computer usage and peak at startup

Welcome to the forum Don!

Lets slow down a bit here... Lots of questions packed in a small space.

I always like to start with knowing your loads first. And conservation... So you are on a good roll here.

Don Mac wrote: »

I have a 100 watt desktop,19 in monitor and a laptop. I am buying a DC refrigerator 72W.12 V led lighting.No TV, no Air. Question - if I change my Desktop to a new TV Box mini computer and 32 inch TV will there be significant power savings? Less peak on startup?

Changing from a 100 Watt desktop to a 20-30 Watt (average, probably around 60-100 watts peak when recharging battery pack), is a good start. 19 Inch monitor--If new LCD type, lot less power than older monitors/tv's. TV Box Mini Computer usually run off a USB type power supply, those are around 5 Watts (1 amp @ 5 volts) nominal maximum, you can find USB supplies that will supply 1.5 to 2.5 amps or more (7.5 to 12.5 watts)--But, again, 5-8 watts maximum is typical for these small USB computers. My smart phone will take ~1.7 amps at 5 volts when recharging (I have a 2.5 amp USB supply).

If you are running from a small AC inverter, get a Kill-a-Watt type power meter. They are very useful for these types of measurements and are also useful for around the house conservation measurements (how many kWatt*Hours per day is your old refrigerator using, the DVR player on "standby", etc.).

I did have a USB power supply that went bad (thought the tablet battery was failing). Found that there are lots of USB volt/amp meter options out there for measuring USB power usage directly:

http://www.amazon.com/s/ref=nb_sb_ss_c_0_15?url=search-alias%3Delectronics&field-keywords=usb+power+meter&sprefix=usb+power+meter%2Caps%2C269

LED lighting--You can get some pretty decent light from 4 to 13 Watt DC Light Bulbs. 12 volt direct LEDs are available and can work find for small system usage. If you are going to use more than a few bulbs, send the power around the house (10's of feet or longer), then 120 VAC LED lights running from a small/efficient AC inverter (along with the rest of your loads) can be a better solution. Yes, you have the losses of the AC inverter, (maybe ~85% efficient), but it is much easier to send 1 amp * 120 volts (120 watts) a 100' down 14 AWG cable than to send 10 amps * 12 VDC (also 120 Watts) 100' down a cable that is a thick as the battery cables on your car battery.

The TV may be in the 25 to 50 Watt range--Run the tv for 4 hours, then 4 hours * 50 Watts = 200 Watt*Hours (a little power to energy math--Watts is a rate of power usage, Watt*Hours is a total amount of power used).

The DC refrigerator, that will be a bit question--And is typically the number 1 power user that drives people from a small/emergency backup system to a relatively serious daily use off grid power system (and significant increase in system costs).

There are lots of options for DC refrigerators... Those that use propane (sometimes with 12/120 VAC backup power), there are Peltier effect coolers ("solid state" cooling), and there are DC compressor type refrigerators.

For occasional/emergency use, a used RV propane refrigerator can be the best choice. A DC (or full size AC) refrigerator/freezer can use anything from 250 Watt*Hours per day (small/very efficient DC or AC compressor) to 1-2 kWH per day (1,000 to 2,000 WH per day) for a full size energy start refrigerator/freezer, to 1-2 kWH per day for a Pelteir type 12 volt cooler.

For long term efficiency, a Compressor type refrigerator/freezer is best--But the compressors (especially 120 VAC types) take a fair amount of energy (typically 120 Watts when running) and can need a 1,200 to 1,500 Watt minimum AC inverter to manage the >600 VA starting current/load. Also, a frost free type refrigerator may run 500-600 Watts a few times a day to defrost the evaporator. The large(r) currents can necessitate the use of a larger AC inverter and DC Battery bank (larger Amp*Hour ratings, and sometimes going from a 12 volt to a 24 volt battery bank).

A small solar power system (power electronics, laptop computer, cell phone, etc.) may be around 500 Watt*Hours per day.

A larger off grid electronics system may be around 1,000 Watt*Hours per day.

A system to power a refrigerator + lights + computer + tv + small well pump + washing machine--Can run upwards of 3.3 kWH per day... (typical 9+ month a year off grid cabin/small home).

So, your load options...

