A general rule of thumb is you get half the daylight hours at 50% output and half the daylight hours at close to 100%. Can generally use 4 hrs. @ 50% and 4 @ 100% to calculate how many amps that panel will put back into the battery during the three seasons. You get a bit more than 8 hrs. but at low output and you generally don't hit 100% so it all works out as a guide.
You have to balance usage against panel amps provided to determine if you can replace all your usage in the battery, and if not how many days you can tolerate a deficit in panel production from rain or clouds and still stay at 50% of charge on battery. Batteries deteriorate if discharged below 50% charge.
Make a list of items that draw power.
- Water Pump
- Furnace blower (if cold enough weather)
- Charging/running technology gizmos (camera, laptop etc.)
- Satellite dish
- Refrigeration (if applicable, as noted before not for 3 way)
- Any appliances such as coffee pot, blow dryer etc.
- Health equipment such as CPAP or Oxygen generators.
Second step figure out how many hours you use said devices per day.
- Lights x hrs.
- Water pump x hrs.
- Fans x hrs.
This stuff is very individual take lights how much you use lights depends on if you stay up late reading or playing cards or hit the sack shortly after you come in. Or get up before full daylight.
Have a shower? Shower every other day or shower daily while camping? Then you will run the water pump more or less than someone else.
We don't even take a TV or have a stereo, many do. We might use the laptop more than most for photo editing, video or music.
At the end of this step you will have a list of how many hours YOU use what devices.
Then look at the device and find its amp usage to add to your list.
- Lights x hrs. times my bulb amps = ?
- Water pump x hrs. times my pump amps = ?
- Fans x hrs. times my fan amps = ?
At the end of all this you can add up exactly how many amps you personally expect to use in a day. Every appliance will have it's power draw listed on it someplace. That is how you know how many amps it draws. You already figured out how many hours it will be running.
bulbs might use 1.9 amps while LED bulb might draw less than a hundred milli amps. You might have an energy efficient fan in the vent or be running a higher draw fan on the counter for a breeze.
The panels are generally listed as being so many "watts" but what you want to know is how many AMPS does it produce. The panel in its description should state how many amps it produces. A 100 watt panel might say it produces 5.2 Amps.
That you can plug into the rule of thumb to be:
4 hrs. producing 1/2 output. 4 @ 2.6 (.5 x 5.2 amps) = 10.4 amp hours
4 hrs. producing full output 4 @ 5.2 amps = 20.8 amp hours
So you can replace a total of 31.2 amps of use from your personal list of power usage per day. If your battery is a 100 amp hr. battery then you can use 50 amp hrs. without charging or draining the battery too low.
I would use very little with 2 LED lights
, no water pump, might run a fan. laptop plus some phone & camera charging so might only use 10 amp hours a day. I could go 5 days just on battery and if I got even one sunny day that put 31.2 amp hours back in that would add 3 more days I could run.
Folks with a shower, furnace
and who watch TV for a few hours each night would find my "perfect" solar panel and battery left them with a dead battery after one day of rain.
I think a lot of people may be over powered. Better safe than sorry I guess but it is only by calculations based on your power use that you can figure out what you need the solar panel to replace, and how much battery storage you require to allow for some rainy days.