How an off-grid solar system works
Six parts that pass energy along, from the sun to your devices. Tap a part to see what it does, the words to know before you shop, and the mistake everyone makes.
Between each part: the right size of wire and a fuse. Learn why
During the day
The panels produce. The controller fills the batteries, and your devices take what they need along the way.
Evening and night
The batteries take over. The inverter turns their power into outlet power for your devices.
Grey days and winter
The generator and charger fill the batteries when the sun isn't enough.
Solar panels
They turn light into electricity, as direct current. Light is what counts, not heat: a panel even produces better on a cold, clear day.
Words to know
- Watts (W)
- Peak power, measured in a lab. Outdoors, count on about 80% of that number.
- Voc
- The panel's voltage with nothing connected. It rises in the cold, and it's the number that must stay under the controller's limit.
- Monocrystalline
- Today's common type, all black. It produces the most for the space it takes.
- Bifacial
- It also collects light from underneath. Useful above snow or light-coloured ground.
The classic mistake : Letting a branch, an antenna or a chimney shade one corner. A small patch of shade can cut a good part of the whole panel's output.
The charge controller
It sits between the panels and the batteries. It sends the right current to fill the batteries without ever overcharging them.
Words to know
- MPPT
- The good kind. It gets 20 to 30% more out of your panels and accepts panels wired in series.
- PWM
- The cheap small model. Fine for one small panel, but it wastes part of the output.
- Maximum input voltage
- The limit never to exceed, even at -30 °C, when the panels give their highest voltage.
- Output amps
- The current it can push into the batteries. Roughly: panel watts ÷ battery voltage, plus a margin.
The classic mistake : Wiring too many panels in series. In deep cold their voltage rises and can fry the controller. Always check with the Voc and the coldest temperature at the site.
Batteries
They're the tank. They keep the day's energy for the evening, the night and grey days.
Words to know
- Amp-hours (Ah)
- Capacity, but only together with voltage: 100 Ah at 12 V is 1,200 Wh; 100 Ah at 24 V is 2,400 Wh.
- Watt-hours (Wh)
- The real measure of energy. It's the number to compare from one battery to another.
- Lead-acid, AGM or gel
- Cheap, but you only use half: below 50% they wear out fast. Heavy.
- Lithium (LiFePO4)
- More expensive up front, light, you use almost all of it and it lasts much longer. It won't charge below 0 °C, except heated models.
The classic mistake : Comparing two batteries by their Ah without looking at the type. 100 Ah of lithium gives almost twice the usable energy of 100 Ah of lead-acid.
The inverter
Batteries give direct current at 12, 24 or 48 V. The inverter turns it into outlet power, like at home (120 V in North America, 230 V in most other countries), so you can plug in ordinary devices.
Words to know
- Pure sine wave
- Clean current like the grid's. That's what you want: motors, chargers and electronics like it.
- Modified sine wave
- Cheaper, but some devices heat up, hum or don't work at all.
- Continuous and surge watts
- Continuous: what it gives for a long time. Surge: the starting kick. A fridge or a pump needs 3 to 5 times its power to start.
- Idle draw
- When on, an inverter burns energy even when nothing is running. Turn it off when you don't need it.
- Inverter-charger
- An inverter that also works as a battery charger when you plug in a generator.
The classic mistake : Buying the biggest inverter “just in case”. It draws more at idle and needs bigger wires. Add up the devices that run at the same time, and keep a margin for starting.
Devices
Everything starts with them. What you want to run, and for how long, decides the size of everything else.
Words to know
- Watts × hours = Wh
- A 10 W LED light on for 5 hours is 50 Wh. Add up all your devices to get your day.
- Ballpark figures
- LED light: about 10 W. Charging a phone: about 10 Wh. TV: 50 to 100 W. Small efficient fridge: 300 to 600 Wh a day.
- 12 V devices
- Lights, water pump, RV or boat fridge: wired straight to the battery, they avoid the inverter's losses.
The classic mistake : Wanting to heat with electricity: space heater, baseboard, coffee maker, hair dryer, oven. Anything that heats needs a huge amount of energy. Propane or wood is much better.
The generator and charger
The backup. In winter, or after a few grey days, the panels aren't enough. The generator then charges the batteries through a charger.
Words to know
- Battery charger
- It turns the generator's 120 V into current for the batteries. Its amps decide how fast they charge.
- Inverter generator (quiet)
- Thrifty: it slows down when little is asked of it. Ideal for charging batteries.
- Contractor generator
- It runs at full speed all the time, even to charge a small battery. It burns more gas.
- Absorption
- The end of a lead-acid battery's charge. It's slow, no matter how big the charger.
The classic mistake : Running the generator to 100% with lead-acid batteries. The last 20% takes hours. Charge to 80%, and let the sun finish the job.
Wires and fuses
They connect all the parts. At 12 V the currents are strong: a wire that's too small heats up and wastes energy, and a short circuit without a fuse can start a fire.
Words to know
- Gauge (AWG)
- The wire's size. Careful: the smaller the number, the thicker the wire.
- Voltage drop
- Energy lost as heat in a wire that's too long or too thin. Devices run poorly and batteries charge less well.
- Battery fuse
- The most important one, right next to the battery's + terminal. It protects against a short circuit, which can melt a wire.
- Disconnect switch
- A switch to cut everything during maintenance or if something goes wrong.
The classic mistake : Not putting a fuse near the battery. In a short circuit, a battery can deliver hundreds of amps at once. To connect the system to a building's electrical panel, call an electrician.
12, 24 or 48 V?
That's your batteries' voltage. The higher it is, the lower the current for the same power: thinner wires and fewer losses.
For the same 2,400 W, the current and the cable you need:
12 V
222 A · cable 2/0
Small systems: trailer, van, boat, shed. 12 V devices plug straight in.
24 V
111 A · cable 2
Medium systems, when the inverter goes past 1,500 to 2,000 W. A simple cabin, a camp.
48 V
56 A · cable 8
Big systems: a fully equipped cabin, a house. The standard for big inverters.
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