When I was first building DAPHNE (our van) I agonized quite a bit over the right amount of batteries have, or really, ultimately, the right amount of battery storage to have. In hindsight, I think I accidentally made the right decision.
200 amp hours of battery storage with 300 watts of solar seems just right (maybe a little overkill on the solar) for us in our van, even during the winter when we are running the heater all day, getting very little solar, and like to park at ski destinations without moving for a few days.
Basic Battery Theory
There are plenty of good resources for information about batteries, electricity, circuits, power, and that kind of stuff. I am not going to go into that. But of the many good pieces of information found in a BatteryStuff.com article, one thing that’s good to note is that deep cycle batteries do not like being discharged below 50% charge – so, simplifying everything else, a 200 AH battery bank will only provide 100 AH of usable storage.
But the basics employed here are this (simplified, condensed, and based on my feeble understanding of electricity):
- Watts = Volts X Amps. So if you know the wattage and you know the volts (12V system for us) you can calculate the amps
(Amps = Watts / Volts).
- Electrical gadgets use power (unit of measure: watts). Solar panels generate power (unit of measure: watts). Batteries store power (unit of measure: amp-hours [the amps used multiplied by the duration of use]).
So as an example, if I have a 20-Watt electrical device on my 12-volt system I can calculate that it requires 1.7-Amps of power and thus will use 1.7-amps every hour. If I run that device for 24 hours I will use 40 amp-hours from my batteries.
And that’s the end of basic battery theory – before I embarrass myself further.
To first start figuring things out, I did what most people do, looked at all the electronics I wanted to power or recharge in the van, calculated how many amp-hours each would take and then summed the total. All of this was of course theoretical, based on how much I thought I would use, and how much the electronics manufacturers documents their power use. But I thought it would get me close. The spreadsheet excerpt at right shows the results of my early calculations, estimating the power use I would have – about 90 amp-hours daily. WOW – that sounds like a lot!
With 200-watts of solar cells on the roof and decent solar efficiency and about 5-hours of full sunlight I estimated that I could recover 50 Amp-Hours per day (based on 60% efficiency). So that’s kinda concerning if I am using close to double of that per day. Now with 300-watts of solar on the roof I got up to 75-Amp-Hours estimated recovery – not bad. With more sun or better efficiency I estimated that I could catch up on a daily basis.
Of course, all of this is just theory – turns out, no bearing in reality.
Actual Van Camping Power Use Performance
Below are some shots of my power monitor for a couple of cold weather examples. These really highlight the real world power draw when camping in cold, dark conditions. In these time periods, here’s generally what we are running:
- Lights on and off throughout the day
- Espar / Eberspacher D-2 diesel heater continuously (its cold out!)
- Vent fan intermittently
- Dometic Fridge
- Charging cell phones and other small electronics
- Water pump – very intermittently
That’s about it. So as you can see, we don’t have a lot of big power users – we very rarely run the inverter and cook using liquid fuels. So it should not come as a surprise that our actual power use is much lower than my early calculated estimates. In reality, we tend to use around 1 to 2 amps per hour or 24 to 48 amp-hours per day. On sunny days, the solar panels have no trouble keeping up. On cloudy snowy days, when the solar panels get covered in snow and ice – well… that’s why we are alternator intertied too.
Unfortunately, at this time I don’t have good data on solar performance. But what I can say, is that after a good day sitting in the sun the batteries are always back to 100%.
All in all, I am happy with out decision to install 200 amp-hours of battery capacity and 300-watts of solar capacity. It seems to fit our needs just fine.