Getting started with PV solar

Re: Getting started with PV solar

Techinically, it is possible to connect solar panels directly to an inverter--but practically it is not a very satisfactory solution. Basically, solar panels output exactly what the sun is shining. Cloud goes by, the panel drops by 1/2 or more in output current capability.

The standard inverter wants to supply a steady 120 VAC 60 Hz (US voltage/frequency) to what ever the load requires (fridge, TV, etc.)...

Problem is that if the load is greater than the input power of the solar panel, you get a brownout (and damage the appliance--especially if it is a motor) or simply an on/off AC power to the device--not very useful or practical.

If you want pure solar power--it is generally better to connect simple DC devices to the solar panel and run the, for example, solar water pump slower, or if a DC boombox the music will drop out if the sun is blocked to the solar panel.

For you home... Most cost effective is the "Grid Tied Inverter System". It, basically, uses the Utility AC Grid as a giant battery unlimited battery. However, there are issues (first is that this requires permits, second, not all utilities allow grid tied inverters or support net metering).

I have to go right now, but this is a start... You have to have an idea of how much money you wish to spend and what type of solutions do you want (best price, AC power if lights go out, etc.).

-Bill

Re: Getting started with PV solar

Partly, I was in a rush at the time and forgot about your request--the other was I really did not know how to answer your questions/statements.

More or less--you have a lot of loads, a large inverter, very little solar PV panel power and not a lot of battery storage. Plus it sounds like you are making your own solar panels?

Basically, I would suggest that you look at your solar loads and understand how many Watt*Hours of energy you want to use--and how much you want to offset with solar panels.

Realistically, just as a SWAG, your 75 watts of solar will probably run just your TV for 1-2 hours a day. And, if you have a standard automotive sized 12 volt storage battery, it would reliably run the TV for about 4 hours (assuming you recharged the battery with solar + generator).

Your 1kW inverter will discharge a typical 100AH 12v storage battery in about 1/2 hour (for long battery life--discharge only to 50% of capacity). If you ran the 1kWatt inverter at full power for 1 hour, it would kill the battery within a few days or weeks of use (discharge/charge cycle).

So--Two places to start--One is you need to measure how much power you want to run on solar/generator by using a kill-a-watt meter or equivalent.

http://store.solar-electric.com/kiacpomome.html (from here or equivalent)

Do you understand how voltage, current power, and energy are measured?

Power=Volts * Amps
Energy=Watt*Hours=V*I*Hours
kiloWatt*Hours = Watt*Hours/1,000 (kWh is the unit of power your utility charges for electricity. Typically about $0.10 to $0.35 per kWhr).

So, example: TV watt * Time = 75 watts * 2 hours = 150 watt*hours or 0.15 kWhrs (about $0.001 of electricity).

Heater Energy = 479 watts * 2.4 hours / 24 hour day (10% cycle) = 1,149.6 WHrs / 24 hours = 1.1kWhrs /day

And so on... Remember that measuring actual power used rather than just nameplate ratting is much more accurate. Also, duty cycle for some appliances (like fridge/freezer/heater) also is important.

Just as a first guess, your solar panels will average something like 2xRating worth of usable energy per day because of inverter, charger, and battery losses. So, a 75 watt panel * 2 hours will give, very roughly, about 150 Watt*Hours per day of usable energy (150 watts for 1 hour or 10 watts for 15 hours, etc.).

You will find that with solar, conservation of power is very important... For example replacing two 75 watt bulbs with two 12 watt CFL's would be top on the list.

Sorry--family is calling again--but does this give you some ideas of what more is needed to really help?

-Bill

PS: just to finish the thought...

2x75w*4 hours = 600 Watt*hours
2x12x*4 hours = 96 Watt*hours

So, in this example, your system would easily run two CFL's with energy left over. The other 75 watt bulbs--your system could only run them for about 1 hour (assuming filament 75 watt bulbs vs 12 watt CFL's--obviously, I don't know your home/usage).

Also, solar does vary over the seasons (and by weather, location, mounting angle, shade, etc.) and you can get more than 150 watt*hours from your 75 watt panel (and you can get less)--but it is a good place to start without losing everyone in the math and other issues (battery choice, charger choice, inverter choice, wire choice, etc.).

I suggested Grid Tie as an alternative because the batteries, losses, additional equipment, etc. usually make Grid Tie less expensive over the long term on a $/kW costs (other than using electricians/permits required for Grid Tie). Unless you need off-grid power (for storms, cabin, etc.) and you need a battery based system.

-BB