Part VII: Solar ElectricityChapter 11. Grid-Tied and Off-Grid PV
Batteries and Small Off-Grid Systems
A modest battery bank for a cabin or backup circuit, with straightforward limits on what it can run.
- Difficulty
- Advanced
- Cost
- Varies widely
A small off-grid system runs a list of loads you wrote down, for a number of cloudy days you chose, from a battery you can actually lift into the design. It does not run a house the way the grid does. A well pump, a hair dryer, and a resistance heater will empty a cabin bank or stall the inverter on the surge. Start from the watt-hours, the way the grid-tied page starts from the bill, and then be stricter, because there is no utility to hide the mistake.
If the building also has grid power, you are not off-grid. You are building backup, and the two sources must be unable to meet. That is an interlock or a transfer switch, on a permit. The wiring page covers that connection. Do not make it with a cord.
Difficulty
- Level
- Advanced
- Time
- A designed cabin circuit, not a car battery and a hope
- Skills
- Adding watt-hours honestly, and fusing a battery
Cost estimate
Varies widely
A small lithium pack and a charge controller can be several hundred dollars and will run lights and a phone. A cabin bank that carries a fridge and a few cloudy days is often a few thousand before the modules and the inverter. The sketch sizes amp-hours only.
| Load | Why it surprises people |
|---|---|
| Lights and chargers | The easy part, if you measured them |
| A fridge | Running watts are modest. The cold start is not. And it runs all night |
| Heat, a well, a microwave | Often the reason the bank was too small |
Materials
| Item | Qty | Notes |
|---|---|---|
| A list of loads in watts and hours | 1 day | Measure. A nameplate is the maximum, not the duty. Phantom loads count all night. |
| A battery meant for the cycle | From the sketch | Deep-cycle, or a lithium pack with a management system. Not a car starting battery. Not a mix of old and new. |
| A charge controller matched to the modules and the battery | 1 | Voltage and current inside the controller's window. A panel clipped straight to a battery is not a controller. |
| A fuse or breaker at the battery | Where the maker specifies | Direct current from a battery can sustain an arc. The fuse is not optional and not far away. |
| An inverter sized for the surge | 1 | If you need alternating current at all. Many cabin loads can stay on direct current and skip the inverter loss. |
Amp-hours for the cloudy days
Daily watt-hours times the days you want to survive without useful sun is the energy the bank must deliver. Divide by the fraction of the nameplate you are willing to use, and by the battery voltage, and you have amp-hours. Flooded lead-acid is often planned around half, because draining it flat shortens its life. A lithium iron phosphate pack with a real management system may allow more of the nameplate. Use the maker's fraction, not a forum's.
The sketch does not know your winter sun. A bank that covers two dark days still needs modules that can refill it in the kind of week you actually get. If the refill takes ten days of December, those days of autonomy exist only on paper. The inverter has to start the largest motor, not just run the average wattage. A pump that needs a surge of several times its running watts will click the inverter off while the sketch still looks fine.
Chemistry, gas, and the room
Flooded lead-acid batteries vent hydrogen while they charge. Hydrogen ignites. The bank belongs in a ventilated space that is not a bedroom, with no spark, no water heater flame, and no inverter sitting on the battery case. They are heavy, they spill acid if they are the flooded kind, and they want the maintenance the maker wrote down. Sealed lead-acid spills less and still does not like to be flattened.
Lithium iron phosphate does not vent hydrogen the way a flooded bank does. It still needs a management system that will actually stop a cell from going outside its limits. Do not charge a lithium pack below freezing unless that pack says it can. Do not mix it with lead-acid on the same controller setting. Do not use a pack with no management system because it was cheap.
Every positive conductor at the battery needs overcurrent protection at the distance the maker and the code allow, which is short. Parallel batteries need that thought on each string. This page will not draw the lugs. If you cannot fuse a battery without a drawing, hire the person who can. A cabin that is truly separate from the grid is still a place where a bad connection starts a fire, and the fire department is far away.
Medical equipment, a sump that must run, and anything else that cannot go dark do not belong on a hobby bank. Those need a design with monitoring and a real backup plan.
Write the loads before you buy the battery
Watts times hours, including the night. Drop the heater and the pump or give them a different energy source.
Run the amp-hour sketch with the maker's usable fraction
Then ask whether December can refill that bank. Add module area or cut loads until the answer is yes.
Match the controller and the inverter to that bank
Voltages agree. The inverter surge covers the motor. Direct-current loads are better when you do not need alternating current.
Fuse at the battery and ventilate the room
Flooded lead-acid gets ventilation and no flames. Lithium gets a management system and a temperature it is allowed to charge in.
Keep the grid out unless an interlock is in
A backup that shares a building with utility power is a permitted transfer, not a cord.
Further reading
These links go to outside sources. This book cites them by link and does not copy their words or pictures.
- https://www.builditsolar.com/Projects/PV/pv.htm
- https://www.builditsolar.com/Projects/PV/EnphasePV/ComponentSelection.htm
- https://www.builditsolar.com/Projects/PV/EnphasePV/Mounts.htm
- https://www.builditsolar.com/Projects/PV/EnphasePV/Wiring.htm
- https://www.builditsolar.com/Projects/PV/EnphasePV/Economics.htm
- https://www.builditsolar.com/Projects/PV/EnphasePV2019/Main.htm
- https://www.builditsolar.com/References/pvwiring.htm
- https://www.builditsolar.com/Experimental/PVShading/
General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.