Savings and key metrics
How savings are two bills, and payback, NPV, IRR, and related metrics.
Reviewed
What this is for
Why the bill did not fall as far as consumption did, and whether the outlay comes back. How a saving is defined lives on Financial models: two computed bills, never production times an average price. This article is the six figures you quote, and the table they reconcile to.
Once a model is computed, those figures sit on the site strip. Until then they are not drawn at all. The list says not computed, never 0.0 yr.
Steps
- Open Financials. Press Save and compute on a model.
- Once computed, the strip carries:
- Payback - years until the cumulative cash flow turns positive, interpolated inside that year. A dash and never means it did not, within the analysis period.
0.0 yearsis immediate (incentives covered the outlay), which is the opposite of never. - Net present value - the net cash flow discounted at the rate on the model, shifted by the install lead time. The unit names that rate.
- Internal rate of return - the discount rate at which that present value is zero. A dash and no sign change means none could be found. That is not a rate of
0%. - Bill offset, year one - the fraction of the without-array bill that the with-array bill removed. It is a money ratio, not a kilowatt-hour ratio.
- Levelised cost - per kilowatt-hour produced. Both the costs and the energy are discounted, or a kilowatt-hour in year twenty is treated as worth the same as one next year.
- Self-consumption - of production, year one. How much of what the array made the site used. It is not self-sufficiency (how much of what the site drew the array covered). When a matching storage model exists (the same run and the same consumption profile), the cash flow prices the dispatched flows. Without one it is solar-only.
- Payback - years until the cumulative cash flow turns positive, interpolated inside that year. A dash and never means it did not, within the analysis period.
- On the model's Results tab, Year one's bill sits the two totals beside each other, then the same charges in three columns: without, with, difference. A charge that cost nothing is still listed, so "this tariff has no demand charge" and "we did not model demand" stay distinguishable. Saved is amber-as-text, never green. Given away appears only when export in hours the schedule does not buy came to something - that is the number that says what a battery would be worth. The monthly bars are the same two series.
- Cash flow is where "why is payback 7.4 years" is answered. Payback is the year the cumulative column crosses zero (those rows are marked). NPV is the discounted total. The lead names the utility escalation and the module degradation the rows used. A zero amount renders as a dash. The first eight years show; Show all years is the rest, already stored. Export CSV is the same rows.
- The knobs that move these figures live on Setup: analysis period, install lead time, discount rate, utility escalation and income tax rate. A new model starts them from the project's profile, which is where your team states its own figures; changing the profile later does not move a model already made. Module degradation is read from the condition set the simulation ran under, not typed here.
Save and compute stays disabled until a design, a completed simulation, a site tariff and a consumption profile are all in place.
Tips and common mistakes
- Do not multiply annual production by an average tariff and call it a saving. Time-of-use, self-consumption and demand charges all go wrong, and they go wrong in the optimistic direction.
- Do not read a dash as zero. Never-pays-back and breaks-even-immediately are opposite answers wearing the same empty box if you fill it with
0.0. - Bill offset will be smaller than the drop in imported kilowatt-hours whenever the tariff has a fixed or capacity line. That is the table working, not a fault.
- Utility escalation is the assumption that moves lifetime savings the most. Check it before you quote the lifetime figure.
- A saving is amber-as-text, never green. Green would read as a verdict.
- The list's not computed and never pays back are different states. A model whose design has moved is STALE - worse than a draft, because it still shows a figure.