Practical guide · stated assumptions and checkable examples
Solar savings: value used electricity separately from exported electricity
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Source and worked-example review; not professional advice or approval. Examples are synthetic unless explicitly identified otherwise. Follow the cited authority for current eligibility and legal requirements.
A solar array can produce the same annual energy at two homes yet create different bill savings. The calculation needs to know when the household can use that energy and how exported energy is credited. Annual generation alone is not an annual saving.
Keep power, energy and money separate
Panel capacity is measured in kW; generation is measured in kWh. PVWatts models production from system and location assumptions. It does not supply this calculator with a live roof survey, your tariff or your export agreement. Enter the annual-generation estimate from a suitable design assessment rather than treating an illustrative default as a prediction.
References: National Laboratory of the Rockies — PVWatts; NIST — Guide for the Use of the International System of Units
A worked household scenario
Assume 8,000 kWh of annual generation, 60% used on site, an avoided import price of 0.20 currency units/kWh and an export credit of 0.05/kWh. The on-site share is 4,800 kWh worth 960. The remaining 3,200 kWh earns 160. Gross modeled annual value is therefore 1,120. Subtract an illustrative 100 annual running cost to obtain 1,020. These are invented scenario prices, not national or local tariff claims.
Calculation: Annual value = generation × self-use share × avoided import rate + generation × export share × export rate − annual operating costs
Test the variable that can actually change
Keep generation and tariffs fixed but reduce self-use to 30%. On-site value becomes 480 and export value becomes 280, leaving 660 after the same operating cost. The 360 difference comes entirely from moving 2,400 kWh from a 0.20 saving to a 0.05 credit. It does not require the array to produce less energy. That sensitivity is useful when comparing household schedules or a battery scenario.
Use bill evidence, not the total-bill average
Separate energy-dependent charges from amounts that remain even when imports fall. A household electricity-use estimate helps describe consumption, but it does not establish export eligibility or a utility credit. Record the tariff date, import and export periods, applicable taxes and any cap or expiry on credits. When a tariff changes by time of day, one blended annual number is only a planning approximation.
References: US Department of Energy — Estimating appliance and home electricity use
Carry the right output into the next calculation
Use the net annual value—not gross generation—as an input to simple payback. Keep grants and other incentives separate from recurring savings. Do not assume an incentive exists because a country has been selected. Obtain written eligibility and settlement terms before including it in the project budget.
Questions about this guide
Does exported electricity always earn the retail import rate?
No. This worksheet uses the export rate you enter. Verify the actual contract and settlement method; a retail import price does not establish an export entitlement.
Is the example a forecast for my roof?
No. Its generation, self-use share and prices are synthetic. Replace them with roof-specific production and household tariff evidence.