I can't find the watt rating for the TV boxes. Reason, buying a solar 4X250w panels,2000w inverter, 2x200 ah batteries. 40 volt controller. I should be able to function with a 1000w inverter with these changes - right?

Getting a Kill-a-Watt type meter and taking it to the TV store can be helpful (if they will let you plug it in). But the small TV boxes (Roku and such) are very small power users... The TV itself, a satellite box, DVR, Internet Box/Router , etc. can all use 25 watts each pretty easily.

Next is designing the off grid solar power system... Looking at your above system, my first comment is to not buy anything until you do a few paper designs first.

Each component of an off grid solar power system has "gotcha's" that can just waste your money if don't wrong.

First, the 250 Watt solar panels... Solar panels have never been cheaper, and the major companies seem to make pretty reliable units. However, many of the >140 Watt solar panels are not really designed for "inexpensive" battery based off grid solar power systems. The typical 250 Watt system has a Vmp rating of ~30 or 36 volts. For a 12 volt battery bank with a PWM (inexpensive) solar charge controller, you need panels with Vmp~18 volt ratings to efficiently recharge your 12 volt battery bank. If you have a 24 volt battery bank, you need Vmp-array ~ 35-40 VDC rating... The typical 250 Watt Vmp~30 volts is too low to recharge a 24 volt battery bank in warm weather (Vmp array falls as the panels get hot in full sunlight/warm days).

2x200 AH batteries--Are you looking at using AGM or GEL batteries? Those are expensive and not a great choice for the first time off grid user... Batteries are pretty easy to "murder" and these are expensive to replace if you do kill them in 6 months. Second, for varous reasons, I am not a great fan of placing two 12 volt batteries in parallel--I would prefer (for example) two x 6 volt @ ~200 AH golf cart batteries in series. Them place two strings in parallel for a 12 volt @ 400 AH battery bank (cheaper batteries, you can measure specific gravity to check their state of charge, etc.).

The 2,000 Watt (at 12 VDC?) inverter--Big inverters can use a lot of power just to turn on... 10-20 Watts is typical--And that is power you have to generate/use (20 watts * 24 hours = 480 Watt*Hours per day before you have powered your first load). Also, 2,000 Watts @ 12 volts need a:

2,000 Watts * 1/0.85 inverter eff * 1/10.5 battery cutoff votlage * 1.25 NEC wiring derating = 280 Amps minimum Branch circuit fuse/circuit breaker rating

Enough copper to run nearly 300 amps to reliably run a 2 kWatt AC inverter on a 12 volt battery bank is a bit of pain.

And drawing a couple hundred amps from a 400 AH @ 12 volt battery bank will kill the battery bank in less than 2 hours... If you do not need the large AC inverter, then go smaller (no large starting loads). If you do need that much power, then you may want to look at a 24 volt battery bank and possibly even more batteries to support those higher loads.

In the end, we try to design a "balanced system". Loads (with lots of conservation) drive the battery bank. The battery bank drives the size of the solar array, and the hours of sun per day (where you live) also drives the size of the solar array (larger battery bank, larger solar array; less sun in cloudy/marine layer coastal region, larger solar array, etc.).

Hope this helps... Trying to do a solar power system in one step/post is very difficult. Focus on the loads first, then a paper sizing of the system next. Then finally pick the hardware needed to make your system.

-Bill

Few places get 7 hours of "noon time equivalent" sun per day--especially 12 months a year... I don't have any data handy for your location, but for Panama City:

http://solarelectricityhandbook.com/...rradiance.html Panama
Average Solar Insolation figures

Measured in kWh/m²/day onto a solar panel set at a 81° angle from horizontal:
(For best year-round performance)

Jan
Feb
Mar
Apr
May
Jun


6.10
6.23
6.12
5.45
4.58
4.33


Jul
Aug
Sep
Oct
Nov
Dec


4.14
4.22
4.40
4.37
4.22
4.94


Of course, we are looking for something that will sustain you loads and meet your needs--Use a genset during bad weather (or turn loads off until sun returns, use utility power if available, etc.).

So, assuming you don't get the marine layer as much, perhaps your place gets 5 hour a day or more minimum sun per day (long term average).

