Your Electricity Needs

How long until my batteries are empty? In this post, we’ll show you how to use your own data to answer common questions like this. Before you continue, we recommend reading Part 1 Electricity Basics for Everyone and Part 2 Power System Components.

If you already have a rig, then you probably know your house battery capacity in amp hours. If you have a RoamRig power system, you can find your capacity in the Victron App (Settings > Battery > Battery Capacity). Everything in this post will be relevant if you have lithium house batteries, even if you don’t have a RoamRig power system.

Knowing your battery capacity is important because without that, it’s impossible to estimate how long it will take to charge and discharge the batteries. After all, it will take longer to fill and use a 50 gallon water tank when compared to a 5 gallon bucket. If you don’t know the size of the tank, then how can you estimate how long it will take to fill it?

Remember, amp hours (Ah) and watt hours (Wh) show the amount of work done over time, as described in our earlier post. Since watts are voltage-agnostic, Wh is our preferred way to measure power over time. If you know the Ah of your system, convert it to Wh with this equation: 

While we may refer to power systems as 12v, if you’ve used a battery monitor then you’ll have noticed that voltage is usually higher than 12 – up to the high 13’s when full. Since voltage fluctuates, we’ll use 12.8v as our reference voltage for 12v lithium batteries, which is a nominal approximation.

Start by converting your battery capacity from Ah to Wh. For our examples, we’ll use a 960Ah system, which is the size of RoamRig Second and Third Generation Base power systems.

When you know the Wh of your system, you are ready to answer some of our most common questions. Here are few.

Estimating power generated over time:
How long will it take my 215w solar panel to charge my batteries? 

We’re starting with a tricky question. Even in OPTIMAL conditions, the panel will probably never supply the full 215w. The strength and angle of the sun, power loss in the wires, and loss when power passes through the DC-DC Solar Converter will all decrease panel efficiency. Never plan to get 100% production out of your solar.

However, we’ll pretend that a 215w panel produces 215w of electricity. If you have 12,288 Wh of battery capacity and your batteries are totally empty, then this is how to estimate how long it would take the panel to charge your batteries to 100%:

12,288Wh ÷ 215w = 57.2 hours to fully charge

Yes, you read that right. Over two days to fully charge via perfect solar, if it were possible to be in the sun 24/7. This is why we always discourage anyone from relying solely on solar power to charge and power their systems unless solar has been drastically upgraded.

Estimating power consumption over time:
How long can I use a _______ without completely draining my batteries? 

This is the equation to use for anything that consumes power. From kitchen appliances to air conditioners, Ebike batteries to space heaters – the list goes on. And the math is simple! 

We’ll assume the appliance always uses a consistent amount of power (not true for stovetops, refrigerators, air conditioners, etc.). Based on that, estimate what percentage of your batteries it will consume in one hour, regardless of how charged they are:

Wattage of Appliance  ÷ Your Wh Capacity  = % of your capacity the appliance will consume in 1 hour

Let’s imagine a 1,500w slow cooker being powered by our battery bank with 12,288Wh:

1,500w ÷ 12,288Wh = 12% of your battery capacity consumed every 60 minutes

Depending on what else needs power, 12% might be a lot of power or barely noticeable. It depends on your particular needs.

Comparing production to consumption:
Will solar be enough to power my fridge and fan when the van is in storage? 

The quick answer is most likely not unless you have significantly upgraded your solar. This time, we’ll consider both the amount of power consumed AND the amount of power generated, then compare the two.

Wh Produced - Wh Consumed  = Must be Positive Or Batteries Will Deplete

A 215w solar panel, even if it produces at peak power for 8 hours during the day, will only generate approximately 1,700Wh of power (215w x 8 hours = 1,700Wh).

We’ll assume that the refrigerator and fan consume an average of 7a. Over 24 hours, they will consume 168Ah (7a x 24 hours = 168Ah). Convert that to watts, and it is roughly 2,150Wh consumed (168Ah x 12.8v = 2,150Wh). 

In our example, every day the fridge and fan will consume 450Wh MORE than the solar panel produces. In this scenario, the difference will be made up by depleting the house batteries until they are out of power, and then everything will shut down.

As you can see, solar is not a guarantee of running anything continuously. If you’ll be storing your van, then please follow our 1st gen storage guide or 2nd & 3rd gen storage guide.

Not sure about the power consumption? 

If you do not know how many watts something consumes, then you can use your battery monitor to check. For vans with RoamRig power systems, use the Victron app to see the NET number of watts/amps being consumed or produced. 

Net power is loads (things like lights, stove, fan, etc. which are power leaving) and power sources (things like solar power, alternator power, shore power, which are power coming into the battery) added to each other. 

If loads are smaller than sources, the Victron display will show a positive number. For example, with the engine running, even if using electrical loads, you might see a positive power number. This means your batteries are currently being charged EVEN with loads turned on!

If loads are greater than sources, the Victron display will show a negative number. For example, even when solar is working, loads often exceed the power that solar generates, so you may see a negative number on the display. This does NOT mean that solar isn’t working, but rather that your solar power generated is smaller than your current electrical needs. Read our article about checking solar for more info.

If you want to determine how much power a specific thing uses, here’s an easy way to do that. To check power consumption of your Starlink (or fan, or lights, or anything), turn it on. The battery monitor might show -200w. With Starlink off and everything else the same, you might see -50w. So, this would mean that Starlink consumes approximately 150w when being used (-200w – -50w = -150w). 

What about real data?

As noted, our examples use OPTIMAL conditions and assume there is no power loss. That’s just not realistic. If you want to monitor your system with more precision, or simply want to skip using equations to make estimations, then use your battery monitor app. 

Start with a full charge then operate your system for a few hours, perhaps with that electric dog bed plugged in. After several hours, how much has the SOC changed? Is voltage still above 13%? If the voltage is still high and the SOC has just changed a few %, then you can confidently keep that bed plugged in all the time. However, if voltage is low or SOC has gone down a significant amount, then you’ll probably want to be judicious and only plug it in when your pup needs the extra heat.