# Battery to computer

I am thinking about using a deep cell battery to run my computer and one 26 watt bulb= 76 watts. Approximatly how much battery time might I have? I hope to work three to five hours a day. I live in the northeast and I am not sure whether I should use solar or AC to charge the battery, or both. Would appreciate your help
Charles

• Solar Expert Posts: 5,436 ✭✭✭✭
Re: Battery to computer

Tis a question without an answer,,,

How much time you can run ~76 watts depends on a bunch of things, such as the size of the battery, how low do you want to discharge it, how fast can you recharge it.

So in rough numbers, (I'm sure Bill will chime in with real numbers)
75 watts from a 12 volt battery would be ~6.25 amps, over 4 hours it would be ~25amp/hours. So if you had a 75amp hour battery, you could run it for 12 hours. Of course you would kill the battery do it that way, since even deep cycle batteries should only be drawn down a maximum 50%. It is considered much better to only draw the down 20%. 20% would then allow you to run your loads about 2 hours.

75 watts for 4 hours a day would average 300 watt/hours/day.

If you wanted to use 300wh you would need to generate ~20% more, or 360 watt hours just to keep the battery even. A general rule of thumb is to consider in the net/net, you would be lucky to average about 1/2 your solar capacity into the batteries, taking into account all the efficiencies and losses. So if you need ~300 wh/day you would need the capacity or ~ 600 wh/day, Say a 200 watt array for 3 hours,,,just to stay even. Add in a reserve for cloudy days etc.

( I realize that I am using watt/hours and amp/hours in this answer and that might serve to confuse, but it is easier for me to talk about batteries in amp hours since we know the voltage (assuming 12volt) and panels in watt hours since we don't know the panel voltage and the question revolves around watt hours. Sorry for any confusion!.

The bigger question is why would you want to do this? If your goal is to save money, there are way better ways to save money. It turns out that the grid is a very inexpensive source for power when you get right down to it. Grid tie systems, watt for watt, are about 1/2 the price of a battery system.

Tony
Re: Battery to computer

Batteries are typically rated in Amp*Hours at a 20 hour discharge rate...

Some of the basic electrical equations:

Volts=Current*Resistance=V=I*R (and all of the algebraic variations; volts, amps, Ohms)
Power=V*I=V^2 / R=I^2 * R (power is a rate in Watts)

Work is simply Power * Time:

Work=P*T (power is in watts or kWatts for home billing, Time is in Hours)

AmpHours is how batteries are rated--and it is Power "without the volts"--assume 12 volts for smaller systems, 24 or 48 volts for larger systems (yours would be a small system):

Amp*Hours=Amp*Hours=Work/Volts=P*T=Watt*Hours/volts=etc.

So, for your case, P=76 watts for 5 hours.

Assume your battery is ~80% efficient and your inverter is ~85% efficient.

Amp*Hours=(76 watts*5 hours / 12 volts) * 1/0.80 * 1/0.85 = 46 Amp*Hours per day.

Typically, for many reasons, we recommend that a deep cycle battery be ~6x the daily usage (3 days of "no power", and 50% maximum discharge for long life). This works out, in your case:

46Amp*Hours * 6 = 276 Amp*Hours for a 12 volt system.

Charging wise, we recommend around 5%-13% for charging current of the battery's 20 hour rating. 10% is a good number, so:

276 Amp*Hour * 0.10 = 27.6 amp charger (around 20-30 amps would be fine).

It sounds like you are making a "UPS" for your home? Rather than a solar powered system and/or backup in case of a power failure?

Depending on what you are after--somebody here can recommend different types of inverters/chargers/solar systems/battery types/etc.

For example, a neat little system for 100% UPS backup (short of buying a UPS), would be a little 300 watt 12 volt True Sine Wave inverter and a 20 or 40 amp charger (with remote temperature sensor).

If you are looking for a system with large capabilities, you could look at one of these 2,000 watt+ from Outback.

There is a wide range of what can be done--the question is what do you need/want?

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset