Solar & Power Forum

What this board is for

Panels, batteries, inverters, charge controllers, wiring, and everything that makes electricity appear in an off-grid building. That includes the design questions before anything is bought, the installation questions during, and the diagnostic questions afterwards, when a system that used to work has started doing something new.

It is also the right board for the questions that sit underneath the hardware: how to build a load list, why an array sized on annual average sun disappoints in December, and when a generator is a sensible part of the design rather than an admission of failure.

The questions that come up most

How many panels and how much battery do I need? Answerable, but only from a load list and a location. Daily watt-hours plus days of autonomy give the bank; daily watt-hours divided by worst-month peak sun hours, plus a loss factor, give the array. Anyone answering without those two inputs is guessing.

Why is my system fine in summer and short in winter? Nearly always because it was sized on annual average sun. In much of the country the worst month delivers roughly half the annual average, and that shortfall lands in the season with the highest loads.

Twelve, twenty-four, or forty-eight volts? Higher voltage means lower current for the same power, which means thinner cable and smaller fusing everywhere. Small systems live happily at twelve; most whole-house builds settle at forty-eight, and it is a decision that is expensive to reverse.

Why does the inverter fault when the pump starts? Almost always surge, either from the inverter's rating or from a bank that sags under the starting current.

What a useful answer looks like here

Numbers and models. System voltage, battery chemistry and capacity, inverter make and continuous rating, array watts and how the strings are arranged, and the location, because the sun resource and the cold both matter. A photograph of the wiring is worth several paragraphs when something is misbehaving.

Say what is measured and what is assumed. A state of charge read from a shunt-based monitor is a fact; one inferred from resting voltage is an estimate, and one inferred from voltage under load is barely that.

And on anything involving conductors, fusing, or the battery bank itself, an answer should be specific about ratings rather than approximate. Direct-current faults do not behave like alternating-current ones, and this is the part of an off-grid system where a wrong answer starts a fire rather than merely disappointing somebody.

The mistakes that cost the most here

Sizing the array on the annual average sun figure instead of the worst month. In much of the country the worst month delivers roughly half the annual average, and that shortfall arrives in the season when heating fans, pumps, and lights all run hardest.

Building a panel string to the controller's voltage ceiling as measured on a warm day. Open-circuit voltage rises as temperature falls, and a string that measures safely in September can exceed the limit at dawn in January, which destroys the controller outright rather than gradually.

Buying far more inverter than the house needs and then paying the idle penalty every hour of the year. Idle draw is small, continuous, and cumulative, and on a modest system it is a real share of the daily budget.

Undersizing conductors on the array run, where voltage drop is a percentage of every kilowatt-hour the system will ever produce, and on the battery run, where it is a safety matter as much as an efficiency one.

And treating a battery bank as a substitute for generation. A bigger bank buys a longer, slower descent if the array cannot refill it; the fix is panels, a generator, or a smaller load list.

Settled arithmetic against genuine preference

The arithmetic is not a matter of opinion. Daily watt-hours divided by worst-month peak sun hours, plus a realistic loss factor, gives array size. Usable capacity is nameplate multiplied by the depth of discharge you will actually use. Battery current at full inverter load is watts divided by battery voltage divided by efficiency. Anyone who disagrees with those is disagreeing with multiplication.

What is genuinely open: all-in-one against separate components, lithium against lead acid in small systems, how many days of autonomy are worth buying, whether to run any circuits at direct current, and how large a role a generator should play. Those depend on climate, budget, and temperament, and the useful answers come with reasoning attached.

Safety is in the first category rather than the second. Fusing at the source, interrupt ratings appropriate to what a bank can deliver, correct grounding and bonding, and connectors rated for direct current are not stylistic choices.

Diagnosing a system that used to work

Start with what changed, because something usually did: a new load, a new shadow as vegetation grew, colder weather, or a connection that has begun to fail. The controller's own logging is the best instrument you have, since it sees the array, the battery, and the charge behavior together.

