Grid-tied solar is sized against your annual bill. Off-grid solar is sized against your worst week. On the BC coast that week is in December, and it is the whole design problem — everything else follows from it.
Why December sets the size
Natural Resources Canada’s photovoltaic potential data puts Vancouver at about 938 kWh per kW of installed panels per year, with the provincial average around 1,062 and Victoria near 1,018. Those are annual figures, and they are perfectly good for grid-tied work, where a strong summer simply banks against a weak winter.
Off-grid has no bank. There is no meter running backwards and no credit to draw down. Whatever the array makes in a given week is what you have that week, minus whatever the battery is holding. So the array is not sized to the annual average at all — it is sized so that the darkest, wettest stretch of the year still covers your loads, or comes close enough that the generator only runs occasionally.
That is why an off-grid array on the coast looks oversized to anyone used to grid-tied systems. In July it will make far more than you can use and simply throttle back. That surplus is not waste; it is the price of not running a generator in January.
Days of autonomy
The battery bank is specified in days of autonomy — how long the property runs on storage alone with no meaningful sun. On the coast, a run of heavily overcast days in midwinter is normal, not exceptional, so the bank has to cover more than a single night.
More autonomy means a bigger, costlier bank that spends most of the year only lightly cycled. Less autonomy means a smaller bank and a generator that starts more often. There is no universally correct answer; there is a point where the cost of another day of battery exceeds the cost of the fuel it would save, and that point moves with how you use the property.
The generator is part of the design, not a failure of it
People sometimes treat a generator as an admission that the solar did not work. On the BC coast the opposite is true: sizing an array and bank to cover a worst-case December with no generator at all usually costs far more than sizing sensibly and letting an auto-start generator cover the handful of genuinely bad stretches.
A properly configured system starts the generator on its own when the battery reaches a set state of charge, runs it at a useful load to charge efficiently, and shuts it down. You should not be walking outside in the dark with a flashlight to pull a cord. More on specifying this in batteries and generators for an off-grid BC property.
Loads are cheaper to fix than generation
Every watt you do not use in December is a watt you do not have to generate or store in December, and load reduction is almost always the cheapest lever on an off-grid property.
- Heat is the big one. Resistance electric heat is brutal on an off-grid system in winter. A heat pump is far better; wood or propane for the coldest stretch is better still, and takes the peak off the design.
- Hot water. Electric tanks are a large, inflexible winter load. Propane or a heat-pump water heater changes the picture.
- Well pumps and septic. Short, sharp draws that size the inverter rather than the array. They need to be in the load list from the start.
- Everything always-on. Fridges, freezers, routers, pumps and standby loads run 24 hours a day in December too, and they add up faster than the occasional kettle.
Seasonal cabins are a different problem
If the property is closed from November to March, the December constraint largely disappears and the system gets dramatically cheaper — you are designing for shoulder seasons instead. What matters then is what happens while nobody is there: keeping the battery healthy, protecting against freezing, and being able to check the system remotely before you drive up.
Be honest at design time about which case you are in. A system sized for a summer cabin will disappoint badly if the use pattern later becomes full-time, and retrofitting more array and battery afterwards costs more than building it in.
What we need from you to size it
- Location and, if you have it, the roof or ground-mount orientation and any shading — trees matter enormously in winter when the sun is low.
- The loads you actually need to run, and which of them are negotiable.
- How you use the place through the year, month by month.
- Whether propane or wood is available for heat and hot water.
We size for the darkest weeks first and then check the rest of the year follows, rather than the other way round.
Sources
- Natural Resources Canada, Photovoltaic Potential and Solar Resource Maps of Canada (municipality database), as compiled by Green Building Canada — Vancouver 938 kWh/kWp/yr, BC average 1,062, Victoria 1,018.
How the array is oriented differently off-grid
A grid-tied array is usually oriented to maximise annual yield, because every kilowatt-hour has similar value whenever it arrives. Off-grid, a July kilowatt-hour you cannot use is worth nothing and a January one is worth a great deal, so the array is aimed at the hard season instead.
- Steeper tilt. A steeper angle favours the low winter sun, sheds rain and debris, and gives up summer output you were going to waste anyway.
- South-facing priority. East/west splitting spreads production across the day, which suits grid-tied self-consumption. Off-grid in December, total harvest in a short window matters more.
- Ground mount deserves a look. On rural property there is often space for a ground array at ideal tilt and orientation, clear of the tree line, and far easier to clean and service than a roof.
- Shading is unforgiving. Winter sun is low, so shadows are long. A tree line that is irrelevant in June can dominate the array in December.
The load audit
Grid-tied design starts from your bill. Off-grid there is no bill, so the design starts from a load list: every device, its draw, and how many hours a day it runs in winter. It is tedious and it is the most valuable hour you will spend on the project.
The audit produces two numbers that size different things. Daily energy in kilowatt-hours sizes the array and the battery. Peak simultaneous draw in kilowatts sizes the inverter. A property can have modest daily consumption and still need a large inverter because a well pump and a kettle occasionally coincide.
Oversizing and what happens to the surplus
An off-grid array sized for December will produce far more than the property can use for much of the year, and the controller simply throttles back — the panels sit idle rather than generating into nowhere. That is normal and not a fault.
It also creates an opportunity. Summer surplus is free energy that would otherwise be curtailed, so it is worth putting to work: heating domestic hot water, running a dehumidifier in a crawlspace, topping up an EV, or timing any deferrable load to the sunny half of the year.
Hybrid heating changes the whole design
Nothing reduces an off-grid system’s cost like taking winter heat off it. Electric resistance heat in December is the single most expensive load an off-grid property can carry, because it peaks exactly when generation collapses.
Wood or propane for heat and hot water through the coldest months lets the electrical system be sized for lighting, refrigeration, pumps and electronics — a far smaller and cheaper problem. A heat pump sits between the two: much more efficient than resistance heat, but still a meaningful winter load, and worth modelling honestly rather than assuming.
What a realistic design conversation looks like
We work through the load list, agree which loads are essential in December and which are negotiable, pick a target for how often the generator should run in a normal winter, and then size array and battery to hit it. Then we test the design against a bad year rather than an average one, because an average year is not what strands people.
The output is a system with a stated behaviour: this is what it does in July, this is what it does in December, and this is roughly how many hours the generator runs. That is a far more useful thing to buy than a headline kilowatt figure.
Frequently asked questions
How many days of autonomy do I need?
It depends on how the property is used and how much generator running you will tolerate. More autonomy means a larger, costlier bank that is lightly cycled most of the year; less means a smaller bank and more generator hours. We set the target with you rather than applying a default, because the right answer for a full-time residence differs from that for a weekend cabin.
Can I run a heat pump off-grid in a BC winter?
Yes, but it has to be in the design from the beginning rather than added afterwards, because it is a substantial winter load arriving exactly when generation is weakest. Many coastal off-grid properties pair a heat pump for the shoulder seasons with wood or propane for the coldest weeks, which keeps the electrical system to a sensible size.
What size array does an off-grid cabin need?
There is no standard answer, because off-grid arrays are sized from a load list rather than from floor area or occupancy. Two cabins of identical size can differ several-fold depending on whether heat and hot water are electric. The load audit is what produces the number.
Designing an off-grid system for your property
This guide is one part of a bigger picture. Our off-grid and remote solar page sets out how we approach a whole system — array, storage, generator and monitoring — for cabins, islands and acreage across coastal BC, and the other guides in this series cover the grid-extension comparison, winter sizing, rebates and PST, and batteries and generators.