But when I reply with – “Well, how much power do you use in a day?”, I almost always get a blank stare in response.
We encounter only a handful of RVers who have any idea of just how much power various electrical devices they own use, and many with little knowledge of how much power they use in a typical day conducting everyday tasks.
Before investing in a solar system, battery upgrades (such as our lithium batteries), or other electrical tweaks – it makes sense to first invest a bit of time into figuring out just how much power you need, where the low-hanging fruits for efficiency upgrades are, and how much consumption you can control by adjusting your usage patterns.
Audit yourself. Dig deep. Learn the loads on every circuit in your rig.
Even if you never intend to try and live for a while boondocking with nothing but batteries and the sun to sustain you, the knowledge you gain in an energy audit is useful when connected to metered power, smaller hook-ups than you’re used to or even just overnight dry camping off batteries.
Knowledge is power. Especially knowledge of your electrical system.
Watt Is Power, Anyway?
Power is defined as “the rate of doing work”, and is measured in a standardized unit called the “watt“.
When talking about electrical systems, many people often use “Amps” as a shorthand for power. If you are comparing two 12V DC circuits to each other, or two 120V AC circuits – the amp measurements (the current of electrons flowing through a wire) does indeed give you a good relative sense of the power demanded by a circuit.
But no end of confusion results when comparing DC to AC circuits – because a 12V DC amp carries just 1/10th the power of a 120V AC amp. For example – a 5 amp DC light on a 12V circuit is burning 60 watts of power, enough to light up a room. But a 5 amp light on a 120V AC circuit is using 600 watts, and likely lighting up the neighborhood.
To keep things straight, you need to rely on the most fundamental electrical equation: V*A=W
In other words, voltage (volts) times current (amps) = power (watts).
If you want to measure and compare power, you need to do it in watts. Amps alone gets you just halfway there.
Measuring Power Consumption
Determining how much wattage a given electrical device consumes can be tricky.
The simplest and crudest way is to just flip it over and look.
Most AC appliances are stamped with a wattage somewhere on them, though this is often the maximum possible wattage it can consume – not necessarily what you will see in typical use. Some other devices (particularly “brick” power supplies) are stamped with a maximum input and/or output amperage – and you can multiply volts times amps to get a rough max wattage.
DC devices tend to be less consistently marked – they may tell you nothing at all, or just the fuse size they need (5 amps, for example) – but only if something is wrong will that amount of current be drawn, blowing the fuse. Multiplying 12V * the fuse size at least gives you a max wattage for doing rough comparisons with.
But it is much much better to actually take measurements, rather than rely on rough estimates and stickers on the bottom of gadgets.
To measure AC power consumption – one incredibly invaluable tool for the job is the Kill A Watt Electricity Usage Monitor, an inexpensive gadget that plugs into a wall outlet – and gives you an accurate wattage readout of whatever is plugged into it.
But not all your AC devices can easily be routed through a Kill-A-Watt for measuring (like a built in microwave or hardwired lighting, for example) – and the Kill-A-Watt has no way to measure DC loads.
To get a real comprehensive picture of the power consumption on an RV, you have to go right to the source. The battery.
And to keep track of the power flowing out of your battery, you need a battery monitor.
A real battery monitor is one of the most essential pieces of equipment on any truly self-contained RV, and it shocks me that they are so rarely standard equipment.
An electrically independent RV without a battery monitor is like a car without a gas gauge. If you fill up at every stop (always going to campgrounds with hookups) you can get by without a gauge, but it is haphazard to venture far from hookups without one.
The most basic battery monitors are very analog affairs – just an ammeter that shows the current flowing from the battery, and a voltmeter that shows the battery voltage. You can do the math yourself to work out the wattage.
Smarter battery monitors actually act like a gauge – measuring every amp that flows in and out of the battery, calculating an accurate “gas gauge” percentage of how much battery capacity you have remaining.
The smartest battery monitors can actually do the power calculation math for you – and thus give a readout in watts directly, and not just separate “volts” and “amps” displays.
We have a Victron BMV-702 battery monitor in our bus, which gives a wattage readout as well as telling us the battery volts, amps, amp-hours consumed, and percentage remaining. It is the smartest battery monitor that I know of, and it works great even if you do not integrate it with a Victron inverter or CCGX display panel.
On our prior two RVs, we used a Blue Sky Energy IPN Pro Remote battery monitor, which integrated with our Blue Sky solar controller. Another solid choice used by many RVers is the Bogart Engineering Trimetric battery monitor, or the Xantrex LinkLite battery monitor. Some inverters have battery monitors designed to integrate into the inverter control panel too, like the Magnum Battery Monitor for Magnum Inverters.
Whether you have a basic ammeter or an actual wattage readout, having some way to measure battery current opens the doors to conducting a real energy audit on your RV.
And if you don’t have a battery monitor… Before you invest in solar panels or new batteries – get one!
