While there are many posts on how to estimate your solar needs this method measures your usage using only inexpensive equipment which also has a lot of other uses. Summary: Set up your rig with fully charged batteries to simulate a boon docking day but with power available. Use the $30 Kill-A-Watt device with adapters on the shore cord to measure your power usage for 24 hours. The watts-hours (Wh) measured would be the same as an equivalent boon docking day including DC device usage and AC device usage using an inverter. This method then provides actual power usage which can then be used to determine the solar equipment requirement. If the usage was 1500 Wh for example then the solar could be sized to supply that power while boon docking.Solar sizing is involved considering weather, location, mounting, time of year and other factors but at least the power usage requirements are know. Details: The Kill-A-Watt device uses a 20A plug so you may need adapters for your rig. It’s limited to a maximum of 15A. It’s very important that your start with fully charged batteries so that you are not also measuring AC amps needed to charge the batteries. You are measuring Wh(or power) and as long as it’s equivalent to a boon docking day the Wh used are what you will need to replace in the battery. Solar Sizing Example: For 1500Wh measured add a 500Wh margin for 2000Wh. Flat mount the panels and using 70% efficiency the solar power needed is 2000/0.7 = 2857Wh. Using 4 hours of solar yields 2857/4 = 714W. 3x 250W panels would be more than needed or perhaps compromise and install 500W. Miscellaneous The Kill-A-Watt is a very useful device for measuring power from any plug in AC device and I highly recommend it. You can also leave it plugged in with no connected loads to monitor your rigs AC voltage. Use it in the rig or at home.Updated to reflect Wh.

Usage is measured in watt-hours, or amp-hours. Watts is the instantaneous draw.

My roof size is going to tell me how much solar power I will need haha...I can mount two 120WATT Panels in the front part of the roof on each side of my fantastic roof fan. Then I can maybe get a 240WATT solar panel across the rear part of the roof. That is going to give me around 480 WATTS of solar panels to use.I don't know how others plan to use their solar panels but for me they will be used to supplement my generator run times to re-charge my battery from their 50% to 90% charge state before I lose the high the high sun each day.My generator will run for about an hour to get my 50 AMPS PLUS initial charge current when the batteries first get hit with the 14.4VDC charge voltage. After that the DC charge demand tapers back to around 8 AMPS or so which means I can shut down the the generator and just allow the 20AMPS of DC charge from my solar panels to run until the high sun goes away in the rest of the day.My WATTS being used will be determined on how many batteries I have in my setup...Been living quite well using just 255Ahs of battery capacity for my one day/night run but want to increase that soon to two banks of 300AH's from each bank with one on my trailer and the second bank in the truck bed... We usually camp at places that all have generator run time restrictions in place so going solar is really going to help out in that respect...My DC Power demand will be restricted to what I can use coming from my battery banks... Using the generator is my first priority followed up by what I can produce from the solar panels. My absolute priority is start each evening with at least a 90% charge state on my batteries if I want to make it through the one day/night running the things we want to be running...Pretty simple for me figuring out what my solar needs will be haha... Two panels in the front and a bigger panel in the rear...Roy Ken

2oldman wrote:Usage is measured in watt-hours, or amp-hours. Watts is the instantaneous draw.Oops - The Kill-A-Watt does measure the accumulated watts or Wh and I'll update the post.

kWhAdd 20% for conversion losses and it'll put you in the ballpark. Inverter efficiency and battery Charge Efficiency Factors are the reason for calculating at 120%.

RoyBConsider mounting the panels above the fan. Works for me.Open fan vent

Measuring Your Solar Needs – An Easy and Low Cost Method

22 Replies

JiminDenver
Explorer II
Sep 28, 2015
Interesting approach but I'd want to run the test on a cold rainy day with the lights, furnace and TV running. Even then I would double it for bad weather and the fact that once you have solar, you tend to relax and use more power. I feel the same about the battery, the freedom the excess gives you makes it easier to keep it in balance.

The other thing is it takes a while to get your routine down to the point that you know how much power you use daily like Roy does. It took a few years of boondocking and paying attention to the morning resting voltage to get a handle on how much power just the trailer uses. That's nothing compared to the inverter, it really sucks the power. Our day to day changes a lot. Run the little AC and it spikes that day, use the pie irons instead of the cook top and it slumps. So we generally use up to 200 Ah a day but that's growing still.
pianotuna
Nomad III
Sep 27, 2015
Hi CA Traveler,

You would need to disconnect the battery bank and run the 'house' on just the converter. Almost always some charging is going to be going on. I wonder how power factor 'fits' into that equation. I know my PD is 0.7 power factor. Does that mean using 30% instead of 10?

CA Traveler wrote:
My thoughts on efficiency of the Kill-A-Watt approach. The converter is only used to supply house loads while on shore power so the Kill-A-Watt power used by the house loads would be higher the power used from the battery for the same loads. I’d probably use 10% for the converter. My Magnum ME2012 specification says 95% peak efficiency so I’d probably use 10% for it.

There are also efficiency in the solar system to consider like controller efficiency and weather, mounting, location, etc.

If I was going to consider efficiency I'd add 10% but overall I’d likely just ignore efficiency because I’d add a margin for the solar panels anyway. :R
CA_Traveler
Explorer III
Sep 27, 2015
My thoughts on efficiency of the Kill-A-Watt approach. The converter is only used to supply house loads while on shore power so the Kill-A-Watt power used by the house loads would be higher the power used from the battery for the same loads. I’d probably use 10% for the converter. My Magnum ME2012 specification says 95% peak efficiency so I’d probably use 10% for it.

There are also efficiency in the solar system to consider like controller efficiency and weather, mounting, location, etc.

If I was going to consider efficiency I'd add 10% but overall I’d likely just ignore efficiency because I’d add a margin for the solar panels anyway. :R
pianotuna
Nomad III
Sep 27, 2015
Nice way to get a number. Once you have the number of watt-hours simply divide by 4 to cover most eventualities.
RJsfishin
Explorer
Sep 27, 2015
I can't imagine setting up solar system w/ running a generator every day as part of it.

I have a generator too, (2 in fact) but on a normal sunny day you won't hear either one of them. I kinda thought that was the whole idea of solar, DUH??

Solar needs is easy to figure out,.....start w/ 300, if that ain't enuff for your average use, go to 4-5 or 600 watts. Perdy simple, huh ? :)
CA_Traveler
Explorer III
Sep 27, 2015
Well it was new to me since I just came up with the idea. There are a lot of posts on these boards concerning using solar calculators, measuring individual device usage, using a battery monitor, etc. All good information to help understand solar.
Naio
Explorer II
Sep 27, 2015
Honestly, this is not news.

No need to spend $30, though. Your local electric utility probably loans them out for free. Or ebay -- that's where I got mine.
CA_Traveler
Explorer III
Sep 27, 2015
RoyB

Consider mounting the panels above the fan. Works for me.

Open fan vent
MEXICOWANDERER
Explorer
Sep 26, 2015
kWh

Add 20% for conversion losses and it'll put you in the ballpark. Inverter efficiency and battery Charge Efficiency Factors are the reason for calculating at 120%.
CA_Traveler
Explorer III
Sep 26, 2015
2oldman wrote:
Usage is measured in watt-hours, or amp-hours. Watts is the instantaneous draw.
Oops - The Kill-A-Watt does measure the accumulated watts or Wh and I'll update the post.

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Measuring Your Solar Needs – An Easy and Low Cost Method

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