A gradual decline in harvest usually means soiling, shading, or a connection degrading. A sudden step down usually means a string has dropped out. A bank that no longer reaches absorption, or finishes charging far too early, is telling you about the battery rather than the array.

Then look for heat. An inexpensive infrared thermometer run over the connections under load will find a joint that is warmer than its neighbors, and a hot joint is a fault in progress rather than a curiosity. Retorque to the manufacturer's figure rather than to feel.

When posting a diagnostic question, say what the system did before, what it does now, when it changed, and what you have already measured. Photographs of the installation and a screenshot of the controller's history are worth several paragraphs of description.

How much of this is regional

More than the equipment suggests. The same array produces very different results in Arizona and Maine, and the gap between annual average and worst-month sun is what decides how much array a household needs. Cold climates also change the electrical design: panel voltage rises in the cold, which constrains string length, and lithium batteries cannot be charged below freezing, which constrains where the bank can live.

Hot climates have their own version. Panels produce less as they heat, inverters derate at high ambient temperatures, and a mechanical room that bakes in July is a system that underperforms exactly when the cooling load peaks.

So post your location. It is not small talk: it changes the array size, the string design, the battery siting, and whether a generator belongs in the plan.

Cost, code, and where to stop doing it yourself

Most of the money in an off-grid power system is in the battery and the labor, and most of the regret is in buying twice: an inverter that cannot be stacked, a system voltage chosen too low, a bank expanded with mismatched packs. Those are design decisions rather than purchases, which is why the planning stage repays effort.

On code, the honest position is that off-grid work is frequently unpermitted and uninspected, and that holding yourself to code standards anyway is the single most important safety decision in a build. Conductor sizing, overcurrent protection at the source, correct interrupt ratings, grounding, and bonding are not optional refinements.

Plenty of capable people wire their own systems well. Plenty of others hire an electrician for the battery connections, the main disconnects, the grounding system, and any alternating-current wiring. Both are reasonable; doing work you do not understand and hoping is not.

The lesson that covers this

Lesson 4: Loads before panels in the free course works through this subject in the order the decisions actually arrive. Build an honest list of what runs on electricity, in watt-hours per day, because every other power number comes from it.

The whole course is twelve lessons, free, with no account required to read any of it: start at the beginning if you are new to this, or jump straight to the lesson above if you are not.

Tools for this topic

Most questions on this board come down to a number somebody has not worked out yet. These do the arithmetic and show their working, so an answer here can be checked rather than taken on faith.

• Tools/solar calculator: size an array against your own load list and worst-month sun
• Tools/battery calculator: size the bank and compare chemistries on cost per usable kilowatt-hour
• Tools/wire gauge calculator: size conductors for the current and the run length
• Specs/batteries: compare battery specifications side by side

How this board works

Posting needs an account, which exists so that answers have a name attached and so that the same person can be asked a follow-up six months later. It costs nothing and it is not used for anything else.

The conventions here are the ones that make a small forum useful. Say where you are, at least to the state or region, because almost every answer on this site depends on climate or on local rules. Say what you have already, and what you have already tried. Give numbers where you have them, and say when you are guessing. And when something works or fails, come back and say which, because the thread is read by far more people than post in it.

Answers here should be able to say where they came from. Experience is welcome and so is a rule of thumb, as long as it is labeled as one. Where a claim is about safety, code, or law, it wants a source, because the reader on the other end may be about to spend money or wire something themselves.

Boards next door

Off-grid questions rarely stay inside one department. A water question is often a power question underneath, and a land question is usually several at once. These boards field the neighboring subjects:

• Tools & Self-Reliance: Chainsaws, hand tools, first aid, and the skills and kit that keep you self-sufficient.
• Water: Collection, storage, filtration, and pumps: getting clean, reliable water when there is no tap to turn on.
• Shelter & Structures: Cabins, tiny homes, insulation, and building for the long haul in a place the grid does not reach.
• Prepping & Resilience: Practical prepping and SHTF readiness without the doom: food, water, power, first aid, and communications so you are rea...

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