How Much Solar Power Do I Need

A solar power system is great for an RV. Nobody wants to hear gas-powered generators running all day long, and sometimes it's not allowed or practical. Solar cells can unleash your RV and keep you running.

I could write a book on all the details and “what if”s involved in solar power. For most systems, however, you don't need all that detail. Understanding the basics and putting together a good system is a reasonable DIY project for most people.

Many people have solar panels to supplement or entirely power their systems. Let's explore how to determine how much solar power you need.

A quick overview of power

I won't go in depth here, but just so we're on the same page:

Volts: most RV electrical devices will run on 12V DC power or 120V AC power.

Amperes, generally called Amps: The amount of current. Think of current as the speed of a river. Slow river, low current. Fast river, high current.

Power: Watts – we're going to simply multiply voltage by amps to determine the amount of power in watts used when the device is running. Watts of power is the energy used at any point the device is used.

Energy: Watt-hours – after we determine how much power something takes and how much we use it, we determine the overall energy needs.

How we use energy in calculations

We convert everything the an equivalent of a one hour time period. Running a 10 watt device for 24 hours continuously takes the same amount of energy as something that takes 240 watts for one hour.

One last note about power. There's a myth that solar panels don't directly power your devices, they just charge the batteries. While this is a useful way to think of the situation for quick planning purposes, it's not true.

I'll spare you the details but the reality is that you could theoretically disconnect your batteries completely when you have sufficient solar power coming in through your charge controller. I say theoretically because many charge controllers do not allow this.

Your solar array runs your electrical devices and charges your batteries when it's bright enough. Your battery bank only supplies energy when there's not enough solar power.

Why does this matter? When you're sizing your battery system, you don't need to account for battery capacity during sunlight. You should, however, consider having extra capacity to account for cloudy and rainy days.

Uh oh, there's math involved

Wait, wait, I'll make it as easy as possible. I'm going to give you a worksheet to guide you through the math, as soon as I explain what needs to be done and why.

Even if you're not afraid of math, the worksheet is a fast and easy way to make sure you install enough power for what you need.

The math isn't all that complicated. We're going to break down everything you want to power and how much it gets used to determine the overall power needs for each item.

Start at the end – where the power goes

I'm a big fan of working backwards. Figure out where you want to be then figure out how to get there.

Let's start out by figuring out how much power everything takes. Walk through your RV and write down everything that takes some sort of electrical power – no matter how big or small.

Make sure to include things that have control panels. My water heater runs on propane but uses a small amount of electricity to operate.

Write everything down for now, we'll trim out things later

Don't worry yet about deciding that some items won't matter. Maybe you can do without that toaster, right? Later we'll discuss how to determine if some don't matter.

Write down the device voltage and amperage used if you can find it. If you can't find it, you can often google the device for an answer.

This is the device input, not output. My laptop power supply outputs 19v at 4.62 amps, but the input – the power I need to provide for it – is 120v AC at 0.8 amps. In my RV I use a 12V power supply. It's best to keep devices on the low voltage system if possible.

Also, my laptop doesn't often use this much power. The power supply could constantly keep up with the most intensive activities. My average usage is less than half of that, but I estimated slightly higher.

It's okay to estimate if you need. I have a couple of USB fans that aren't labelled. In the plan I just assume they take 5v at 2 amps.

Some devices might specify the watts used – this is better yet. Fill in the voltage used because we'll use that information later when we talk about inverters. You can skip the amp column and simply input the watts used in that column.

Let's talk duty cycle

Duty cycle is a percentage of time something is actually being used. Almost everything will only be used a fraction of the day. Instead of percentage we're going to list is as number of hours in a day. If your furnace runs at approximately 10% duty cycle, that's 2.4 hours per day.

You can see that I'm using a simple 24 hour day as a basis. This is logical since we're going to be determining the amount of electricity from solar energy you can get in a day. Your battery bank will even everything out so you'll have power at night.

My fast phone charger takes about 15 watts, but I only need to charge my phone for about two hours per day when camping (poor signal drains the battery quickly). Even if I leave my phone charging overnight, after it's charged it no longer takes a significant amount of power. Thus my phone charger takes about 30 watt-hours per day.

How are you supposed to know duty cycle? It's okay to guess if you need

Some things are difficult to determine. What's the duty cycle of the refrigerator? RV fridges use a different technology than residential fridges, so google could steer you wrong on this one.

