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BacklogCalculations explained

The piDSS calculations, in plain English

Every number the app shows comes from the steps below, and each step names the story that builds it. Open any story on the board and its detail view links straight to the section here that explains it. If you can talk through these at a demo, you can answer any “where did that number come from” question.

1. How much electricity the business uses (stories 5.1 and 5.2)

  • 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 (story 5.2): pretend everything switches on at the same moment and add up all the watts. That worst-case number is the peak simultaneous draw.
  • Day versus night (story 5.2): the day's kWh are split into daytime hours (8am to 5pm, when the sun is up) and nighttime hours, using the site's operating profile. A business that only runs in the day should never be told to buy nighttime batteries.

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 (stories 5.4 and 5.8)

  • 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.
  • VAT: 17.5 percent for businesses.
  • The order: energy charge + fixed charge + FCA gives a subtotal. VAT is charged on the subtotal. That gives the total.
  • Story 5.8 shows this bill next to the after-solar bill, so the difference is the saving, line by line.

4. How big the solar system needs to be (stories 5.5, 5.6 and 5.7)

  • 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.
  • Any solar gear the business already owns is deducted first (story 5.5): never recommend capacity the client already has.
  • Number of panels = array kW x 1,000 / panel watts, always rounded up.
  • The mounting-area clip (story 5.6): panel count times each panel's footprint must fit inside the site's panel mounting area. If it does not fit, the recommendation is capped at what fits, and the screen says the area limited it.
  • Inverter: sized to match the array, with extra headroom above the peak draw so surges (motors, compressors starting) do not trip it.
  • The energy balance (story 5.7) obeys one physics rule: exported energy can never exceed generated energy. Solar generation is split into what the building uses itself (capped at what it actually uses in the daytime), what charges batteries, what is exported, and what is curtailed (wasted because it can be neither used, stored nor exported). Under the BLPC interconnection pause, a Zero-Export system exports nothing: the surplus is curtailed instead.

5. Money from exporting (feed-in tariff, story 5.9)

  • Whatever solar makes and the business does not use itself goes out to the grid, and the utility pays per kWh.
  • The exported kWh comes from the energy balance (story 5.7), never from a number typed by hand.
  • The rate depends on the size band the system falls into.
  • 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. What the system costs up front (stories 5.10 and 5.11)

  • Hardware (story 5.10): for each supplier that stocks every part of the recommended system, add up that supplier's current prices. Every total is shown with its as-of date and an “indicative, not a binding quote” label, because prices move.
  • Everything else you pay once (story 5.11), itemised: the certified survey fee, engineering and consultation, installation labour (a benchmark rate per kW times the system size), the BLPC interconnection and inspection fee, and physical security (anti-theft fasteners, plus locking cages when batteries or outdoor inverters are in the design).
  • Up front means paid once. None of these belong in the yearly running costs below.
  • Each supplier's hardware total plus these one-off costs gives that supplier's Total Upfront Cost.

7. What the system costs every year (story 5.12)

  • The maintenance plan price for a year.
  • Insurance: a flat 1.5 percent of the Total Upfront Cost each year, covering hurricane and theft for a commercial system in Barbados.
  • Replacement reserve: the kit's inverter cost and battery cost, each divided by a 10-year expected lifespan. The component prices come from the kit itself, so the reserve follows the build. Ten years matches the battery lifespan the payback story (5.15) already uses.
  • Panels are not in the reserve: they last 25 years or more, past the payback window, so nothing is set aside for them. Real panels also lose a little output each year (about 0.5 percent); modelling that degradation is documented future work, not part of this review.
  • These are subtracted from the year's savings so the savings number is honest. Skipping them is how solar quotes fool people.

8. The tax incentive, exactly (story 5.13)

  • Method: it is a deduction from taxable income, not a discount and not a rebate. The business deducts 150 percent of the eligible spend (the Total Upfront Cost from story 5.11) from its taxable income. The actual cash it keeps equals that deduction times the company's tax rate.
  • Worked example: 20,000 BBD of eligible spend gives a 30,000 BBD deduction; at a 25 percent tax rate the business pays 7,500 BBD less tax.
  • Cap: the deduction is capped per year at 25,000 BBD for corporations and 10,000 BBD for individuals. If 150 percent of the spend is bigger than the cap, the deduction is the cap, and the screen says so.
  • Eligibility: the platform serves businesses, so the corporate cap and the corporate tax rate apply. Until the client record carries an entity type and tax rate (story 1.7, Iteration 4), the tax rate is an input on the calculation, and the cap used is the corporate one.
  • Timing: the deduction is claimed once, against the year the money was spent (year 1). It is NOT added to every year's savings, and it is NOT subtracted from the invoice. It reduces the payback calculation's starting cost, once.
  • The rate and cap live in the tax rules data with their source and effective date, so a change in the law is a data edit, not a code change.

9. Net annual cash savings (story 5.14)

  • One formula: annual bill reduction (from 5.4 and 5.8) + export earnings (from 5.9) − yearly running costs (from 5.12).
  • What does NOT belong here: hardware and one-off costs (5.10 and 5.11 are capital, paid once, not yearly) and the tax incentive (a year-1 one-off, see section 8). Putting either into the annual figure is the classic mistake this story exists to prevent.
  • This is a recurring, every-year number. It feeds the payback story, which handles the one-off money.

10. Payback and the verdict (story 5.15)

  • Net cost = Total Upfront Cost (5.10 + 5.11) − the year-1 tax benefit (5.13).
  • Payback in years = net cost / net annual savings (5.14). Displayed in years, one decimal.
  • The verdict: beyond 25 years the system is flagged “not economically viable”; for off-grid designs with batteries the threshold is 10 years. If savings are zero or negative, the app shows that payback never happens instead of dividing by zero.

11. Loan costs (stories 5.16 and 5.17)

  • 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.
  • Payback with financing (story 5.17) 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.

12. What a budget buys today (story 5.22)

  • Only ever shown after the full recommendation, never as a gate: the client has already seen what solar can do before anyone asks what they can spend.
  • The client enters a capital cap and the site shows the largest kit or partial system inside it, the share of their energy it covers, and the gap to the full system.

13. The no-login quick estimate (story 0.7)

  • A visitor types two numbers from their bill: the monthly amount in BBD and the monthly kWh. Effective rate = amount divided by kWh, which doubles as a sanity check on the inputs.
  • Daytime share: all of it for daytime-only businesses, the daytime fraction for 24-hour ones. Kits carry no batteries, so solar can only offset daytime use.
  • Required size = (monthly kWh x daytime share) / (30 x peak sun hours). The site recommends the smallest kit that covers that, or the largest kit if none does.
  • Offset = the smaller of what the kit generates and what the business uses in the day. Annual savings = offset x effective rate x 12. Payback = kit price / annual savings, with a “does not pay back” message when savings are zero.
  • Every figure is labelled preliminary, hardware price only, because installation and ownership costs are what a registered client gets to see properly.