Off-grid systems fail in one direction: they run short in winter. Almost always the cause is the same, sizing against a summer average and discovering that December delivers a fraction of it.

These off-grid solar guides work through the decisions in the order that avoids that, because each one constrains the next.

What I look at, in this order

  1. Your daily energy budget, in kilowatt-hours. Everything else is derived from this number. Get it from actual appliance consumption over 24 hours, not from an estimate of what feels reasonable.
  2. Worst-month peak sun hours, not the annual average. Peak sun hours are location-specific and vary enormously by season. Sizing against your worst month is the difference between a system that works all year and one that works until October.
  3. System voltage. Twelve, twenty-four or forty-eight. This determines your cable sizes, your component options and your expansion ceiling, and it's the hardest choice to reverse.
  4. Controller class and headroom. MPPT or PWM, and rated comfortably above your array rather than exactly at it. A controller running at its limit is one cold sunny morning away from clipping.
  5. The wiring, fusing and mounting budget. The line beginners leave out entirely, and the reason otherwise well-planned builds come in over budget.

Three build sizes, and what changes between them

The trickle build. One panel, a small controller, one battery. Keeps a vehicle battery topped up and runs lights and USB charging. Cheap enough to be a learning exercise, useful enough to keep afterwards.

The van build. A few hundred watts of panel, a proper controller, a lithium bank and an inverter. Fridge, lights, laptops, water pump. The most common band, and where an MPPT controller starts clearly earning its cost.

The cabin build. Approaching a kilowatt of panel and beyond, running a fridge continuously plus tools or larger appliances. This is where system voltage stops being an academic question and starts determining whether your cable sizes are reasonable.

The step from trickle to van buys you independence. The step from van to cabin buys you continuous operation, and it's where planning errors get expensive.

Which guide you want

If you need to turn a list of appliances into a panel count and a battery bank size, that's the off-grid solar sizing guide — the full version of the method above, including how to apply sun hours and system losses.

If you're stuck on a single component rather than the whole system, the head-to-heads are in my off-grid comparisons, starting with MPPT vs PWM. For specific gear I've used, see the reviews. And to find out whether the build pays for itself, the off-grid calculators.

Frequently asked questions

How do I work out how many panels I need?
Divide your daily energy use in kilowatt-hours by the peak sun hours your location receives in its worst month, then add margin for system losses. The two figures people get wrong are using an annual average instead of the worst month, and forgetting that real systems deliver noticeably less than their nameplate rating. Work through the sizing →
Why do off-grid systems run short in winter?
Because they were sized against a summer or annual average. Available sunlight in December can be a fraction of what June delivers at the same location, while consumption often rises at the same time. Sizing against the worst month is what prevents it.
What do beginners most often leave out of the budget?
Wiring, fusing, mounting hardware and monitoring. Panels, battery and controller are the parts people plan for, but correctly sized cable, overcurrent protection on every source and a decent shunt-based monitor add a meaningful amount to the total and are not optional.

All off-grid solar buying guides