If we don't know, we simply guess but guess high. We could estimate that it runs 50% of the time, or we could even estimate that it runs 100% of the time if we want to be really conservative.

The current can also be difficult to find sometimes. One option is to find the fuse for the device and assume it draws that much power. My fridge uses a 5a fuse on the 120v line, so I could assume it draws 600 watts.

Another option is to measure the power. There are various devices that can do this, from a Kill-a-watt plug-in power meter models, to a whole RV Electrical Management System (EMS) like we use, to an AC/DC clamp-on multimeter models. These options should provide you with a more accurate number.

Use Cases

We want to come up with different scenarios – cases – that we'll encounter. This can also be helpful if you're considering a large system but need to compromise due to space or budget.

The best case scenario we're going to ignore – beautiful weather and you're out hiking all day, not really using power. There isn't much point in calculating this, because you can't assume this will happen every day.

Your cases should include very hot weather and very cold weather. Just how hot or cold depends on where you like to RV, but consider the most extreme areas and times of year.

Complete all possible use cases so you can see which one will drive your solar panel needs

In my case, my cold weather case assumes a furnace duty cycle of 25%, or 6 hours per day. No fans, and a higher duty cycle of laptop use.

When it comes to hot weather, air conditioning is a massive power consumer. We didn't want to purchase a system for this. In summer we make sure our camp site has power available, and we have a generator if the power is out.

We still have a warm weather plan, in case it gets unseasonably hot when we're camping with no shore power. This case assumes all fans on all day and night.

You might have more use cases than these. Perhaps you sometimes work while on the road. Maybe you sometimes bring guests along. Each of these scenarios should have its own use case.

Determine the daily energy needs for each use case

If you plan on using the spreadsheet I provide, this will perform this calculation for you. Even if you're doing this on your own the math is pretty easy. For each line, determine the energy needs.

Energy (watt-hours per day) = voltage * amperes * duty cycle (hours per day)

Inverter losses

All of the 110V items will run through your inverter. That inverter takes in 12VDC and turns it into 120VAC power.

It does a great job at this conversion, but there is some heat generated, which is energy loss. Some inverters are more efficient than others. If you can't find values from the manufacturer data, use a value of 90% as an estimate.

Divide the item's energy need by this efficiency, not multiply. We need the final result to be bigger because we need extra power due to this loss.

Converting from 12V to 110V creates heat as a by-product - requires more energy

For example, 100 watt-hours divided by 0.9 is 111 watt-hours. That extra 11 watt-hours is the heat generated from the inverter.

Side note, inverters can draw power if they're on and not being used. Think of it like a car idling, you're still using gas.

Some inverters barely use any electricity when idling, but some draw quite a bit of power. If you plan on leaving your inverter on all the time (versus only when using a 110V device), then be sure this is included in your energy plan.

Add up everything for each use case to determine the total energy needed in a day. For example, our cold weather case requires watt-hours per day.

I have a number! I know my energy needs. Now what?

All right, you've determined the power consumption for a daily basis, in various cases. Now let's determine the solar panel system you need to power this.

There are two things that will drive your system. How many watts of solar panels do you need? What capacity of RV batteries do you need? Let's take those one at a time.

How many solar panels do I need?

As I mentioned earlier in the post, there's enough detail that could fill a book. For this calculation you're getting generalized averages. These are typical values you can expect if you're not in the extremes.

Solar panels come in many different sizes, but an extremely common size is rated at 100 watts. Let's assume that is what you'll use.

It might be daytime for 12 or more hours in a day, but you can't expect 100 watts for all of that time. We can't assume you'll always have ideal conditions.

The reality is that you can expect 300-500 watt-hours on an average day from that 100w panel. Yes, that means the equivalent of only 3-5 peak sun hours.

I recommend shooting right for the middle of that range, 400 watt-hours on each of the 100-watt panels. Different locations have different average conditions.

Location definitely matters!

If you're in Arizona where it's very sunny you might choose to estimate higher. However, if you're in Seattle where it's often cloudy you might choose to estimate lower.

For each case, you simply divide the required energy (watt-hours) by your estimates watt-hours per panel. Then you round up to determine the number of solar panels to purchase.

Let's say you determined that you need 3400 watt-hours per day. You expect each of your 100 watt solar panels to provide you with 400 watt-hours per day. 3400 / 400 = 8.5, so you round up and need 9 solar panels to provide this much energy.

