Solar Powered Small Home - Tech Notes.
* Totally 'Off-Grid'?: If you have frozen meat, it is NOT practical to go Totally 'Off-Grid' with a small solar setup.. With two rainy day, you will need grid power to keep those items preserved. And, power from a petrol Generator is very expensive.
* 'Solar' is NOT totally Free Energy..
Our 'tiny house' uses about 3 kWh/day - Which, at 2017 grid prices, saves $1/day..
The Solar equipment cost about $2300 and so (at $1/day) will take 6 years to pay for itself.
However, the Batteries ($1000) might only last 5 years! So, they will be an ongoing cost of 40c a day.
Thus, with the odd service call, solar power is not cheaper than the exorbitant 'grid'!
Solar equipment used in our 'Tiny House' project..
We needed about 3 kWh/day to run a Fridge; Freezer; Microwave; Led Lights; Small TV.
The 3 kWh/day on Solar will require a 400Ah deep cycle battery bank. ($1500) (Note: You should only plan to use 60% of the battery capacity.)
Solar Panels need to be able to replace that 3kWh in 4 hours - so we used 1kW of panels. ($800)
The Inverter is 2kW pure sine: ($200)
The Charge controller is 40A, 3 stage, programable. ($220)
Auto backup mains charger: $50 (cuts in if the batteries drop below 11.5V)
Our 400Ah battery bank costs about $1500 - How soon will it pay itself off??
The sun might only be 'useable' for about 9hrs per day - so the batteries have to 'fill in' for 15 hrs/day. This takes a heavy toll on their life span.
Over their life time, 'our' average capacity will be about 180 Ah or 2kWh per night. At 30c/kWh, this is about $4/week - Or, about 5 years to 'pay back' $1500.
'Battery + Solar' is supposed to be cheaper than Grid Power! But, if your batteries don't last 5 years, then the Grid is cheaper!
The most common destroyer of battery life is a poor quality charging controller.
The Charge Controller should be at least a 'Three Stage' charger (eg. Bulk/Boost, Constant Current, Float) plus a monthly 'Equalize' program. (see diag. 1.5 below)
With a good charger (eg. $250), you should be able to get 5 years from your batteries. With a cheap charger, you might only get 2 years!
Note 3: The 'start up' current of a freezer or fridge can be 8 times the running current - so you need a 1000w inverter to safely start the average (100watt) fridge/freezer.
Microwave ovens require double their quoted power - so an 800w oven will need 1600W inverter power) The minimum inverter for a tiny home is about 2500 watts.
Note 4: If you use the standard 'PWM' controllers, a "100watt" PV (solar) panel will only give 70 watt. (6 amp at 12v) (The full 100w requires the more expensive 'MPPT' controllers)(17v x 6 amp)
(MPPT = Max.Power.Point.Tracking - 'PWM' = Pulse.Width.Modulation)
Note 5: When buying a Freezer, go for a 'High Efficiency' rated one. They have 3" (75mm) thick walls and can survive the whole night without power. They also need less running power (eg. 80watt for 20 min in an hour)
Note 6: Even a 'dinner plate' size shadow on a solar panel will stop all power from that panel! With multiple panels, ensure they all have 'bypass diodes' fitted.
Note 7: If you don't get many hours of full sunshine - you can get 30% more 'panel power' by just changing to the more expensive 'MPPT' controllers. They might cost an extra $50 each, but that's cheaper than an extra panel. Also, if you are using more than 3 panels, you should use 'MPPT' controllers.
Note 8: Use "Pure Sine Wave" type Inverters.. Very few appliances function well on the cheaper "Modified Sine wave" Inverters.
Note 9: Battery Type?
a. Normal car batteries do not suit solar projects.. Their ability to stand daily deep discharge is severely compromised in order to cheaply deliver a high Starting Current.
b. 'Lithium Ion' is currently 'best' but very expensive and they are soon to be superseded by Graphene Batteries. The mining of Lithium (rare earths) also causes enormous damage to the environment.
c. Lead acid "Deep Cycle" batteries (eg. Caravan Batteries) are also soon to be superseded but currently are the best value for money. They can stand up to 10 years of being taken down to half charge every day. The four batteries in this project were $240 each. (N70T 100ah).
Note: Even if you only want to power a freezer while the sun is out - you still need at least a cheap car battery to keep the power stable. And, the freezer needs 75 amp for a second to start-up (10 times what a panel can provide.)
Note 10: Roof Mounted panels or Ground mounted?
Roof mounting has two disadvantages: a. The panels are hard to clean and 'dirty' can cause a 10% loss.. b. The roof is a very hot place, and the low air gap under the panels will keep them too hot to touch. A hot panel can give 20% less power and has a shorter life. So, a dirty + hot panel can be 30% down on power.
Ground based, on an angled (and possibly rotatable) frame can be easily cleaned; will be cooler (free air underneath); and can be better angled to face the suns track.. So, ground based can require 40% less panels.
Note 11: "Equalizing Mode".. Many first time purchasers of 'Charge Controllers' notice their expensive 12v battery being overcharged - they post the on-line question..
Why is my Charge Controller overcharging the Battery - up to 16volts?
Most 'Charge Controllers' initially (and at regular intervals - eg monthly) run 'Equalizing Mode' for a few hours.. This involves taking a 12v battery up to about 14.5v to remove sulfate crystals and prevent the acid settling into density layers. This initial 'overcharge mode' prompts many purchasers to believe their controller is faulty.