Off-grid solar systems

Who actually needs one

Sites with no mains connection at all: land outside the distribution network, a tube well in a field, a room or a workshop where bringing a supply in costs more than generating your own.

Sites where a connection exists but cannot be relied upon for the work being done. That is a judgement about consequences rather than about hours: an outage that stops a business is a different problem from one that is merely irritating.

If you do have a usable supply and what you want is lower bills and power through load shedding, a hybrid system is the better buy. It uses the same equipment, is sized far less conservatively because the grid is behind it, and costs considerably less for the same comfort. We will say so rather than sell you the larger system.

Why the sizing is different

A grid-connected system is sized against an average. If a week is poor, the grid quietly makes up the difference and nobody notices. Off-grid, nothing makes it up: the battery runs down and the lights go out.

So the design question is not what you use in a normal week but how many days the system must carry you through bad weather. More days of autonomy means a larger battery bank, and the battery is the expensive part. This is the decision that sets the cost of an off-grid system, and it is worth taking seriously at the survey rather than discovering in January.

Everything is also sized to the real peak. There is no grid to absorb the surge when a pump or a compressor starts, so the inverter must supply it alone. Motors are the common cause of an off-grid system that trips under a load the owner thought was modest.

What we build them from

The hybrid inverters we fit run a system with no grid connected — that configuration is what the inverter is designed for, and it is the same unit we install on a grid-connected hybrid system. Lithium storage from Solax, Itell and Ziewnic, in 25.6V and 51.2V.

Lithium matters more off-grid than anywhere else. You can draw nearly all of a lithium battery's rated capacity, against roughly half of a tubular one, so a bank sized for several days of autonomy is both smaller and longer-lived. On a site you cannot reach easily, a battery that needs no watering is worth more again.

Panel capacity is generally greater than a grid-connected system of the same load, because the array has to refill the battery as well as run the house, and has to manage it on a short winter day rather than a long summer one.

What we will tell you honestly

An off-grid system is a budget decision about how much bad weather you want to ride out. Nobody can size one for every conceivable week without the cost becoming absurd, so the right design states plainly what it is built to carry and what would exhaust it.

We would rather set that expectation at the survey than have you find the edge of it in the middle of winter. If the honest answer is that your site is better served by a grid connection and a hybrid system, that is what we will quote.