Cut power usage where you can to reduce your solar power needs!

Can you reduce the number of panels you need? The spreadsheet is a great way to find out where you can trim things.

In our case, we like to have our air purifier running all the time if the windows aren't open, but it consumes over 1000 watt-hours per day. We don't truly need it, it would cost a lot to keep that running all the time.

We can drop down to just 6 solar panels if we decide not to use the air purifier when on solar power. Furthermore we could reduce our needs to just 4 panels if we didn't use our electric kitchen devices and used the propane stove and grill.

This cost saving strategy will also apply to battery power, which we'll calculate next.

Many people ask if they can run their RV air conditioner with solar power. The answer is yes, but you need an extremely large array.

If you pursue this, you will find that it's best to replace your standard air conditioner with an extremely efficient model. Further options include only air conditioning the bedroom with a small A/C.

How much battery storage do I need?

Here is another area where it could get really complicated, trying to optimize battery life and account for multiple cloudy days. This article is focused on the amount of solar panels, so I'm only going to provide a very brief discussion of battery power.

For the purposes of this article, we're going to again focus on reasonable estimations. With lithium batteries we'll assume you choose to use 80% of the rated capacity. For lead-acid batteries (including AGM batteries) let's assume you choose to use 50% of the rated capacity.

Those values will lead to long battery life, and are common values used. It's worth looking into the details of your battery technology so you can determine what's best for you.

You don't need to rely on batteries 24 hours a day

When it's full sun out, your solar charge controller is providing your electrical system with power for your devices and will charge your batteries.

Once again, even thought it's daytime for many hours, we act as if the sun isn't out that long. We can expect at least 6 hours per day when you won't need battery power, or 25% of the time.

For simple calculations, let's assume your power needs are constant. That is, you use the same amount of power at 2am as you do at 2pm. This definitely might not be the case, but the math does get more complicated if you want to be more precise.

We need battery power to supply 75% of your calculated value. We also need to account for the depletion amount, as mentioned above.

Let's go back to our example where we want to supply 3400 watt-hours from the solar array. 75% of this amount is 2550 watts. My RV batteries are lead acid, and I want them to last so I'm only going to let them get 50% depleted. Side note, this is also a nice way to have a buffer in case of a cloudy day.

To supply 2550 watt-hours but only use 50% of capacity, I need 2550/0.5 = 5100 watt-hours from my batteries. Since this is a 12V nominal system, 5100 watt-hours / 12V = 425 amp-hours (aH).

Less solar needed, less battery needed!

That's a pretty large battery bank. As mentioned above I could choose to skip the air purifier and reduce my energy usage to 2300 watt-hours per day. This leads to a battery array of only 288 amp-hours.

If I switched to a lithium battery system I would only need about 150 amp hours of battery power since the nominal voltage of lithium-ion batteries are about 14V. If my budget allows for it, it would be a good idea to switch to a lithium battery bank, but this means I'd have to update my entire system.

As a side note, there's a common rule of thumb that your battery bank size should be (in amp hours) half the size (in watts) of your solar panel array. Amp hours and watts don't relate directly, but they correlate well.

Don't get stuck on this value rule of thumb. It can be a quick-and-dirty way to estimate budget and size needs.

What's next?

The next step is to determine if your desired solar setup will fit on your RV roof! The size of your RV clearly makes a big difference here. Our motorhome has a lot of available roof space which could hold many roof-mounted panels.

A small travel trailer can't accommodate many panels due to its small roof space. There are flexible solar panels available to accommodate curved surfaces.

Another option are portable solar panels, where you set up panels on the ground when you set up camp. This has the advantage where you can move them into the sun if you have partial shade.

They often come with legs that can adjust the angle to optimize light capture. The best choice would be to include a combination of panel types for versatility.

The worksheet you've completed will also help determine if your inverter has enough power. Add up the total wattage of devices that could run concurrently. Your inverter power should be larger than this number.

Final thoughts

It can seem like a big chore to determine your needs. It's critical to maintain your budget yet ensure you get the power you need.

Use my downloadable Solar Panel Worksheet as a guide and determine your needs. Check out different scenarios and determine areas where you can cut power needs. Take a look at the tab with my analysis, and begin your own analysis on the prepared tab.

Note: calculations include inverter losses for all lines. Simply omit the inverter losses for 12v devices, or leave it in for a little extra cushion in your analysis.