# Newbie question .. please answer if you can

Posts: 2,511 admin
hey guys ,
i am really interested in solar energy ..and solutions i have read up on seem to big for me ..
i am looking for help to see what i would need to

* power a 800W heater ( battery , inverter , solar panel )

* power a thinkpad laptop ( battery , inverter , solar panel )

* Power a fan ( i can get the specs if needed )

I would really like some help in figuring out what i need to run these things for 8 hours or so .

Please help me understand if you can

Thanx

## Comments

• Posts: 27,905Super Moderators, Administrators admin
Re: Newbie question .. please answer if you can

I guess you are in the Reno, Nevada area? And you want solar power...

First question is "how much power"... Simple way is in Watt*Hours or kWatt*Hours (same units as your home electric power meter).

For example:

2 hours * 800 watt heater = 1,600 watt*hours (1.6 kWhours)
8 hours * 60 watt laptop = 480 watt*hours
4 hours * 100 watt fan = 400 watt*hours
==========================================
2,480 watt*hours (2.48 kWhrs) guesstimate of load.

Now, you can either go grid tied or off-grid... Guessing that you want off grid battery power system...

The battery should be 2x the capacity that you want to run (a normal flooded cell storage battery should not be discharged by more than 50%).

So, for yours (assuming 24 VDC system)...

Batt Cap = 2,480wh/(0.50 cap * 24 vdc system) 207 amp hours @ 24 volts DC...

Roughly, that is about the size of 4 good sized car batteries. And it will run your loads as listed above... If you need more/less, adjust the formulas...

Now, you need to recharge the battery with solar panels... Your power needs have to be derated by solar panel / inverter/ battery inefficiencies... In this case, assume 80% / 85% / 80%):

Power = 2,480 WH / ( 0.8*0.85*0.8 )= 4,844 Watt*Hours

Looking up Reno in a solar isolation table (fixed collector tilted to noon summer or winter sun):

Best summer month = 7.7 Hours of Sun (July average)
worst winter month = 4.2 Hours of Sun (December average)

And assume that you want to do this every day (using average numbers)

4,844 W*H / 7.7 Hours = 630 watts of solar panels
4,844 W*H / 4.2 Hours = 1,153 watts of solar panels...

Pick a solar panel, battery, and inverter from here (sponsor of this site, good prices and good products)...

Solar panels are roughly \$5/watt--\$3,150 to \$5,767 (summer/winter panels)

4x AGM deep cycle batteries \$255 each--\$1,020 (AGM are very good batteries, flooded cell may cost 1/2 that)...

Exeltech Pure Sine inverter 1,100 watt @ 24 VDC input = \$612

MX-60 solar charge controller ~1,400 watt max @ 24 vdc = \$498

~\$5,000 to \$8,000 to power your system as estimated for 1 night of battery use, and one day of solar charging...

If you want three days of no-sun, use 3x the number of batteries.

If the batteries are not around room temperature, but exposed to very cold weather, you will need to derate them and add more capacity.

If you only need to run on batteries for 1 day at a time, and can wait up to three days to recharge--then cut the number of panels by 1/3.

If you have to have power and need to ride through bad weather, you probably will need a generator and a battery charger or an inverter/charger unit.

If this is a full time use, solar is very nice--it generates power day in and day out all year long (weather permitting). If this is for a summer or 2 weeks out of the year type deal, a nice generator is hard to knock (price wise).

Generally, going solar would require you to do as much conservation as possible... For example, electric heat from solar PV panels is horrendously expensive. You can buy or build solar thermal panels at \$0.50 per watt (vs \$5 per watt for PV panels) and they would be 1/4 the area for similar wattage from solar PV panels.

The laptop--I just took a wild guess here--but I have seen numbers that range form 8 watt to 60+ watts depending on the "power" of the computer and what it is doing (back light on/off, heavy processing or just typing, used for 8 hours or on standby for 6 of the 8 hours, etc.).

I hope that this helps you narrow down your requirements... The chances that I have guessed even 1/2 way close to your needs are probably pretty remote.

Once you have firmer requirements, I am sure that everyone here can give you a much tighter system design recommendation.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 5,090Solar Expert ✭✭✭✭
Re: Newbie question .. please answer if you can

Is that 8 hours a day?

If so that is 6400 watt hours or 6.4 Kwh @ ~ 100 wats per panel = +- 64 panels!!! Just for the heater...

the laptop and fan are much easier (cheaper) to deal with but not heat.

cheers
Eric

KID #51B  4s 140W to 24V 900Ah C&D AGM
CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM
Cotek ST1500W 24V Inverter,OmniCharge 3024,
2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
West Chilcotin, BC, Canada
• Posts: 202Solar Expert ✭✭✭✭✭
Re: Newbie question .. please answer if you can

I agree with Bill, but here's another idea for heat:

Consider a pellet stove (if pellets are available where you live). They require some electricity for their operation, but not nearly as much as trying to use solar panels for resistance heating. Once your PV panels are in place, I'll bet that you'll want to use the electricity for lights, communications, entertainment, and things other than what you already have on your list. I've found that a small table-top oscillating fan requires 35-watts, and I use 13- to 19-watt compact fluorescent bulbs for light. Bill has provided some excellent examples. You can use those to do your calculations.

John
• Posts: 5,108Solar Expert ✭✭✭✭
Re: Newbie question .. please answer if you can

The heater is the killer. 800 watts of continious load is huge for a small system. In addition to looking at a pellet stove consider a hi ef propane or natural gas direct vent space heater. Some use no electricity at about 80% afu, some like the rinnai's use a small amount for the controls and the fans. My Rinnai is better than 85% ef with a 55 watt draw. Puts out ~17,000 btu/hour. Way more than 800 watts of electrical heat. (Don't have the conversion table handy but you can google it). The money saved on batteries and panels, could go a long way to increasing the effeciency of the rest of your house/building/what ever.

Good luck,

Icarus
• Posts: 10,311Solar Expert ✭✭✭✭
Re: Newbie question .. please answer if you can

1 watt is about 3.4btus.
• Posts: 5,108Solar Expert ✭✭✭✭
Re: Newbie question .. please answer if you can

Ergo, 800 watts=2720 BTUs

Icarus
• Posts: 27,905Super Moderators, Administrators admin
Re: Newbie question .. please answer if you can

Errr... ergo... 800 Watt*Hours = 2,730 BTU or...

800 Watts = 2,730 BTU/Hour...

Sorry... Just trying to keep the units correct.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 5,108Solar Expert ✭✭✭✭
Re: Newbie question .. please answer if you can

Bill,

Once again I sit humbled. A watt means nothing, a watt hour on the other hand,,,,,

I often have a thought, but as I was told in school long ago, I need to pay attention to detail.

Icarus
• Posts: 10,311Solar Expert ✭✭✭✭
Re: Newbie question .. please answer if you can

ditto as i had forgotten to place the time factor to it.
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