The power surges for computers is usually not too much--Mostly for larger systems, it was the power needed to spin up multiple disk drives--As long as you are not running a server farm, your "surge" is probably on the order of 10-20 watts for a typical computer. And for a laptop, they may run 20-30 watts nominal and use 60-70 Watts when recharging the battery bank.

Inverter air conditioners--The few that have been discussed here actually have quite low surge current--A few actually would not surge at all... Set to "low speed", the start near 300 Watts and run at 300 Watts. But--That is why you need test equipment--So that you can measure your true loads. And since you have (I think) utility power (or at least genset power for your day to day loads), you have the ability to measure the actual power usage before you design/build your solar power system.

From what I have seen, many of the more efficient refrigeration appliances tend to run something around 40-50% of the time--But if you are in a warm climate, they certainly can run 70% or more duty cycle (on time). And, as they age (condensers get dusty, refrigerant leaks, wear on the compressor, etc.), they will tend to use more power over the coming years.

You don't want to design your solar power system to be "on the edge"... Besides your power usage gradually climbing over time (aging, you add new appliances), your batteries are aging, get dust on the array, and you get a streak of bad weather--You want some extra power capacity just in case (typically, you don't want to run your "daily/maditory" loads over ~65-75% of predicted available power from solar). If you run 'on the edge', it will mean that you have to watch the system closer and spend more time managing your loads. For some people, this is "fun". For others, they have no desire to manage their off grid power system on a daily/hourly basis--Your choice.

For example, looking at your original system:

• 12 volts * 400 AH * 0.85 inverter efficiency * 1/2 days of storage * 0.50 maximum battery discharge (for longer battery life) = 1,020 Watt*Hours of "stored" energy for 2 days

The recommended array size for 5% to 13% rate of charge for your lead acid battery bank:

• 400 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 377 Watt array minimum (weekend/season usage)
• 400 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 753 Watt array nominal (daily off grid use)
• 400 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 979 Watt array typical "cost effective" maximum

And then there is the amount of daily solar power available to you... Say you get 5 hours a day minimum (long term average) and you have a 1,000 Watt array:

• 1,000 Watt array * 0.52 typical off grid system efficiency * 5.0 hours of sun per day (typical minimum) = 2,600 Watt*Hours per day

So, you can use ~1,600 Watt*Hours per day (when the sun is up) and another ~1,020 Watt*Hours per night off of the battery (or ~1,020 WH per day during dark/stormy days).

Just rough estimates--But it does sort of set some expectations of what your system should be able to do (conservatively).

Of course, if you are looking at very common 12 hour daily blackouts... A very standard setup is to basically make a "house size" UPS system first, then add solar later as funds/time permits:

1: AC to DC Battery charger -> Battery bank -> AC inverter + AC automatic transfer switch
2: Add solar panels+solar charge controller as time/money permits

There are simple AC to DC Battery chargers, and DC to AC inverters--And there are Inverter-Chargers with automatic transfer switches built it.

Either are a good solution for your needs. The one issue (I see) with using an automatic transfer switch (basically a fancy AC relay)--Is the power fail detection and AC transfer is not always fast enough to switch from AC Utility to AC Inverter and it can cause your desktop computer to crash/reboot. Laptops are nice because they have their own internal battery.

Of course, there are other options... You can get a small UPS system just for your desktop computer. Or you can run your AC inverter 100% of the time to your computer and run an AC transfer switch to the rest of your AC loads (less power wasted through the battery charger -> AC inverter circuit path when the main power is available).

-Bill

It was not a "Don Forum" comment... He was trying to slow you down a bit. Questions of on/off grid, was this a backup power system, and it turns out you have fairly regular 12 hour outages of your utility power.

We all have been down the solar road before, and we don't want to waste your (or our) money. He was just trying to be light hearted (no body language, and things get read the wrong way at times).

As I said, it works much better if we start with a simple question (I have 1234 Watts of load OR I have 200x 200 AH battery bank, etc.). When somebody puts loads + solar + batteries + etc... All into the first post, it is much more difficult for us to answer questions in a logical manner.

We all want to help, but are trying to avoid creating a 1/2 dozen more posts of confusion.

To have a "successful" off grid power system--It is a system and each part of the system needs to be sized (roughly) to play well together.

The system you are asking about will get fairly complex fairly quickly. So, we expect a lot of discussions as the details are hammered out.

-Bill