Solar & Off-Grid Battery Storage | Self-Consumption & Autonomy

Size for the worst week, not the average

Off-grid systems fail in winter, and they fail because the battery was sized against average generation. A location with an annual average of four peak sun hours may have a week in December averaging under one.

Design the autonomy around that week. With a generator as backup, two to three days is usually sufficient. Without one, the answer is seven days or more, and the cost rises sharply — which is why most practical off-grid systems include a generator, even one that runs rarely.

Charge current is the constraint people miss

A battery accepts a maximum charge current. A 15 kW array at realistic output produces more than a single 100 Ah 48 V pack can absorb, and the excess is not harvested.

If the array is sized to maximise winter generation — which is what off-grid design usually requires — the bank must be sized so the accepted charge current matches the array's realistic peak. Otherwise a significant part of the investment in panels is idle for much of the year.

Cycle depth and system life

An off-grid battery cycles every day, which is a harder duty than a grid-connected system that cycles only when the tariff makes it worthwhile.

Two things protect the life of the bank: keeping daily depth of discharge at or below 80 %, and keeping the cells at a reasonable temperature. Where the battery is installed in an unheated outbuilding, the winter combination of cold cells and deep discharge is what ends a bank early.

We will size the bank so that daily depth stays within the design window at your consumption, rather than selling you the smallest bank that technically covers the average day.

FAQ

How many days of autonomy do I need?

Two to three days for a system with a generator as backup, seven or more for a system with none. The figure is driven by the worst consecutive run of low-generation days in your location, not the average. Tell us the location and whether a generator is available and we will size accordingly.

Why does my battery never reach full charge in winter?

Because the array's winter output is a fraction of its summer output, and often less than the daily consumption. The battery is fine; the generation is the constraint. The fix is more array or less consumption, not a larger battery — a bigger battery simply spends more time partially charged.

Does charge current matter for the battery size?

Yes, and it is frequently ignored. A 15 kW array can produce more charge current than a single pack accepts, in which case the surplus is simply not harvested. If the array is sized to maximise winter generation, size the bank so the accepted charge current matches the realistic peak array output.

What depth of discharge should I design for?

Aim for a maximum of 80 % in normal operation and treat 90 % as an occasional event. Cycle life is specified at a stated depth, so a system that discharges to empty every night will not reach its rated cycle count. A slightly larger bank cycled shallowly usually costs less over its life.