This should be the top item on every RVer aiming for solar’s electrical upgrade list!
Conducting A Comprehensive Energy Audit
To get an accurate sense of how much power your RV can consume, you need to break it down one circuit at a time.
Start by pulling the plug – disconnect from shore power and disable any solar charging. Turn off your inverter, turn off every power switch, close all AC breakers, and pull all DC fuses. Shut off everything you can find.
Hopefully you’ve set aside plenty of daylight to do this – because now you are sitting in a very dark RV. If you’re relying on a water pump for fresh water, also fill up a few water bottles, and set aside a reserve for manually flushing your toilet. You’re going to need it to last through the day ahead. And make sure you’re starting with an ample charge on your mobile gadgets.
The only thing that you should leave powered up is your battery monitor.
DC Power Auditing
Start by taking an initial baseline “everything off” reading – recording the voltage, current, and watts reported by your battery monitor.
Hopefully this will reveal an extremely small amount of power being used – since this overhead is being consumed 24/7. In our case – our baseline drain is 3.5W, which powers our battery monitor and lithium BMS.
If you see more power being consumed here then you expect, track it down – there must be some circuit that you didn’t find to pull the fuse to.
Once you have every circuit shut down, it is time to enable them again – but only one at a time.
Start with circuit #1, and put the fuse back in. Record any changes in the baseline – does it jump up more than a trivial amount? Do you know why?
Now it is time to turn on everything that is attached to circuit #1, one at a time. Record the current (and wattage) for each device, light, fan, or gadget. Be sure to give things enough time to stabilize – the power may bounce around a lot when something is first turned on.
If your RV has poor documentation on the electrical wiring – now is your chance to play detective to figure out just what devices, lights, and outlets are tied to each circuit. Take copious notes – if you ever have an electrical problem in the future, this information can come in quite useful.
For every device that you measure – record the full range of usage scenarios. Measure the lights at their dimmest, brightest, and typical levels. Measure the roof vent fan at high, medium, and low. Don’t forget to measure how much power it takes to run water for showers and doing dishes – it takes power to run a water pump. And so on.
Once you have tracked down every device on circuit #1, pull the fuse again. And then move on to circuit #2.
Once you are done – you can subtract the baseline from every measurement to calculate the actual power needs of every DC-powered device on your RV.
You may discover that you have some mystery circuits in your rig that do not appear to be connected to your house batteries. They may be connected to your starting battery instead, so try disconnecting the starting battery to know for sure. It is good to identify these circuits, because over-using them might leave you unable to start your rig someday!
AC Power Auditing
Now it is time to fire up the inverter.
Some RVs have almost all the AC outlets fed through the inverter (we do), while others have only a few outlets and appliances capable of being powered via the inverter off the battery. You will only be able to accurately audit circuits that can be powered by the inverter, but if you are preparing for solar-powered boondocking these are the only ones that will matter to you anyway.
Once the inverter is powered up – leave all the downstream circuits switched off, and take a new baseline reading. This will give you a sense of how much overhead power your inverter consumes just being on, and idle. With many inverters, this overhead can be substantial.
When connected, our Victron MultiPlus 3000 inverter ups our overall baseline to 7W even when switched off – and all the way up to 29W when on, even without a single downstream circuit connected!
In other words – it makes sense to shut it down overnight if we are being energy frugal!
Now that you have a new baseline, the process works exactly the same as the DC audit. Turn on one circuit at a time, measure any changes to the baseline to find and understand any parasitic loads that you can not turn off, and then turn on and measure one AC device, appliance, or gadget at a time that you might use on that circuit.
Be sure to test a range of situations – we measured every cooking mode on our convection / microwave, the power consumption of actually watching a BluRay movie, and even the different brightness settings on our computer monitors. Test every variation you can think of – you may discover some surprising corner cases where power consumption spikes unexpectedly!
And you may find other places where you are being unnecessarily frugal – for example, Cherie’s monitor uses essentially the same power at medium and low brightness, so there is no gain to be had suffering by dimming it all the way.
If you are testing the power consumption of things with thermostats – let them cycle on/off and get both readings. You will see huge swings in refrigerator or air conditioner power consumption depending on whether or not the cooling compressor is engaged.
When you are measuring AC circuits – be sure to record the DC current recorded by the battery monitor. Your inverter may also display the AC current or even calculated AC wattage being used, but this is not taking into account the inverter overhead that comes from converting DC battery power into AC power.
You can compare the power from the battery to the power reaching your appliances to get a better sense for just how efficient your inverter is. Typically, inverters are 80% – 90% efficient at turning DC power into AC power, so always remember that AC appliances are wasting up to 20% of your power to inverter overhead!
