Most off-grid capsule projects are designed backwards. The building is chosen, the panels are sized on a summer figure, the battery is bought on price, and the generator is added after the first winter. This is the system-level view: what each component is actually for, where the sizing decisions interact, and which choices you cannot change later.
For the basic load and array calculation, the solar sizing guide covers the method. This article goes past it, into the parts that decide whether the system survives February.
Start with the load profile, not the total
Two sites with the same daily kWh can need completely different systems depending on when the energy is used. A capsule with a heat pump running overnight draws most of its power in darkness, which means the battery does the work and the array only recharges. A capsule whose main load is daytime air conditioning can use the array directly and needs a smaller battery. Same total, different system.
So build an hourly profile rather than a daily sum: heating or cooling overnight, hot water in the morning and evening, cooking at mealtimes, everything else spread thin. Then check the worst realistic day — cold, overcast, guests in residence — rather than the average. Systems are sized by their worst day, and paid for by their average one.
Solar: the number that matters is winter yield
Array output in December at 45° latitude can be a third of July output, and no amount of panel quality changes that ratio. Two design responses exist. The first is to oversize the array so that winter production meets winter load, which produces surplus in summer that can be dumped, diverted to hot water, or used to charge an electric vehicle. The second is to accept a winter deficit and cover it with wind or a generator, which is usually the cheaper of the two if either resource is available.
Tilt angle is a free optimisation that is frequently got wrong. A shallow tilt sized for year-round average production underperforms in winter; steepening the tilt toward the winter sun angle improves the months that matter, at the cost of some summer output you do not need. On a ground mount, make the angle adjustable and change it twice a year — the cheapest upgrade in the system.
Wind: only where the resource is real
Wind is the most oversold component in small off-grid systems. Power available scales with the cube of wind speed, which means a turbine rated at 1 kW in a 12 m/s wind produces a small fraction of that at the 4–5 m/s that most sites actually see. Before considering a turbine, get an actual measurement at hub height for three months, or use a validated wind atlas for the exact coordinates — not the nearest town.
Where wind is genuine — exposed coast, ridge lines, open plains, islands — it complements solar well, because it produces during the grey, windy weather that flattens PV output. Three practical requirements: the tower must put the rotor well above the capsule roof and any nearby obstruction, the turbine must have an automatic furling or braking mechanism for storm protection, and there must be a dump load for surplus generation, or the turbine will over-speed when the battery is full and the wind is strong.
A small turbine on a short mast beside a building is a maintenance item, not a power source. If the tower cannot be tall enough, do not buy the turbine.
Battery: chemistry, temperature, and usable capacity
| Aspect | Lithium iron phosphate | Flooded lead-acid |
|---|---|---|
| Usable depth of discharge | 80–90% | 50% without shortening life |
| Cycle life | Long — thousands of cycles | Short — hundreds at depth |
| Partial state of charge | Tolerated | Degrades the bank |
| Ventilation | Minimal, but thermal control matters | Required — hydrogen venting |
| Cold performance | Charging inhibited below freezing; needs heating or a warm location | Capacity falls sharply in cold |
| Upfront cost | Higher | Lower |
The line that is always underestimated is temperature. A lithium bank at 0°C will not accept charge, which is precisely the condition in a cold climate when the array is producing. The fix is either to locate the bank inside the thermal envelope or in an insulated enclosure with a small heating pad on a thermostat — and that heater is itself a load that has to appear in the sizing.
Usable capacity is what you buy, not nameplate. A 10 kWh lead-acid bank at 50% depth gives 5 kWh; a 10 kWh lithium bank gives 8–9. Compare on usable kWh and cycle life, not on the label.
Inverter: off-grid or hybrid
An off-grid inverter runs the site from the battery and manages the generator. A hybrid inverter adds the grid, so the same unit can operate connected and later switch to backup, or operate off-grid with a grid input as a fallback. If there is any chance a grid connection will arrive at the site later, specify hybrid now — the price difference is small and the retrofit is not.
Two ratings matter. Continuous rating must exceed the simultaneous load, which on a capsule means the heat pump surge plus everything else. Surge rating must cover the compressor start, which can be several times running current for a fraction of a second. An inverter sized on running load alone will trip when the heat pump starts, and it will do so on the coldest night of the year.
Generator integration
A generator is not an admission of failure. A modest unit that runs a few hours a week in the worst month is usually cheaper than the array and battery needed to cover that month on renewables, and it extends battery life by absorbing the deep discharges.
It has to be integrated properly: a transfer switch so it cannot back-feed the inverter, a dedicated external enclosure with exhaust directed away from the units, and a charging profile that the inverter controls rather than one the generator imposes. A generator that is wheeled in and plugged into a socket is a safety problem and will eventually damage the inverter.
Earthing, protection and monitoring
Off-grid systems need an earthing arrangement designed for the site, not inherited from a grid-tied pattern — particularly where the inverter is the source of supply and there is no utility neutral. This is a licensed electrician's job, and it is where a self-installed system is most likely to be unsafe.
Monitoring is the other item that pays. A system with remote monitoring tells you the battery is fading before a guest does, and most inverters and charge controllers expose that data already. If the site is remote, connect it — a fault discovered remotely is a phone call, while the same fault discovered by a guest is a refund and a review.
What to specify
- Hourly load profile for the worst month, per unit type
- Array kWp, tilt, orientation and mounting location
- Wind resource evidence and tower height, if wind is included
- Battery usable kWh, chemistry, location and thermal control
- Inverter continuous and surge rating; off-grid or hybrid
- Generator rating, transfer switch and enclosure position
- Earthing arrangement and protection scheme, signed off by a licensed electrician
- Monitoring and remote access
Related
The building side of the same problem — insulation, thermal break, glazing and shading — is covered in the solar sizing guide and the shell materials comparison. If you are planning a remote site, the extreme climate deployment guide covers cold-weather specifics. Send us the site coordinates, the number of units and whether a grid connection is possible, and we will come back with a system architecture rather than a shopping list.
