Why this article exists
If you've read the common DIY solar mistakes, you know sizing is where off-grid builds most often go wrong — in two directions. Too small, and the system dies at night. Too big, and you've paid for capacity the rest of the system can't use.
This article teaches the logic of sizing — what to calculate, why, and what factors affect each number. For the actual figures, run your specific loads and your specific location through a real off-grid sizing calculator. We won't hand you a one-size formula, because a wrong formula is worse than no formula — the precise answer depends on your battery chemistry, your panels, your location, and how you'll actually use the system.
Sizing flows in one direction: loads determine your energy budget, the energy budget determines your battery and your panels, and the battery and panels determine the controller and inverter that handle them. Everything else falls out of the first honest answer about what you intend to run.
Step 1 — Start with your loads
The whole calculation starts with a load list — every device you intend to run, its power draw in watts, and how many hours per day you actually expect to use it.
The honest part of this step isn't the arithmetic. It's being realistic. Three things people consistently miss:
- Phantom or idle draws. Devices that pull a small amount even when "off" — chargers left plugged in, electronics in standby, smart devices keeping their radios alive. Small wattage, but it runs 24 hours a day; the daily watt-hours add up.
- Inverter overhead. A pure-sine inverter consumes power just being on, even with nothing plugged in. If yours stays on around the clock, count it.
- Cycling loads. A fridge compressor doesn't run continuously — it cycles. The right number isn't nameplate running watts × 24 hours; it's running watts × the fraction of the day the compressor is actually drawing.
An optimistic load list produces an undersized system that disappoints you forever after. Better to err realistic-pessimistic here — it costs nothing on paper.
Step 2 — Daily energy budget
Once the load list is honest, the math for this step is a unit definition, not a rule of thumb:
watts × hours = watt-hours
Multiply each load's draw by its daily run-time to get its daily watt-hours, then sum across every load. That single number — your daily watt-hour budget — is what the rest of the calculation hangs on. It's the amount of energy your battery has to deliver over a typical day and the amount your panels have to refill while the sun is up.
Don't move on from this step until the load list and the budget feel like an honest representation of how you'll actually use the system.
Step 3 — Battery: how big does the bank need to be?
The battery has to store at least one day's watt-hour budget, and almost always more. How much more depends on four factors that vary by your specific equipment and use. Each one pushes the battery bigger; none has a multiplier I'll invent here.
- Days of autonomy — how many cloudy days in a row should the system keep running? More days = bigger bank.
- Usable depth of discharge — lithium banks tolerate deeper discharges without damage than typical lead-acid banks. The usable capacity is meaningfully less than the nameplate amp-hours, and the gap depends on chemistry and on how long you want the bank to last. Get the number from your battery's spec sheet.
- Temperature — capacity drops in cold weather. A bank that lives in an unheated shed or RV bay has less usable capacity in winter than at mild temperatures.
- Round-trip efficiency — storing and retrieving energy isn't lossless. Some of what you put in doesn't come back out; the loss depends on chemistry and charge profile.
The actual amp-hour or kWh number that falls out of these factors is what a sizing calculator is for. Plug in your chemistry, your climate, your autonomy preference, and your daily watt-hours, and you get a defensible bank size.
Step 4 — Panels: how much do you need?
The panel array has to refill the daily watt-hour budget during the hours the sun is actually producing usable power. The variables:
- Local sun hours. "Peak sun hours" — the equivalent number of hours per day at full rated insolation — varies dramatically by location and by season. Southern Arizona and coastal Washington don't get the same number, and neither does the same site in July versus December. Use solar-resource data for your specific location, not a national average.
- Orientation and tilt. Fixed mounts use compromise angles; portable kits let you reposition. Off-axis orientation reduces what you collect compared to optimal.
- Real-world losses. Panels rarely produce their full nameplate wattage. Heat lowers output, dirt and shading lower it, wiring and conversion lower it. Cumulative losses are real.
- Worst-month design. If the system needs to keep running through winter, sizing to an average sun-hour figure isn't enough — size for the worst month you'll actually use it, or accept that the system underperforms in that month.
Actual panel wattage falls out of a calculator that knows your location's sun-hour data, your tilt and orientation, and your daily watt-hour target. Don't trust generic sun-hour numbers from the internet for sizing decisions — your location is your location.
Step 5 — Controller and inverter: match the components
Once the battery and panel sizes are known, the charge controller must handle the panel array's output (voltage and current) and match the battery's charging profile. The inverter must handle your largest simultaneous AC load — including the surge when motors start, which can be several times the running wattage. An inverter that runs your fridge fine when it's already going may not be able to start it from a stop.
Manufacturer datasheets specify what each unit can handle on input and output; matching is a comparison exercise, not arithmetic. If you're not confident matching them from datasheets, that's a stop-and-ask-a-pro moment — bad matches show up as overheating, premature failure, or systems that work until you turn on the wrong combination of loads.
For what each component does at a conceptual level, see the components overview.
Common sizing errors
Most of what makes DIY off-grid sizing go wrong is on the mistakes list:
- Optimistic load list → undersized everything.
- Forgetting phantom and inverter-idle draws.
- Oversizing panels the charge controller and battery can't absorb.
- Using a national or generic sun-hour figure instead of your location and worst-use month.
- Ignoring temperature, depth-of-discharge, or round-trip efficiency.
Each of these is the difference between a system that quietly works and one that disappoints.
The bottom line
The logic is one direction:
Loads → daily watt-hour budget → battery (storage) + panels (refill) → controller and inverter that match both sides.
Understand that flow and a sizing calculator's output stops being a black box. You can sanity-check it, and you'll know what changed if you swap one variable later.
For the actual numbers, plug your specific loads, location, and target equipment into a real off-grid sizing calculator. The variables in this article are exactly what those calculators ask for.
If you're comparing an off-grid system to a grid-tied install that earns net-metering credits — those are different questions with different answers. The per-state payback calculators cover the grid-tied side.