some common appliances:

  • Clock radio : 10 watts
  • Dvd player : 40 watts
  • Small tv : 54 watts
  • Light bulb : 60 watts
  • Lap top computer : 72 watts
  • Ceiling fan : 120 watts
  • Lcd tv : 200 watts
  • Hand-held blender : 350 watts
  • Refrigerator : 500 watts
  • Coffee maker : 800 watts
  • Toaster : 1000 watts
  • Microwave oven : 1000 watts
  • Hot plate : 1100 watts
  • Power saw : 1350 watts
  • Vacuum cleaner : 1600 watts

Although some appliances like the hot plate may seem to have a higher than normal watt rating compared to a tv, these appliances are typically used for smaller periods of time so the overall wattage used balances out and isn't as big as you may think.

Basically, the more watts your solar system has, the more power you can produce & store in your battery bank for use whenever you want.



Solar power system sizing - VoltsQuestion 2
:

How many volts should my system produce for my specific appliances?

Volts represent the pressure of electrical flow (the push).

When it comes to pv system sizing, you need to make sure you have enough volts in your system to power your specific appliances (which also have a volt rating on them).

If you are operating an appliance with a high voltage rating, you will need your system / battery bank to have that same voltage (actually a little higher) to supply enough power to it for it to work.

Increasing or decreasing the voltage is accomplished through the arrangement / wiring of your solar panels and your battery bank.

So let's say you want to power your laptop computer with your solar system.

Volts

You need to check it's volt rating. This should be on a sticker located on the underside of the computer itself.

If your computer is rated at 24 volts, then your solar system must also be able to produce up to 24 volts or more in order to power that device.

It's ok to power a device rated at lower voltage with a system that produces higher voltage, but if you tried it the other way around you wouldn't have sufficient "push"  to power the higher voltage appliances.

The different voltage ratings are: 12 volts, 24 volts, 48 volts, 120 volts and 240 volts.

If your solar system is rated at 36 volts, you'll be able to power appliances up to 24 volts, but not 48 volts, 120 volts or 240 volts. If your solar system is rated at 54 volts, you'll be able to power appliances up to 48 volts, but not 120 volts and 240 volts. If your solar system is rated at 126 volts, you'll be able to power appliances up to 120 volts, but not 240 volts. If your solar system is rated at 252 volts, you'll be able to power appliances up to 240 volts.

Always check your own electronics or appliances for the right volt rating but just to give you an idea of what to expect, here are some common voltage ratings for some common appliances:

  • Plug in burner element (Hot plate) : 12 volts
  • Clock radio : 12 volts
  • Lamp : 12 or 24 volts
  • Lap top computer : 24 volts
  • AC contractor unit : 24 volts
  • LCD TV : 120 volts
  • Table top oven : 120 volts
  • Water heater : 240 volts
  • Dryer : 240 volts
  • Oven : 240 volts

Basically, the more volts your solar system has, the more varieties of higher voltage appliances you can power with it - provided you have the watts to supply the power. Don't worry, we show you how to best balance everything out in the next section of this website.

However, one way of getting around this is to use (or buy) appliances with low voltage ratings. Imagine how much money you would save if you bought some of your home's appliances at the RV and camper store. You may be surprised at what's available in very low voltage ratings.

Solar power sizing - AmpsQuestion 3:

How many amps do I need in order to be able to produce solar energy fast enough for my usage needs?

Amps represent the intensity (and amount) of current and thus determine the size of the wire needed.

When it comes to pv system sizing, you need to make sure you have enough amps to make / store power as fast as (or faster than) you use it. And you also need to insure you have the right size wire to handle the current.

So if the solar system you build has a total of 7 amps, then you would need to buy 7 amp wire. Actually, to be on the safe side, you should buy 8 or 9 amp wire, just so you know it can handle the current.

The more amps your system has, the faster you can make/store energy and therefore the more energy you will have available to use - that is if you have enough batteries to store it.

When you increase the amps of your solar system, it's like you're literally using a bigger wire that allows more power through at once.

If you have enough amps as well as enough batteries, you can increase your production and storage capacity so you never run out of solar power.

Increasing or decreasing the amps is accomplished through the arrangement /wiring of your solar panels. You'll also need more batteries to store the extra power.

Amps

So let's say you needed your solar system to produce power super fast because you had high energy needs and you wanted to take advantage of your huge battery bank's large capacity by charging it back up faster after use.

To do this, you would need to arrange the panels in your solar system to increase the total amps. Don't worry we show you how to do this (while best balancing out everything else) in the next section of this website. But first let's learn about "amp hours" and how they affect our solar system.

Solar system sizing - amp hours.


Amp Hours

Amp hours represent how much electrical current can flow per hour, so if your battery says 105 amp hours on it, then this means you could charge it to produce 105 total amps during the course of an hour.

This is meant to give you an indication of the storage capacity of the battery and how long it takes a battery to discharge.

The more amp hours you have in your battery bank the longer your total power reserve would take to deplete.


Now that you know how solar system sizing works, you will have a better idea of what size solar energy system you will need - to handle your specific appliances and household loads.

In the next section, we show you how you can add multiple batteries to your system and increase your energy reserve at any time in the future. However, if you want to learn more about what types of solar batteries are available and how to choose the right ones, click here first.

You will also learn (in the next section) how to add multiple solar panels together, how to wire your panels and battery bank to produce different desired volt & amp results and how to balance everything together so you can get the most out of your solar system.

Now that you have learned about solar system sizing, click here to continue to the next section, Solar Panel Wiring.