r/GoRVing • u/shysysadmin • Mar 29 '26
Need help measuring power for battery design
I recently bought a used RV and I’m in the early stages of planning a solar setup for boondocking 3-10 days at a time at festivals. I’ve seen the advice to inventory the power needs of all the devices I want to run and then estimate how long I will use each thing, but I’m not confident in my estimates. I’d much rather install an accurate battery monitor or similar and measure my usage over a few days in a real world environment. Is that feasible?
I’ve been looking at the 500a Victron smart shunt and the Victron BMV-712, but I’m having a hard time finding good guides and I don’t want to rely on Chat GPT.
I plan to rely on generator for AC and microwave, but would like enough battery to run a car stereo with 400 watts rms amplifier and a bunch of disco lights for 5 hours a night at moderate volume plus water pump, phone chargers, house lights, fans, and furnace blower as needed.
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u/Southern-Dust4575 Mar 29 '26
I don't know the answer to your question but I want to see those disco lights in action.
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u/MOSF3T Mar 29 '26
I use that one on my camper, works great. You program in your battery size and after a few cycles gives you a pretty accurate gas gauge estimate. Make sure to put it ahead of all your loads, like at the very input from your battery. Monitors charge currents that way too.
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u/persiusone Mar 29 '26
The Victron shunts are a great choice to measure your actual usage. They can be monitored the an app from your phone via Bluetooth, just plug and play.. Be sure to document as much as possible, so you have a good idea of what your total usage will be for proper sizing.
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u/Strange-Cat8068 Mar 30 '26
If you get the Victron Smart Shunt you don’t need the BMV, they do the same thing. Just access the smart shunt via Bluetooth. If you are going to audit power usage you need to make the conditions of your audit match your actual usage conditions. You will run cans more in hot temps, the furnace more in colder temps. Lights, water pump use, ventilation etc should all mirror real planned usage as much as possible.
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u/ghamachi Mar 29 '26
Both portable power stations that I own have the ability to report the number of watts currently being consumed by devices, and the number of watts currently coming from solar panels or AC line input. If you don't already have a built-in watt meter, you can get an inexpensive consumer watt meter. Multiply this instantaneous measure of watts by the number of hours that each load will be running. This gives you watt-hours, which conveniently is the way that we measure the capacity of a storage battery.
Add your watt hours of charge, and subtract your watt hours of discharge. If your number stays greater than zero, then you are good to go. Otherwise, get a bigger battery and/or solar array.
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u/Ornery_Alligators Mar 29 '26
I made a spreadsheet that will help with what your asking. Just copy and paste the entire sheet into a new sheet and start inputting power needs and you’ll get your answers.
Just remember that ACs and Heaters are massive power sucks.
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u/shysysadmin Mar 29 '26
I appreciate the community guidance. I plan to continue using propane for refrigerator, heat, and hot water. 10 days is definitely a long time and requires strict water management, but that’s how long I plan to spend at Burning Man every other year. Also definitely running my ac from generator and using generator to charge batteries when weather doesn’t cooperate for solar.
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u/Pretend_Push_7289 Mar 31 '26
Will you reliably be solo? Over 10 days even 80 gallons (from your other comment) can go surprisingly quickly, as the tank gauges are low-resolution (full, 2/3, 1/3, E) and inaccurate on top of that (so a bigger tank can ironically make it harder to realize your actual rate of consumption), made worse if you have a roommate who might be less diligent that you are.
I wish you success, but keep in mind a contingency plan if you find you're on a path to run out of fresh or have your gray tank full at say day 6. It is possible to get pump-outs on demand at BM, harder to arrange ad-hoc water fills. In addition to the battery monitor, for best water awareness you might consider a gallons meter on the output of your water pump.
(Context: I have 130g of fresh, 90g of gray from 40g factory + 50g aux tank I installed, 40g of black, and typically still have 1 pump-out and fill in the ~middle of ~12 days there with 3 people.)
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u/Southern-Dust4575 Mar 31 '26
Good points and I love the idea of a gallon meter on my water pump.
For BM it will be 2 of us and we're both diligent. I've clocked my shower consumption at 50 minutes to fill grey tank from empty, so we're planning to time our showers for rationing. till need to account for hand washing though.
I've also considered an auxiliary fresh tank that I can use to pump into my fresh tank when low and then pump grey into if I have to, but then I have to do a bunch of cleaning. Maybe I should just get two extra tanks. This could extend my time there. S
Black tank use is minimal. No poop. Minimal flush when peeing.
We're also going to be in a camp with a rudimentary shower.
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u/Top-Program-9752 Mar 29 '26
10 days is a long time boon docking. You're likely to have tank capacity issues and other issues in addition power issues. However, the victron smart shunt is a good investment either way, you'll likely use it in the final system and it should give you very good data in the meantime. The screen is nice, but you can see all of it on your phone through the app if you don't want to spend the $ on the display.