Help with a small off grid system

Re: Help with a small off grid system

You can use this link to estimate the amount of power per day/month/year your system will generate (note: it does not go down to 0.1 kW panel size--so you will have to enter 1.0 kW panel and divide all the results by 10).

OK, Richmond VA, 0.1 kW panel (100 watts), use a 0.50 derating factor because you are "designing" an off-grid system and there are battery+inverter losses.

You will generate, roughly 5 to 7 kW hours per month (depending on season) or (divide by 30) 167 to 233 Watt*hours per day (useful energy from inverter to your AC load).

Load is 2 x 13watt x 4 hours per day = 104 Watt*hours for lighting.

Remaining power for TV, roughly 63 to 129 watt*hours per day for your TV.

The 1,000 watt inverter is probably way overkill for this system. Also, an inverter that large is probably a modified sine wave (really mod. square wave) inverter... Most loads will sort of work OK--some will either fail from overheating (motors) or have nasty RF interference (some TV's, radios, etc.).

You would be better off all the way around looking for a smaller "true sine wave inverter" sized closer to your load (in this case, roughly 2x times your normal rated load--unless you are planning on using electric motors--but this is a pretty small system to even run one of those for any length of time).

Exeltech is one brand that has been mentioned several times here as a good true sine wave inverter.

The other thing you did not ask about was the type/size of battery you would like to run this system... If you can justify it, an AGM type sealed lead acid battery, roughly 3-6x your daily power requirements would be a good fit.


233 watt*hours / 12 volts = 20 amp hour... Or ~60-120 amp*hour rated 12 VDC deep cycle storage battery would be fine.

You could also use regular wet cell batteries. More maintenance, cheaper to purchase (and mess-up with if this is your "training" solar PV system).

You do want to not try and drain your system to less than 50% state of charge for best battery life. If you drain it deeper than that, the battery will not last near as long (and wet cell batteries may fail in weeks or a few months if discharged that deeply).

On the other hand, AGM batteries don't like over charging at all because you cannot replace water if they over charge and vent. Wet cell lead acid can have water replaced (using distilled water).

-Bill

Re: Help with a small off grid system

The issue with Mod Sine Inverters is that probably 90% of your electrical loads will work OK (may be a bit more buzz and a bit more heat). And 10% will fail within minutes or hours of use...

Problem is you don't know which device will fall into the 90% or 10% bucket...

Fridge/freezers and other consumer items with motors generally will run hot and can fail sooner than expected.

Cheap/mass produced consumer electronics can fail with Mod Sine (somebody here said their Black and Decker cordless drill battery charger died quickly--and I think somebody else said their B&D worked fine).

If you need cheap, portable power, and don't care too much about what you are powering (flood lights, resistive heating elements like ovens, hand tools, etc.)--mod sine is fine.

However, if this is going to be for ongoing use (like a solar powered home), bite the bullet and get a real sine wave inverter.

The smaller inverters are not that expensive and the cheap but large mod sine units are usually way to big for most people's applications--and frankly, way too big to clip onto a 12 volt car sized storage battery (running a 1kW 12 vdc input inverter will probably completely flatten a standard sized storage battery in 10 minutes +/-... This is pretty much the equivalent of sitting in your driveway and cranking your car engine with the coil disconnected--a continuous 100+ amp load at 12 volts).

For outdoor wiring, you should look at the NEC (National Electric Code)... But basically, most wire should be in conduit when exposed to sun and weather. You can get cable/wire that is rated for UV/outdoor use to wire solar panel to solar panel.

-Bill