For a small solar company, the warehouse is often just "the back of the workshop": a few pallets of modules, inverters on a shelf, cable drums in the corner. As the number of projects grows, that stops working. The installation crew arrives on site and finds a pack of clamps missing, the same inverter has been promised to two customers, or nobody can say what a project really earned because nobody knows what the modules actually cost. This article covers the basics of warehouse and inventory management for solar installers, with a worked cost example and practical routines.
What goes wrong without stock control
- Site delays: A missing connector or rail splice sends the crew home with the job unfinished, and a working day is lost.
- Duplicate purchasing: Material that is already in the warehouse is ordered again because there is no record of it.
- Wrong proposals: An item that is out of stock goes into a proposal, and the delivery date promised to the customer slips.
- Unknown profitability: Without the real cost of materials, you cannot tell afterwards whether a project made money.
Item categories
Good inventory control starts with the right categories. Each one needs to be tracked in a slightly different way:
| Category | Typical items | What to watch |
|---|---|---|
| Modules | Panels of different makes and power classes | Each model and power class should be a separate item; pallet and serial numbers matter for warranty claims |
| Inverters | Single-phase, three-phase, hybrid, off-grid | High unit value; track by serial number which project each one went to |
| Batteries | Lithium, gel | Shelf life and storage conditions should follow the manufacturer's instructions |
| Mounting | Rails, hooks, mid and end clamps, rail splices, bolts, ground-mount profiles | Many small items; units (pieces, metres, lengths) must be consistent |
| Cables | DC solar cable, AC cable, earthing | Tracked in metres; offcuts from drums should be recorded separately |
| Protection and boards | Fuses, surge protection, RCDs, DC/AC boards, connectors | Cheap items whose absence stops the job; always set a minimum level |
Linking receipts and issues to projects
The point of recording stock movements is not only to answer "how many are left?". If every issue is linked to a project, you also know the true material cost of that project when it is finished. Practical rules:
- Record every receipt with the supplier and the purchase price.
- Link every issue to a project or customer; "general use" issues should be the exception.
- Keep shipment details (date, vehicle, person who received the goods) with the issue, so there is a record if something is reported missing on site.
Reserving stock from approved proposals
The most common gap between sales and the warehouse is this: a proposal is approved, but the warehouse only hears about it weeks later, on installation day. By then the same material may have gone to another project. The fix is to reserve or deduct the items in an approved proposal immediately, so that the "available" quantity shows what can really be used elsewhere. If the proposal is cancelled, the material must go back into stock; otherwise the warehouse looks empty on paper and full on the shelves.
How weighted average cost works
Module and inverter prices change from month to month. If you bought the same module at different prices on different dates, what does a module leaving the warehouse cost? Under the weighted average cost method, every new receipt is combined with the stock on hand and the unit cost is recalculated:
New unit cost = (Quantity on hand × unit cost on hand + Quantity received × purchase price) ÷ (Quantity on hand + Quantity received)
With sample values (the prices are only there to illustrate the arithmetic):
| Movement | Quantity | Unit cost | Total value |
|---|---|---|---|
| Stock on hand | 100 | 3,000 | 300,000 |
| New receipt | 50 | 3,300 | 165,000 |
| After receipt | 150 | 3,100 | 465,000 |
| Issue to project | 40 | 3,100 | 124,000 |
| After issue | 110 | 3,100 | 341,000 |
The arithmetic: 300,000 + 165,000 = 465,000; 465,000 ÷ 150 = 3,100 per unit. Issues do not change the unit cost; they only reduce the quantity and total value: 40 × 3,100 = 124,000, leaving 465,000 − 124,000 = 341,000. This way a project's material cost reflects the real average in the warehouse rather than an extreme such as the latest or the earliest purchase price.
Returns from site
After almost every installation some material is left over: a few clamps, half a drum of cable, an unused fuse. If it comes back to the warehouse without being recorded, two things go wrong: the project cost looks higher than it was, and the stock count stops matching. Record returns from site as a separate movement, note which project they came from, and make the return part of the crew's job close-out. Keep damaged returns apart from sound stock.
Low-stock thresholds
Set a minimum level for every item. Two questions help: how much of this item do we typically use in a week, and how many days does the supplier need to deliver it? The quantity used during the delivery lead time plus a small safety margin is a reasonable starting threshold. For cheap but critical items such as connectors and fuses, be generous; running out costs far more than the items themselves.
Stock counts
However good the system, records and shelves drift apart over time. Regular counts catch this early:
- Count high-value items (inverters, batteries, modules) often and small items less often.
- Avoid receipts and issues during the count, or record the exact moment of the count clearly.
- Before correcting a difference, look for the cause: an unrecorded return, the wrong unit (lengths instead of metres) or the wrong item selected are the usual suspects.
Slow-moving and fast-moving items
Items that have not moved for months are tied-up capital. Older module models and inverter sizes nobody asks for lose value over time. Review a list of slow-moving items at regular intervals; prioritise them in suitable projects or adjust your purchasing. The opposite list, the fastest-moving items, tells you where thresholds need to go up.
Warehouse and stock management with PVAGE
In the PVAGE warehouse module you record stock receipts and issues, and costs are tracked using the weighted average method. An approved proposal moves in one step to a contract, the work schedule and a stock deduction; if the proposal is cancelled, the deducted material is returned to stock. Low-stock alerts, returns from site and shipment details sit in the same module, along with slow-moving and fastest-moving item analysis and bulk price updates via Excel.
See the stock and inventory management page for details, and our article on 13-stage solar project tracking to see how procurement connects to the rest of the project.