A commercial solar quote's savings projection is only as credible as the billing data it's actually built from — and checking that connection yourself, rather than trusting the headline number, is one of the more useful things a business owner can do before committing.
Pull at Least 12 Months of Billing History
A single month's bill doesn't capture seasonal variation in your own usage, and a quote based on one atypical month can be meaningfully off. Twelve months gives a proper picture of your actual usage pattern, including any seasonal peaks the quote should be accounting for.
Check What Usage Figure the Quote Actually Used
Ask your installer directly what usage figure their savings projection is based on — your average monthly bill, your highest month, a full year's total consumption? As covered in our piece on how installers size systems, this should be explicit, not left implicit in the summary numbers.
Compare the Quoted Self-Consumption Assumption to Reality
If the quote assumes a specific self-consumption rate (how much generated power your business actually uses directly, versus exporting), check whether that assumption is realistic given your actual operating hours and load pattern — a quote assuming high self-consumption for a business with significant after-hours or weekend closure may be overstating achievable savings, a concern covered more broadly in our piece on commercial solar ROI and load profiles.
Does the Quote Account for Your Actual Tariff Structure?
Commercial tariffs, particularly time-of-use structures, can be genuinely complex — confirm the quote's savings calculation uses your actual current tariff, not a generic or outdated rate. A quote built on stale tariff assumptions will overstate or understate your real savings.
Run a Simple Sanity Check Yourself
Take the quote's projected annual savings and divide by your actual annual electricity spend from your billing history — does the resulting percentage offset look plausible given the proposed system size relative to your roof space and usage? A projection claiming an unusually high percentage offset from a modest system size is worth questioning directly.
Ask for the Math, Not Just the Conclusion
A credible installer should be willing and able to walk through exactly how they got from your billing data to their savings projection — if that explanation is vague or the installer seems reluctant to show their working, treat that as a meaningful signal, echoing the broader red-flag guidance in our piece on red flags in an installer's proposal.
Getting an Independent Reference Point
Our solar calculator gives an independent estimate based on your own inputs — a useful cross-check against any specific quote's numbers before you commit. Compare installers willing to show transparent, billing-based calculations on ADEO.
Frequently Asked Questions
How many months of billing history is genuinely enough to benchmark a quote properly? Twelve months is ideal to capture seasonal variation — anything significantly less risks a projection based on an unrepresentative period.
What if my business's usage has changed significantly in the past year? Flag this directly to your installer — a quote based on outdated usage patterns (before an expansion or operational change, for instance) needs to be recalculated against your current, realistic usage.
Is it reasonable to ask an installer to redo their calculation with different assumptions? Yes — a legitimate installer should be able to adjust their projection if you provide more accurate or updated billing data, without treating the request as unreasonable.
Writes about solar and renewable energy for South African homes and businesses at SolarTally, and cross-checks every guide against current tariffs and installer standards.
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