Factoring in the Power Factor: On DC circuits, watts are always equal to volts * amps. But things get a bit more complicated on AC circuits, because some loads (particularly motors) cause the alternating current sine waves measuring the volts and the amps to shift out of alignment with each other, making the V*A reading actually higher than the real power being delivered. This shift is called “Power Factor” of the load. If the power factor is 1, watts and V*A are aligned. But as the power factor gets smaller (and close to zero) the measured V*A will become more and more distorted. The Kill-A-Watt can display both VA, watts, and the calculated power factor. Some inverters only display the VA output (since it is easier to calculate). But if you look to the DC readings from a battery monitor, you can always determine the actual wattage being pulled from your batteries.
The higher the VA, the less efficient an inverter can operate due to power factor losses. A Kill-A-Watt can help you identify these inherently inefficient loads.
The All-On Baseline
Now that you have taken a look at every circuit individually, it is time to put all the fuses and circuit breakers back to their normal positions.
Now take a new baseline reading – first with the inverter off, and then with it on. These will give you a sense for your minimal usage scenario – letting you know just how frugal you can be without needing to pull fuses and flip breakers. This will show you all the parasitic loads that you have hanging around – like microwave clocks, standby circuits for devices with remote controls, and any other overhead that comes from just having stuff plugged in even if it is turned off.
You might discover that you can save a lot of power by plugging some things into a power strip, and flipping the power strip off when not needed.
In our case, our DC baseline is 8W, and our AC baseline with the inverter powered on is 53W.
Bigger Picture Auditing: Set Up Some Typical Scenarios & Measure Some Tasks
Now that you have a solid grasp of the power usage of the individual circuits, you can go even further and set up some typical usage scenarios to get measurements of. For us – we were concerned with how much power we use while both of us are working on our computers, with our big monitors on, a cellular booster boosting, maybe a light or two in the living room, and some music on the stereo.
We also tested scenarios like “movie watching in bed” and set up tasks like “cooking a favorite meal” or “doing a sink full of dishes”.
A lot of scenarios and tasks use a variable amount of power – so you can take an average reading, or record a range, or use your battery monitor to determine how many amp-hours (or percentage points) are drained from your battery for a given task.
We’ve learned that baking a pizza in our convection oven can use 5% of our battery capacity. Knowing that – we can decide when we have the spare capacity to treat ourselves.
Ongoing Audits & Sanity Checking
Even after you have done an energy audit, pay attention to your battery monitor and ongoing usage. You will find that some things use different amounts of power in different situations than you initially measured, and you will eventually begin to develop an intuitive feel for your RVs electrical system.
For example – over time you will be able work out reasonable estimates for daily power consumption for devices that cycle on and off – like a residential fridge, which uses a lot more power when the compressor is on than when it is off. On warm days in the summer, the compressor will be on a much larger percentage of the time than on cold winter days – and it will make a huge difference in the daily cumulative energy drain.
As you get a better feel for your electrical system, you will naturally become much more energy efficient.
Consider your energy audit a living document – measure new things that come into your household, and every so often double-check your old readings too.
Working Out Your Energy Budget
Just like with personal finance – some people do best with a detailed plan.
If that is your style – once you have all this data, you can work out a “typical day” daily energy budget.
Set up a spreadsheet that calculates how much time you expect to use a given device each day, and multiply it by how much power it uses. It is easiest to work in watts and hours, but some prefer to use amps and hours. If you do use amps – make sure you only use the DC amps measured by the battery monitor!
Microwave: 1755W * 0.25 (15 minutes) = 439 Whrs
Microwave: 135A * 0.25 (15 minutes) = 33.75 Ahrs
Total up your daily estimated usage, and compare that to your battery capacity. You can roughly convert your battery capacity in amp hours into watt hours by multiplying by 12 – so if you have a 250Ahr usable battery capacity (always keep in mind your battery type’s recommend depth of discharge), that means you have 3,000 watt-hours of battery to play with given a full charge.
And that means that 15 minutes of daily microwave cooking will use up nearly 15% of your usable battery capacity!
If you are planning to rely on solar entirely – once you have an energy budget, this gives you a goal of how much energy you need to recoup every day to keep your batteries full.
Most people, once they actually work out an energy budget, realize that they need way more solar than they initially thought. And – way more focus on energy efficiency!
But the upside of having an energy budget is that you know when you are ahead and can splurge. When we know we have energy to spare we treat ourselves to something that takes a lot of power to cook. Or we treat our friends to a movie on the side of the bus with the power-sucking outdoor video projector.
Our Energy Audit
Your results will of course vary, but it can be helpful to see what power consumption others have measured. So here is our latest energy audit that we spent all day yesterday collecting:
(if you’re reading this on RSS or e-mail, you may need to click through to the full post to see the embedded spreadsheet)
It can take a full day to dive this deep into your electrical system and power measurements (the data collected above took nearly 10 hrs to test & compile), but the information is so worth it. We’ll continue to measure various scenarios, and add them to the spreadsheet as we continue to learn and tweak our own systems. As well as fine tune where exactly our sweet spots are usage versus solar collection.
We already have some tweaks inbound before our next boondocking adventure – including a 12v television.