The piDSS calculations, in plain English

1. How much electricity the business uses (load analysis)

• Every item of equipment has three things: a power rating in watts (how fast it eats electricity), how many of them there are, and how many hours per day they run.
• One item per day: watts x hours x quantity = watt-hours.
• Add up every item and divide by 1,000: kilowatt-hours (kWh) per day. Times 30: kWh per month.
• Peak draw: pretend everything switches on at the same moment and add up all the watts. That worst-case number is the peak simultaneous draw.

2. Which tariff class (BLPC)

• Monthly usage up to 1,500 kWh: General Service rates.
• Above 1,500 kWh: Secondary Voltage Power rates.
• Above 1,500 kWh the Secondary Voltage Power rates also charge for demand: the highest power drawn during the month, measured in kVA, not just the energy used.

3. The bill today, without solar

• Energy charge: BLPC charges in blocks. The first block of kWh costs one rate, the next block costs another, and so on. Add up the blocks.
• Fixed charge: a flat amount every month just for being connected.
• Fuel clause adjustment (FCA): an extra charge per kWh that moves every month with fuel prices. This number is typed in as data, never baked into the code.
• On time payment discount: a small percentage off if the bill is paid by the due date.
• VAT: 17.5 percent for businesses.
• The order: energy charge + fixed charge + FCA gives a subtotal. The discount comes off the subtotal. VAT is charged on what is left. That gives the total.

4. How big the solar system needs to be (sizing)

• Array size in kW = daily kWh / (5.5 x 0.86). The 5.5 is the peak sun hours for Barbados (published solar data) and the 0.86 is the loss allowance: after heat, wiring, dirt and inverter losses, only about 86 percent of the rated power actually arrives.
• Number of panels = array kW x 1,000 / panel watts, always rounded up.
• Inverter: sized to match the array, with extra headroom above the peak draw so surges (motors, compressors starting) do not trip it.

5. Money from exporting (feed-in tariff)

• Whatever solar makes and the business does not use itself goes out to the grid, and the utility pays per kWh.
• The rate depends on the size band the system falls into.
• Systems up to 3 kW and systems above 3 kW follow different billing setup rules.
• The rates are data, not code: the old schedule expired December 31, 2025 and the 2026 rates have not been announced, so the numbers stay editable until they are confirmed.

6. Tax incentive (the 150 percent deduction)

• The business deducts 1.5 times the system cost from its taxable income.
• The actual cash saving depends on the tax rate that business pays.
• The deduction is capped: 10,000 per year for individuals, 25,000 per year for corporations.
• The deduction is claimed through the tax return, so treating it as an immediate discount is a simplification, fine for a first estimate.

7. Net cost

System cost (equipment plus installation) plus grid connection costs, minus the cash value of the incentive. That is what the business would actually pay. Grid connection costs (the application, inspection and metering fees for tying the system to the grid) come from the original project brief and are editable data.

8. Savings and payback

• With solar, the business buys less from the grid during the day and earns credits on exports.
• Yearly savings = (old monthly bill minus new monthly bill) x 12.
• Simple payback in years = net cost / yearly savings.
• If savings come out at zero, payback never happens. The app must show that as a message, never crash on the division.

9. Loan costs (financing the system)

• The monthly payment is the fixed amount that pays the loan off exactly over its term, with interest charged each month on whatever is left of the balance.
• The app shows the monthly payment, the total interest over the term and the total paid.
• Interest rates are editable data, not baked into code.
• The comparison that matters: if the monthly loan payment is smaller than the monthly bill savings, the system pays for itself from day one.

10. Payback with a loan

• Payback with financing is the point where the savings built up so far catch up with everything paid so far (loan payments plus any up front costs).
• The app shows payback with and without a loan so the business can compare paying now against financing.

11. What a budget buys today

• The customer enters what they can spend right now, and the site works out how much solar that buys from the catalogue.
• It also shows the cost of the full recommended system next to it, so the customer can see the gap and decide what to add later.
• A budget is what someone can afford today, not a promise about next month, so the site never assumes the same money will be there again.

12. Commissions and listing fees (the supplier layer)

• Commission: the accepted referral's system cost times that supplier's commission rate.
• Listing fee: a flat monthly amount for suppliers that carry one.
• Both rates are editable data and everything is worked out on paper with simulated data: no real money is processed.