A useful estimate of solar savings starts with the electricity your household buys and when you use it. A system’s annual generation is only part of the calculation: power used in the home can avoid retail electricity charges, while exported power is usually credited at a separate rate. To estimate solar savings realistically, combine your bills or interval data with a site-specific generation forecast, the retailer’s import and export rates, and the full installed cost. Then test how the result changes with a battery, a different system size or shifts in household energy use. This gives you a sounder basis for comparing quotes than a headline savings figure.
Solar savings come from two different uses of the system’s output. When solar electricity powers appliances in your home, you buy less electricity from your retailer. When generation exceeds household demand, the surplus may be exported to the grid and credited to your account. These two amounts should not be valued at the same rate unless your electricity plan genuinely charges and credits them equally.
A practical annual estimate is:
Estimated bill benefit = solar electricity used at home × import rate + solar electricity exported × export rate
Use the variable import rate you would actually avoid, rather than treating every part of your bill as a solar saving. Daily fixed charges generally remain while you stay connected to the grid. If your plan has time-of-use rates, use the rates that apply at the times solar is likely to supply your home, and check whether adding solar would change your plan or other charges.
| Energy outcome | How it affects the bill | What to estimate | Common mistake |
|---|---|---|---|
| Used directly in the home | Reduces electricity bought from the retailer | Solar generation that coincides with household demand | Valuing all generation at the full import rate |
| Exported to the grid | May earn an export credit | Surplus generation and the plan’s export rate | Assuming exported power earns the same as avoided imports |
| Not used or credited | May have little or no bill value | How the system and retailer treat surplus energy | Counting every generated kilowatt-hour as a saving |
For New Zealand households, the import tariff and export credit depend on the electricity plan and retailer. Check the rates and any plan conditions that would apply to your home, rather than relying on a rate quoted in an online example. Also confirm whether the estimate refers to lower electricity charges, export credits, or both.
Start with a full year of electricity bills so seasonal changes are represented. Record the household’s consumption in kilowatt-hours (kWh), not just the dollar total. Bill amounts can change because of tariff changes, discounts or plan features, so energy use provides a more stable starting point.
If your retailer provides interval or half-hourly usage data, use it. Annual totals show how much electricity the household consumes, but interval data reveals whether that demand happens during daylight hours, when panels are generating. If detailed data is unavailable, note the regular daytime loads and be cautious about any estimate that assumes the household uses a large share of solar power without explaining why.
Consider future electricity use, but separate confirmed plans from possibilities. An electric vehicle that will be charged at home could change the value of daytime generation, especially if charging can be scheduled while the system is producing. A possible future renovation is less certain and should not be treated as guaranteed demand.
The next input is the electricity the proposed system could generate at your property. Ask for an estimate based on your location and roof, not simply a generic output figure for a system of a particular size. Roof orientation, tilt, shade from trees or nearby structures, equipment and local weather all affect production. A site assessment should identify the assumptions behind the forecast and any roof areas excluded from the design.
Request monthly as well as annual generation estimates. A single annual total can hide a mismatch: production may be plentiful in sunnier months and lower in winter, while household consumption may follow a different pattern. Monthly estimates help you see when the system is likely to cover more of your use and when you may still rely heavily on grid electricity.
Ask whether the generation forecast includes expected system losses and shading. Clarify the proposed panel capacity, inverter arrangement and any limits on export. The purpose is not to judge a design by one technical figure; it is to understand what assumptions drive the predicted energy output and whether they match your property.
When comparing installers, give each the same household usage information and ask for the same forecast period and assumptions. Otherwise, two savings estimates may differ because they use different inputs rather than because one system is genuinely better suited to your home.
Annual generation and annual consumption cannot simply be subtracted to find your bill savings. If panels produce power while the home is using electricity, that output can displace some grid purchases. If the home is quiet during the day and demand rises in the evening, more generation may be exported instead. The value of the system depends on both the amount and timing of production and use.
Look at the household’s routine and regular loads. Appliances that can be run during the day may use more of the system’s output directly, but only if their timing is practical and their electricity demand overlaps with generation. Do not assume every appliance will operate at full solar coverage: production varies with weather and time, and household demand changes from day to day.
A simple estimate may divide annual solar generation into two parts: electricity expected to be used in the home and electricity expected to be exported. For a more refined estimate, ask the installer how those shares were calculated and whether the model uses your household’s load profile or a general assumption. A high self-consumption assumption deserves an explanation, particularly if the home is usually empty during the day.
