We were out in the boonies…far out in the boonies….and Paul was daydreaming
“I have a dream” gushed Paul “a dream of football”
“ooooookay” I responded, trying my best to sound supportive. Clearly we’d been spending too much time out here and Paul was going nuts…but I thought it safest to humor him at this point.
“No, no, no”, he answered excitedly (and with a bit of crazy eye in his expression, I thought). “You don’t understand”. “My dream is to watch 3 football games in one day while boondocking and using the internet without draining the batteries…and I think we can do it” he chirped, skipping happily around the RV
Several things fell into place in my noggin at once. The fact that it was football season (that would be American Football for my European friends -> where they carry the ball, believe it or not), the fact that Paul had been doing covert solar calculations for several days, and the fact that he’d been grumbling about needing more Amps. At least he wasn’t going crazy…in the normal sense I mean.
As a fellow geek I was immediately interested. Not in the football, mind you, but in the concept of the usage plan. Paul explained that he wanted to watch 3 football games in a day (so, TV/satellite on) plus have the internet on all day (because knowing me well, he knew I would need something to do), plus normal boondocking usage….all fully supported by our solar system…and with the least extra expense. Now, that was an interesting problem, from a geek point of view.
In order to fully understand the scale of this dream I’m going to throw some numbers at you. Those of you of technical persuasion will no doubt be gripped with excited interest at this point. For the rest of you, go ahead and skip to the bottom-bottom line at the end, and spend the rest of your day blissfully math-free until tomorrow’s post.
In numbers terms Paul’s dream was about a management of Amps that needed to follow the universal and oft-quoted rule of “what goes in must come out”. In this case our generation (from solar) would have to match what was going out (to usage). Ideally we wanted to end the day with fully charged batteries too, but that was what we in the business call a “stretch goal”.
The Generation Model
Our system has 600Watts of Solar Power. Using solar flux models (see our post on panel tilting HERE and the model HERE), Paul calculated the approx. solar generation we would expect in January in the SW desert with 45-degree tilt of the panels.
Approx. solar flux in SW in Jan at 45-degrees tilt = 5.95 kWhours/m2
Our measured solar surface (6 total panels) = 6 x 0.78 = 4.68 m2
So, total expected generation on a sunny day = 5.95 x 4.68 = 27.85 kWhours
Our panels/system are ~10% effecient (this is very typical of solar systems in general. Panels only really convert ~10% of incoming flux)
So actual generation = 2.785 kWhours = 2,785 Watt Hours
This translates into ~232 Amp Hours (divide the above by 12V)
Bottom Line = We expect to generate ~232 Amps Hours of solar energy from our panels on a good, sunny day. Lots and lots of assumptions in here, but it’s a decent, simple start.
The Usage Model
Here’s where we get into the really fun stuff. We calculated our potential usage model, and backed it up with measurements done at night with our Xantrex LinkLITE monitor (LOVE that thing).
(i) Our Inverter is a Magnum ME2012
(ii) For TV/satellite we have Direct TV, a satellite dish on our roof and a Sharp Aquos 32-inch LCD TV
(iii) For internet we use a Verizon modem, MBR1000 Router, Wilson RV Antenna and Wilson amplifier (see our full set-up HERE).
(iv) We have 2 computers which each run ~3 Amps
(v) This is a “fudge factor” which includes anything else we might want to do during the day (e.g. run lights, a bit of microwave, a few hours of furnace etc.)
Bottom Line = In this usage model we expect to use ~360 Amp Hours in a full day (24 hours)
The Bottom-Bottom Line
If you put it all together this is what you get
Total Solar Generation in one day = 232 Amp Hours
Total Usage in one day = 360 Amp Hours
Total Drain on the Batteries = 232-360 = -128 Amp Hours
Now, that’s the number we wanted to be zero, ideally, and clearly we were WELL over. We have 440 Amp Hours of AGM batteries (=220 usable amp hours if you don’t go below 50% discharge, which should always be your goal with deep-cycle batteries) so we can handle the load, but it’s not where the dream wanted us to be. Clearly this needed some creative geek-thought, especially if we didn’t want to pay out the wazoo to get it done.
Coming Next -> The Final Chapter. How we solved the elusive 128 Amp Hour Gap….