Suppose a household uses 8,000 kWh in a year and a proposed system is forecast to generate 6,000 kWh. For illustration only, assume the household uses 3,000 kWh of that generation directly and exports the other 3,000 kWh. If the avoided import rate were NZ$0.30 per kWh and the export credit NZ$0.10 per kWh, the estimated annual bill benefit would be NZ$900 from avoided imports plus NZ$300 from exports, or NZ$1,200 in total.
These rates and energy figures are invented for the calculation, not presented as typical New Zealand prices or system performance. The example also leaves out fixed charges, financing costs and other ownership expenses. Use your own tariff and a property-specific forecast to produce a personal estimate.
A savings estimate is not a financial decision on its own. Compare the expected bill benefit with the total cost of installing and owning the system. Review the quoted installed price and what it includes, such as panels, inverter, mounting, electrical work, monitoring and any site-specific requirements. Ask whether additional work could be needed and how it would be priced.
For a basic screening calculation, divide the net upfront cost by the estimated annual bill benefit. The result is a simple payback estimate, not a guarantee of when the system will pay for itself. It assumes the annual benefit remains unchanged and ignores factors such as financing, future electricity rates, maintenance, equipment replacement and the value of money over time.
If you are borrowing to pay for solar, compare the loan repayments and fees with the estimated bill benefit over the same period. A system can reduce electricity purchases while still putting pressure on monthly cash flow if repayments are higher than the bill reduction. For a cash purchase, consider whether the expected savings justify using the money for solar instead of another household priority.
Do not assume an incentive or subsidy will apply. If a financial support programme is available to you, verify its current terms, eligibility and effect on the net cost before including it in your calculation. Compare financing offers and incentives separately from the system’s predicted generation so that the basic energy economics remain clear.
A battery stores some solar electricity for use later, which can reduce the amount exported during the day and the electricity bought from the grid in the evening. It does not create more solar generation, and its added cost has to be weighed against the value of the extra energy you can use. The result depends on your usage pattern, battery settings, tariffs, system design and the amount of solar surplus available to charge it.
| Option | Potential advantage | Main limitation | Useful question |
|---|---|---|---|
| Solar without a battery | Uses daytime generation directly and may earn export credits | Evening demand may still require grid electricity | How much generation will be used at home or exported? |
| Solar with a battery | Can shift some stored solar energy to later household use | Adds upfront cost and energy is lost during storage and delivery | What extra annual bill benefit is attributed to the battery? |
| Solar with load shifting | May increase direct use by scheduling flexible demand for sunny periods | Depends on household routines and appliance flexibility | Which loads can realistically run during generation hours? |
Ask for separate savings estimates for solar alone and solar with a battery. The difference between them is more informative than a combined headline figure. Check the forecast battery size and operating assumptions, the expected extra self-consumption, warranties and any likely replacement costs. A battery may suit a household seeking more evening use of its own generation, but it is not automatically the cheapest way to improve the financial return.
Put each quote beside the same household data and compare the assumptions, not just the promised savings. A useful estimate should distinguish annual generation, direct household use and exports, then show how each is valued. If a quote gives a savings figure without showing the import rate, export rate or assumed self-consumption, ask for a revised breakdown.
Be cautious when a proposal assumes that all solar generation offsets electricity bought at the retail rate, or when it presents payback without disclosing its inputs. A strong estimate does not need to promise a precise future bill. It should make its assumptions visible and allow you to test how the result changes when usage, tariffs or export credits differ.
Annual use is a useful starting point, but it does not show when the household consumes electricity. Because the value of solar depends partly on how much generation is used directly versus exported, interval data or a realistic load-profile estimate gives a better result.
Your import rate is what you pay to buy electricity, while the export rate is the credit your retailer offers for surplus electricity sent to the grid. They are separate parts of your plan, so use the actual rates that would apply to your home rather than assuming exported energy is worth the full retail price.
Not necessarily. A grid-connected home may still buy electricity when solar production is low or household demand is high, and fixed charges may continue. A realistic estimate should show the likely reduction in charges, not assume the entire bill disappears.
A battery can shift some solar energy for later use, but its extra cost may outweigh the value of the additional electricity used at home. Compare separate forecasts for the same solar system with and without a battery, and check how the battery’s costs and operating assumptions affect the result.
Simple payback compares the net system cost with estimated annual bill savings, while financing adds repayments and fees that affect cash flow. Compare total costs and expected benefits over the same period, and treat simple payback as a screening measure rather than a promise.
To estimate solar savings credibly, use a year of household electricity data, a site-specific generation forecast and the import and export rates on your actual plan. Separate direct use from exported power, then compare the resulting bill benefit with the full system cost and any financing or battery expense. If a quote cannot show those inputs, ask for a clearer breakdown before using its headline number to make a decision.
